An advanced programming language like Java has thousands of points to be asked in interviews to justify candidates when a company needs to hire Java developers. Your bunch of certificates can do nothing, if your answers can’t satisfy the interviewers. However, being a bit strategic and optimum preparation can help you to gain success. This is why we have come up with several core Java interview questions and answers to be a part of your success.
Java Interview Questions & Answers
Basic Java Concepts
1. What is Java?
Java is the most famous programming language in the world. Developed by Sun Microsystems (now owned by Oracle), it is a robust language that can build websites, desktop apps, and games on the go. Perfect solution for the enterprise-related works.
2. What are the features of Java?
With the slogan of “code once, run anywhere,” Java offers a handful of features for developers. It is simple, platform-independent, and allows an object-oriented approach in all works. Along with this, it is known for its enhanced security, high performance, and dynamic multithreading.
3. Explain the Java Virtual Machine (JVM)
Java Virtual Machine is a tool that provides the runtime environment to execute the program. While running the code, JVM calls the main method present in the code. That means every time you write code in the prompt and run the Java class, an object of JVM is created.
4. What is the difference between JDK, JRE, and JVM?
JDK: JDK or Java Development Kit is the entire software development environment necessary to program Java apps. It also consists of an interpreter, compiler archiver, documentation generator, and so on.
JRE: JRE stands for the Java Runtime Environment. It provides the runtime environment for the program and is the actual interpretation of the JVM. It contains libraries and other files that JVM can use.
JVM: JVM does not physically exist; instead, it is a virtual machine. It converts bytecode into machine-readable code to execute the program. Overall, it provides the specifications required to run the program.
5. What are the different data types in Java?
In Java, two data types are mainly available- primitive and non-primitive. Primitive data types include int, double, char, float, etc. while, the non-primitive data types are strings, arrays, and classes.
6. Explain the concept of a class and an object in Java.
In the OOP of Java, the class and object have some sheer differences. The class is like the blueprint or the design of the object, and it states its attributes and features. On the other hand, the object is the instance of the class. For example, we can make a general class named “Car” for the cars, while the specific “Audi” will be the object of the class.
7. What is the difference between static and non-static methods?
Static methods allocate the memory once at a time. On the other hand, the non-static methods allocate the memory with the creation of the object. That means you can’t call the static method without creating the object of the class, unlike the non-static ones.
8. What is inheritance?
Inheritance is the feature of Java by which the class obtains all the attributes and the behavior of another class, mainly called the parent class. This helps to save a lot of time and enables the reuse of methods and fields.
9. What is polymorphism?
Practically speaking, polymorphism means to exist in many forms. You can create an abstract or an interface and implement it in any class you want. Polymorphism mainly can used for method overloading (compile-time polymorphism) and method overriding (runtime polymorphism)
10. Explain the concept of encapsulation.
Practically speaking, polymorphism means to exist in many forms. You can create an abstract or an interface and implement it in any class you want. Polymorphism mainly can used for method overloading (compile-time polymorphism) and method overriding (runtime polymorphism)
11. What is abstraction?
Abstraction masks the program’s detailed implementation, allowing the user to see only the necessary functionalities. It is more like using a remote control which hides the complexity of the process by simply exposing the buttons to manipulate. In Java, abstract classes and interfaces are used to achieve complete abstraction.
12. What are the different access modifiers in Java?
Access modifiers in Java are specific keywords that control the visibility and accessibility of the class and its members. These are used to keep the methods, variables, and classes inaccessible to the users. There are four types of access modifiers in Java: Public, Protected, Default, and Private.
13. What is the difference between == and equals()?
In Java, ‘==’ is regarded as an operator while ‘equals()’ is a method. However the ‘==’ is used to compare the reference of the objects but the ‘equal()’ method compares the content of the string or any object.
14. Explain the concept of constructors in Java.
Constructors are special bundles of codes that are used to initialize objects. Every time the constructor is invoked, an object of the class is created. The constructor can be of two types – default and parameterized. No matter what, they always have the same name as the class name.
15. What is the purpose of the super keyword?
In Java, the super keyword is used to call the fields of the superclass or parent class. It can be used to call the parent class’s methods, constructors, or variables. This is essential as it removes the confusion when both the superclass and the subclass have the attributes and methods with the same name.
16. What is the "this" keyword?
The primary function of the ‘this’ keyword is to call the methods or variables of the individual class. When there are attributes and methods with the same name in different classes, like the parent class, this keyword removes the place of confusion.
It can also be used to invoke the default constructor of the class.
17. What is a package in Java?
The package is like a container where classes, subclasses, interfaces, and abstracts can be encapsulated. These built-in and user-defined packages ensure no naming conflicts appear and make it easier to locate and store the related classes.
18. What is the use of the final keyword?
The ‘final’ keyword is used to finalize a value for the attribute that can not be overridden by other values or methods. This ‘final’ keyword can be applied to variables, methods, and classes.
For example, we can give the keyword ‘final’ before the variable ‘PI’ and set the value to 3.14159. This will always store this value of the variable and cannot be modified anymore.
19. What are interfaces in Java?
Interfaces are like abstract classes that store the attributes and the functions of the classes like a blueprint. The interface does not contain programmable codes; it is just implemented in other classes. This feature enables multiple inheritance, which is not supported by inheritance in Java.
20. What is the difference between an interface and an abstract class?
Having quite similar functionalities, the interface and an abstract class have some distinct features.
Abstract class
- It cannot be instantiated
- It cannot have both abstract and non-abstract methods.
- Other classes can implement only one abstract class.
- All types of access modifiers can be used – public, private, and protected.
Interface
- It can be instantiated
- It can have only abstract methods by default.
- Multiple implementations of interfaces are possible.
- All the variables have to be ‘static’ or ‘final’.
21. What is the purpose of the static keyword?
The main purpose of static keywords in Java is memory management. Using static keywords helps to use the memory more efficiently by creating a single copy of our input variable or method instead of repeating them individually. This is quite useful, especially when dealing with a massive database.
22. How do you create a singleton class in Java?
To ensure the proper application of the singleton class, first, it must be ensured that the class deals with only one instance to provide global access to the instance. It consists of three main phases.
In the beginning, the constructor was made private. After that, a private static instance of the class needs to be created. Finally, a public static method is provided to return the instance.
23. What is the difference between ArrayList and LinkedList?
ArrayList uses a dynamic array for internal element storage, while LinkedList uses a doubly linked list. Also, the ArrayList class only acts as a list, on the contrary, the LinkedList class can act as either a queue or a list.
However, as the ArrayList uses less memory it makes it suitable for data storage, unlike LinkedList which is better to manipulate lists.
24. Explain the concept of method overloading.
Method overloading is a feature to assign multiple methods with the same name to a class while parameters (type, number, or both) will be different. Using this feature enhances code readability and flexibility.
For instance, when the user wants to add given numbers, but the number of arguments is not fixed. In such cases, if methods are written individually it will make the work more difficult. Here, method overloading helps to figure out the program quickly.
25. Explain the concept of method overriding.
Method overloading is a quite useful feature in Java that helps to assign multiple methods with the same name yet having different parameters for a specific class. For instance, we want to create a new class or subclass, from the superclass, using the same method. Now, method overriding will allow the subclass’s method to override the method of the superclass.
26. What is the purpose of the transient keyword?
The ‘transient’ keyword is used to identify the attributes of the data and stop the serialization process. Serialization mainly converts an object into a stream of bytes.
When this keyword is added to the variable, JVM ignores the variable’s value and stores a default value. Most of the time, it is done when the program needs to accept the user’s login details and password.
27. What is the purpose of the synchronized keyword?
As Java supports multithreading, multiple threads can often access and modify the same resources. This can lead to errors and unexpected results. To prevent this issue, the ‘synchronized’ keyword is used so that a single thread can access the resources at one time, blocking other threads.
28. How do you handle exceptions in Java?
To handle runtime errors, exception handling can be used to prevent unexpected errors. Invalid user input, network issues, or problems in code are some of the reasons for this.
To handle the exceptions, the ‘try’ and ‘catch’ block can be used.
public class Task2 {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int sum = 0;
int count = 0;
while (count < 10) {
try {
System.out.print("Enter a positive integer: ");
int num = scanner.nextInt();
if (num < 0) {
throw new IllegalArgumentException("Input positive integer only");
}
sum += num;
count++;
} catch (Exception e) {
System.out.println("Error: " + e.getMessage());
scanner.nextLine();
}
}
System.out.println("Sum of the 10 positive integers: " + sum);
scanner.close();
}
}
This example shows the basic exception handling of Java. Here, when the user mistakenly enters negative numbers, the ‘try’ and ‘çatch’ block handles the error and the system does not crash.
29. What is the difference between throw and throws?
The ‘throw’ keyword is used when dealing with unchecked exceptions, while the ‘throws’ keyword can be used for both the checked and unchecked ones. However, the ‘throw’ keyword is usually used inside the code block, unlike the ‘throws’ used in method signatures.
30. What is the finally block?
The ‘finally’ block is always followed by the ‘try-catch’ block. Regardless of whether the program throws an exception, this block is being executed. Important statements like closing files or connections are often written in this block.
OOP Concepts
31. Explain the concept of object-oriented programming (OOP).
Object-oriented programming is related to the objects rather than just the values or logic of simple programming. Precisely, the object is a data field that has a set of attributes and features.
This programming model simplifies complex and lengthy software developments using the concepts of abstraction, encapsulation, inheritance, and polymorphism.
31. Explain the concept of object-oriented programming (OOP).
Object-oriented programming is related to the objects rather than just the values or logic of simple programming. Precisely, the object is a data field that has a set of attributes and features.
This programming model simplifies complex and lengthy software developments using the concepts of abstraction, encapsulation, inheritance, and polymorphism.
32. What are the four main principles of OOP?
The four main principles of OOP are encapsulation, abstraction, polymorphism, and inheritance which help the developers to create robust applications.
33. What is a class?
The class is like a blueprint of the object. The class mainly includes the attributes and the methods that the object will have. For example, “Circle” can be the name of a class that will create objects of a circle having different dimensions.
public class Circle {
private double radius;
public Circle(){
radius=1.0;
}
public Circle(double radius){
this.radius=radius;
}
public double getArea(){
return radius*radius*Math.PI;
}
}
This is a class of a circle that creates the objects of a circle taking the user input of radius.
34. What is an object?
Objects are the instances of the classes. The specific object generated has the mentioned attributes when a class is created. It can be interacted with by invoking methods.
35. What is the use of the ‘protected’ access modifier while dealing with inheritance?
The access modifier ‘protected’ is used for the attributes and the methods that need to be made accessible only by the classes of the same package. However, even after locating in different packages, the subclasses can access these variables and methods. This not only helps to achieve proper inheritance but is also useful in preventing access from irrelevant classes.
36. How does composition differ from inheritance in object-oriented programming?
Inheritance is a process where the subclasses inherit all the properties and features of the parent class. In comparison, the composition does not extend to another class. Instead, it occurs when one object contains another object that is dependent on it.
The relationship is known as IS-A in inheritance, but the composition implements the HAS-A relationship.
37. What is encapsulation?
Encapsulation is mainly hiding the data attributes of a specific class so that it cannot be accessed by the users easily. The keywords like ‘private’ and ‘protected’ are used to encapsulate the attributes.
38. What is the difference between shallow copy and deep copy in the context of object-oriented programming?
Shallow copy happens when the user copies the references only into the objects or variables. In this case, new memory is not allocated. However, the new memory is allocated in deep copy, but the reference is not copied.
39. Can you explain the principle of "Single Responsibility" in object-oriented design?
‘Single Responsibility’ means to give a single module or class a single responsibility only. Thus, the class will do only a single job, using methods related to that, reducing any ground for complexities.
40. What is a constructor?
A constructor is like a method that is invoked to instantiate an object. The constructor will have the same name as the class name. Users have the flexibility to use parameterized and non-parameterized constructors. A default constructor is generated without an argument or parameter when the user doesn’t define it.
41. What is method overloading?
Method overloading is simply having the methods with the same names but different parameters. They can be of different data types or have different numbers of arguments. This increases the readability of the program as the same method can be called with different attributes.
42. What is method overriding?
Method overriding is a form of runtime polymorphism. It means a superclass (parent class) and a subclass (child class) can have methods with the same name and parameters. It provides the specific implementation of the method that is provided in the superclass.
43. What is the super keyword?
The ‘super’ keyword is used to refer to the methods of the superclass. This avoids the confusion when the overridden methods are invoked. Another use of this keyword is to call the constructor of the superclass from the subclasses.
44. What are the advantages and disadvantages of using inheritance in object-oriented programming?
In Java, inheritance allows the user to reuse the code by transferring all the attributes to the subclass from the superclass. This also helps with method overriding, saving time, and simplifying maintenance.
Apart from this, inheritance also has a major drawback, restricting the extension of subclasses from more than one superclass. This can sometimes reduce the efficiency of the projects.
45. What is a static method?
A static method belongs to a single class; it can’t be overridden. This method can only be invoked from the specific class without creating an object. That’s why the static method can only access the static methods and variables.
46. What is a final class?
In Java, the final class is used to restrict class inheritance. The final class cannot be inherited or extended to another class. When done so, the compiler throws an error.
47. Can you describe the difference between method overloading and method overriding?
Method overloading means having the same name of methods but with different data types or parameters or even both. For example, the methods below have different parameters and data types.
int area (int side)
double area (double side)
double area (double side1, double side2)
On the other hand, method overriding is having the same method with the same name and parameter but with different implementations and working. For example, the parent class named ‘Shape’ may have a method similar to this –
double area (double side1, double side2){
return side1*side2;
And, the subclass “Triangle’ can have the same method as below which shows method overriding-
double area (double side1, double side2){
return ½*side1*side2;
48. What is an access modifier?
Access modifiers are used to provide access to the attributes and methods and determine who can access them. It also specifies its limits so that the JVM understands if it can invoke it from outside the class or not.
Some of the access modifiers are public, private, default, and protected
49. What is a static variable?
Static variables belong to the classes not to the objects of the class. This can be declared once and can be shared with all the objects of the class.
50. What is a final variable?
When the final keyword is used for a variable then it becomes a final variable. The value of the variable can not be modified. These variables are more like constants that have a fixed, unchangeable value.
51. What is an abstract method?
Abstract methods are used when the implementations of the methods are determined in the subclasses. Many times, the implementation of the same methods can vary based on the classes. In that case, the abstract methods are created in the parent class.
52. What is a concrete method?
The concrete methods are the regular methods that mention the implementation of the methods in the class. They can also be defined in the abstract classes and can also be overridden by the subclasses.
53. What is a nested class?
Nested classes are like inner classes which can be declared inside the other classes. Apart from general classes, interfaces can also be nested. Helping the classes to group logically in one place enhances readability.
54. What is an inner class?
Inner classes are the nested classes that can be declared inside another class. They have access to the objects’ fields and methods of the existing class to which it belongs.
55. What is an anonymous inner class?
Anonymous inner class is quite similar to the general inner class. The difference is that this class does not have any name. It is usually used to extend the subclasses or to implement the interfaces.
56. What is a local inner class?
Local inner classes are not the members of the enclosing classes. They are declared inside the method body to access the local methods and the variables. Also, the scope of the local inner class is restricted within the method block.
57. What is a static nested class?
Similar to the nested classes, the static nested class is also an inner class that is declared within another class. However, like the other nested classes, the static ones can not access the variable and the methods of the enclosing class.
58. How does polymorphism contribute to code flexibility and extensibility in object-oriented programming?
When polymorphism is used, it enables features like method overriding. It helps to reuse the code, saves a lot of time, and makes the entire project’s development faster with unique features.
59. What is a functional interface?
Functional interfaces or Single Abstract Method Interfaces only contain one abstract method. It can have any number of static or default methods, but the abstract method should be only one. It can be used in functions like changing a list of objects of one kind to a list of objects of another.
60. What is a lambda expression?
Lambda expression is like a method without a name but has the implementation inside the method. This helps to implement the functional interfaces so that one method can represent the expression.
Java Data Types and Variables
61. What are primitive data types?
Primitive data types are the fundamental data types of the programming language. Commonly, there are eight primitive data types – int, double, float, char, long, boolean, short, and byte.
Here, the int, double, float, long, and short are all used for declaring the numerical values. While the byte and char are used for textual primitives and the boolean is used for true or false (boolean values).
62. What are reference data types?
The non-primitive data types are the reference data types as they store the memory address of the data instead of the actual value. Some of the common examples of reference data types are string, array, object, class, and interface.
63. What is a variable?
Variables are mainly the assigned names where the user can store the value. Each variable can only store one data type which is essential to declare before the variable. The variable can be primitive or non-primitive type.
64. What is a constant?
A constant is a type of variable whose value can not be changed once it is assigned to a variable. It is more like the ‘final’ keyword. The ‘static’ and ‘final’ keywords together are used to declare a constant variable.
65. What is typecasting?
Typecasting is a process of converting one data type to another data type. In Java, two types of typecasting are possible – widening casting and narrowing casting. This casting system allows the conversion of data by preventing data loss.
66. What is autoboxing and unboxing?
Autoboxing and unboxing are the features that convert primitive types to Wrappers automatically. When the primitive data types are converted to equivalent Wrapper types, it is called autoboxing. On the other hand, unboxing is the opposite process where the conversion of Wrappers occurs.
67. What is the char data type?
Char data type is used to store the character data in a variable. It can be strings, letters, numbers, or any other symbol. It can be both single byte or multibyte. However, the variable with the char data type has to be surrounded by single quotes.
68. What is the boolean data type?
Boolean data type is used in variables to store true and false. It is not case sensitive and it only stores data using only 1-byte space. By logic values, it is worked by 1 representing true and 0 representing false values.
69. What is the byte data type?
Used for containing binary data, the byte data type is a group of eight bits. They can hold values ranging from -127 to +127 for whole numbers. An example of the simple byte data type can be ‘a’ or ‘7’.
70. What is the short data type?
Short is another data type of 16-bit signed two’s complement integer. It has the size of two bytes and holds values from -32768 to 32767. This data type can help to save the memory while working with the large arrays.
71. What is the int data type?
This data type has the size of 4 bytes used to store the integer values in a variable. For integers that data type came to be named as “int”. An unsigned 32-bit integer can be stored in this data type from -2,147,483,648 to 2,147,483,647.
72. What is the long data type?
Long is used to store the largest integer values up to 64 bits of memory. It has the size of 8 bits. Storing values from -9, 223, 372, 036, 854, 775, 808 to 9, 223, 372, 036, 854, 775, 807, long is essential to store integer values that exceed the integer data type.
73. What is the float data type?
To store floating points or decimal values, float data type is used. Up to the precision of 6-7 decimal points, the float can store the value. In order to store the value to float, along with declaring the variable to float, the letter “f” should be added to the end of the value to save it as float.
74. What is the double data type?
Double data type is used to store 64 bits of decimal points in a variable which means it can store double size as much as the float holds. When the letter f is not mentioned in the float data types, JVM automatically categorizes it as a double value.
75. What is a string in Java?
The string is nothing but a sequence of characters. In other words, the string is an array of characters. In Java, string is not a primitive data type, rather it is an object.
76. How do you declare a variable?
To declare a variable means to assign a value to something to store it in the system. In order to do that, at first the data type needs to be mentioned. After that, the name of the variable according to the user’s preference should be given. Then the value followed by the semicolon is enough to create a variable.
Here’s a simple example of declaring a variable.
int num=10;
77. How do you initialize a variable?
Initializing a variable is the same as declaring a value. However, in order to initialize the variable, the value must be assigned, unlike the declaration. A simple example can solve the confusion.
int num=10;
Double num1;
Here, the first variable is declared and initialized as the value is assigned to it. While the second one, ‘num1’, is only declared; it is not yet initialized as there is no value given to the variable.
78. What is a literal?
In Java, literals are simple values of integers, boolean, characters, or strings. They can be assigned directly to a variable. For instance, here the 10 is the literal of the variable ‘num’.
int num=10;
79. What is a type conversion?
Type conversion is another name for typecasting in Java. It is used to convert one data type to another data type. It is necessary when one type of data needs to be stored in another variable. The type conversion must be done by mentioning the variable that makes the program more readable.
80. What is type promotion?
As the name sounds, the type promotion promotes the data type from smaller data to larger size of datatype. For example, changing the integer values to double or float data types can be a type promotion as the size of the data is increased here.
81. What is a type inference?
Type inference is provided by Java 8 in its improved versions. It is the automated ability of the compiler to look at the method declaration and the invocation to determine the type of argument. Without mentioning the type of the argument, users can call specific methods in Java 8.
82. What is a scope of a variable?
The scope of the variable means the areas where the variable is accessible. According to the location of the variables, the scope of them is determined. The variable can have local, instance, and static scopes based on their needs.
83. What is a lifetime of a variable?
The variable’s lifetime is the time a variable uses in the system’s memory. For instance, for variables, the lifetime of the instance variable is until the end of the program, while for the local variable, it is until the control leaves the block. Again, the static variable has a lifetime until the object stays in the memory.
84. Explain the difference between local variables, instance variables, and class variables in Java.
The local variables are the ones that are declared within the method block, conditions, or loops. They are only recognized by that part of the program and can not be interacted with or invoked from another part.
The instance variable is the opposite of this. They are declared outside these methods or conditionals, usually at the beginning of the class. That’s why all the parts of the program recognize them.
Lastly, the class variables are the general variables present inside a class. They can be both local or instance.
85. What is a local variable?
Local variables are the variables that are declared inside specific methods or functions. These variables are not globalized; that means they are not recognized by the other parts of the class except that block. Simply declaring a variable inside a method can make it a local variable.
86. What is an instance variable?
Variables that are declared within the class but outside the methods are the instance variables. They can be accessed by all parts of the class. They are quite different from static variables and have unique values for the particular object unlike them.
87. What is a class variable?
Class variables are the simple variables of the class. They are static variables and belong to the class rather than specific objects. Declared with the ‘static’ keyword they are often located outside the methods.
88. What is a constant variable?
A constant is a type of variable whose value can not be changed once it is assigned to a variable. It is more like the ‘final’ keyword. The ‘static’ and ‘final’ keywords together are used to declare a constant variable.
89. How does the final keyword affect variables in Java, and what are its implications for the program?
The final keywords make the value of the variable unchangeable. Hence, they are fixed and can not be modified. This is done to fix and make the variables constant so that the work related to them can be done efficiently.
90. What is a variable declaration?
The variable declaration means to give a name to the data type where the user wants to store a value. It is like a container. For example, here the ‘num’ is the name of the variable having the data type of double. Here, the users can store any decimal values by initializing it.
double num;
Control Flow Statements
91. What are control flow statements?
To control the flow of the code, Java has specialized control flow statements to help the user to make the decision to provide a smooth program flow. The common control flow statements are ‘if’, ‘loop’, and the ‘jump’ statements.
92. What is an if statement?
If statements are decision-making statements. If the conditions are fulfilled according to the program, then the code inside its block is generated otherwise the system closes. Most of the time, it gives a boolean value of true or false.
if (num > 0) {
num=num*2;
);
}
This example shows a basic if-statement. Here, if the number the user has input is bigger than 0, then the number will be multiplied by 2. When the conditions are not met, the program will not function.
93. What is an if-else statement?
If-else statement enables the use of more than one condition in the program. If the first one is not fulfilled, the program will check if it satisfies the rest of the conditions. Oftentimes the last condition is ended with an ‘else’ clause.
if (num > 0) {
num=num*2;
);
if else(num<0){
num=num/2;
}
else{
num=0;
}
This example shows that if the num variable is greater than 0, it will be multiplied by 2. If the first condition is not fulfilled, and the num variable is less than 0, it will be divided by 2. If none of the above conditions are satisfied, the value of num will be 0.
94. What is an if-else-if ladder?
An if-else-if ladder is a nested if-else. It is similar to the previous example. When many conditions are included in the program, the if-else-if ladder is used. The program checks if the conditions are met. When the conditions are met, the program skips the rest of the ladder and runs it.
95. What is a nested if statement?
A nested if statement is similar to the if-else statement. This is always used to give conditions inside one condition. When both conditions are fulfilled the program is executed.
96. What is a switch statement?
The switch statement is exactly like the if-else statement which is used to check if the conditions are fulfilled. One of the striking differences is that the return type of the switch statement can be only int, double float, short, long, enum, and strings, unlike the if-else statement.
An example can help to understand the basic differences.
case '+':
return operand1+operand2;
case '-':
return operand1+operand2;
case '*':
return operand1*operand2;
case '/':
eturn operand1/operand2;
}
default:
System.out.println("Result: Invalid”);
}
This is a simple calculator that takes the user input in characters and based on the symbol it decides if the operands need to be added, subtracted, multiplied, or divided. When the conditions are met, it does not check the rest of the blocks. However, if none is correctly aligned, it goes to the default option and executes the code inside it.
97. What is a break statement?
A break statement is usually placed after the condition or loop to break the loop or the condition. When it reaches the break statement, it stops its work in the condition or the loop and then the rest of the code outside that block is executed.
98. What is a continue statement?
The continue statement is mainly used inside the loops. It is used when we don’t want the current iteration anymore and need to jump to the next one of the loop. Therefore, when the continue statement is used, it skips the rest of the code that it was executing and continues the loop functions.
99. What is a for loop?
A for loop is used to iterate or repeat a given set of instructions multiple times until a specific condition is fulfilled. To use the for loop, there must be initialization, condition along with the function like increment or decrement, and the statement inside the block.
100. What is an enhanced for loop?
The enhanced for loop is also popular as the for-each loop is used in Java to make the code more readable and avoid silly errors and bugs. Its functions are the same as the traditional for loop, but it is more suitable for working with the traverse arrays.
for(int i: array ){
System.out.println(i)
}
101. What is a while loop?
The while loop is also similar to the for loop, but it is used to iterate until a specific condition is fulfilled. When the user does not know how many times the task shall be repeated, using a while loop is the best practice.
while (i=!10){
i++;
}
Here until the value of variable i becomes 10, the while loop will be repeated and the task inside will also be repeated. Here the task is to increase the value of the i by one.
102. What is a do-while loop?
The do-while loop is the same as the while loop. Used to repeat until specific conditions are not fulfilled, it executes the code at least once before checking the validity. Thus, when there is a need to check the condition yet perform it at least once, even if the conditions are incorrect, do-while should be used.
103. What is an infinite loop?
An infinite loop is an endless loop that iterates until the terminating conditions aren’t met. This type of infinite loop is usually unintentional and errors though it can be featured for some applications.
104. What is a nested loop?
A nested loop is the inner loop inside a loop. This is essential when we perform interactions twice with two different conditions. Usually for calculating the two or three-dimensional matrices, the nested loop is used.
105. What is a labeled loop?
Labeling a loop means to include continue or break statements inside the loop. When these break statements are used in the nested loop, for example, the inner loop will be terminated. Then it will jump to the outer code and execute it.
106. What is a return statement?
When the execution of the block is completed, whether it is a method or a conditional statement, the return statement can return the value. This will stop the further execution of the rest of the code.
107. What's the difference between a while loop and a do-while loop, and when would you use each?
The while loop and do-while loop functions are similar – they are used to iterate the code block until a specific condition is fulfilled. However, do-while is especially used when the user wants to iterate at least once even if the conditions are satisfied.
108. What is a fall-through in a switch case?
For switch cases, there must be a break or default statement to stop the execution when the conditions are fulfilled. If the break statement is not used in the specific case of the switch case, the program will be executed until it finds the next break keyword. This is called the fall-through in a switch case.
109. When do you choose a switch statement over if-else, and vice versa?
Switch statements are often used when the conditions are neither true nor false (boolean value) but relatively certain specific values, like the calculator example. When the operator is the symbol ‘+,’ the program should add, and so on.
On the other hand, the switch statement is better when a faster and easier approach is needed with few values. For instance, the operation is performed if the number is greater than 0. Otherwise, it is excluded.
110. What is a try-catch block?
A try-catch block is used to handle the exception that is thrown at the compile time. Any error inside the try block is handled and immediately terminated. It then jumps to the specific catch block of that try block and then executes the command inside there.
Every try block must have a catch or finally block.
An example of a try-catch block –
try{
System.out.println("Enter an number: ");
double num=input.nextDouble();
System.out.println("The sqaure root of "+num+" is "+getSquareRoot(num));
}catch(Exception e){
System.out.println(e);
}
111. What is a finally block?
Finally is a block of code that is executed after a “try” block. It’s typically used to clean up codes like closing files or releasing database connections.
112. What is a throw statement?
The throw statement in Java is used to throw a user-defined exception or create a custom human-readable error. You can use a throw statement to signal that something has gone wrong in your code, this way it can be caught and handled properly.
113. What is a throws clause?
The throws clause is used to specify certain exceptions executed from the applied methods. It informs the programmers about handling or propagating those exceptions. For example, you can use it to throw a warning sign like “This method could cause these types of problems.”
114. What are checked exceptions?
Checked exceptions in Java are exceptions that are checked or declared in the throws clause during the method execution. These exceptions represent conditions that are outside the control of the application such as trying to read a file that doesn’t exist.
115. What are unchecked exceptions?
Unchecked exceptions are exceptions that don’t need to be declared in a throws clause or caught in a try-catch block. They are subclasses of RuntimeException and represent programming errors that are unrecoverable such as logic mistakes or improper use of an API.
116. What is a custom exception?
A custom exception is a type of exception that is defined and customized by a Java developer through extending the Exception class (for checked exceptions) or RuntimeException class (for unchecked exceptions). Custom exceptions allow you to create meaningful error messages specific to your application’s needs.
117. What is a try-with-resources statement?
A try-with-resources statement in Java is a try statement used to ensure each resource is closed after finishing the execution of a statement.
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
// Use the resource
} catch (IOException e) {
// Handle exception
}
Here, “BufferedReader” is automatically closed when the try block finishes, even if an exception occurs.
118. What is a multi-catch block?
A multi-catch block is a type of block used to catch multiple exceptions in a single catch block. This simplifies the code by avoiding duplication. A multi-clutch block is effective when you need to handle different types of exceptions generated from a single try block.
119. What is an assertion?
An assertion is a statement used in Java programming language to verify whether your assumptions are correct or not. If your state of a program or assumption is false, it will throw an “AssertionError”, which helps catch bugs during development.
120. How do you use assertions?
Run your program with the -ea (or -enableassertions) JVM option. Then, use the assert keyword followed by the assumption you want to test. If the expression is false, the AssertionError will be thrown. For instance:
int age = getAge();
assert age > 0 : "Age must be positive";
Here, the assertion checks if age is positive. If not, it throws an AssertionError with the message “Age must be positive.”
Java Collections Framework
121. What is the Java Collections Framework?
The Java Collections Framework is one kind of package that combines multiple classes and interfaces. It provides a standardized way to simplify the process of storing, retrieving, and manipulating data in Java programs.
122. What are the main interfaces of the Java Collections Framework?
The Java Collections Framework comprises five major interfaces, which include List, Set, Map, Queue, and Deque.
Lists maintain ordered collections and permits duplicates, while Sets ensure unique elements. Maps manage key-value pairs and each key is unique. Queues follow the FIFO (First-In-First-Out) principle, and Deques support insertion and removal from both ends.
123. What is a List?
List is an interface used in Java to store elements in ordered formation. It is a child interface of the Collection that provides methods to access, add, remove, and manipulate elements by their index.
124. What is an ArrayList?
An Arraylist class is a part of the collections framework in Java which represents a resizable array. Arraylists are dynamic in nature, which means you can change elements according to your needs.
125. What is a LinkedList?
A LinkedList in Java is a class that implements the List and Deque interfaces using a doubly-linked list. It allows you to insert and delete elements efficiently since each element of the LinkedList is linked to the next and previous ones.
126. What is a Set?
A set is an interface in Java that restricts data storage within unique elements. In other words, you can’t store duplicate elements, facilitating mathematical operations like union, intersection, and difference.
127. What is a HashSet?
A HashSet is a part of the Collection framework that uses a hash table to store unique elements. It prevents storing duplicates to ensure fast performance for operations like adding, removing, and checking for elements.
128. What is a LinkedHashSet?
LinkedHashSet is a class used in Java to extend the HashSet through adding new elements while maintaining the order as inserted. It uses a hash table and a link simultaneously to maintain insertion orders.
129. What is a TreeSet?
TreeSet is an implementation of the Set interface to store elements in a sorted order. For this, TreeSet uses a red-black tree structure where all elements are stored in ascending order.
130. What is a Map?
In Java, a map is an interface which can be accessed from the “java.util” package. A map stores key-value pairs, while ensuring each key is unique. It is commonly used as HashMap, LinkedHashMap, and TreeMap, when a quick access to data is required.
131. What is a HashMap?
HashMap is a part of the Collection framework of Java that stores data in key-value pairs. Using the HashMap class, you can efficiently store and retrieve elements based on the unique keys.
132. What is a LinkedHashMap?
A LinkedHasMap class in Java is an extended form of the HashMap. Similar to the HasMap class, it stores key value pairs, while also remembering the order in which elements were added. This is because a LinkedHashMap uses both a hash table and linked list.
133. What is a TreeMap?
TreeMap is a class in Java which is the implementation of a red-black tree. It stores key-value pairs in stored order based on the natural ordering of keys or a custom operator.
134. What is a Queue?
A Queue is a first-in, first-out (FIFO) data structure used in Java to define methods for adding, removing, and inspecting elements in the order they were added. It is a subtype of the Collection interface.
135. What is a PriorityQueue?
A PriorityQueue is an implementation of the Queue interface, which is used to order elements based on the priority. Using this class, you can order elements based on their natural ordering or a custom comparator. This way, elements with higher priority are dequeued before those with lower priority.
136. What is a Deque?
A Deque also known as a “double-ended queue” is an interface that extends the Queue interface. It allows elements to be added or removed from both ends, making it useful for implementing stacks and queues. Deque is part of the java.util package and provides methods for efficient insertion, removal, and access at both ends of the collection.
137. What is an ArrayDeque?
An ArrayDeque class in Java is an implementation of the Deque interface that uses a resizable array. It allows elements to be added or removed from both ends efficiently and provides a flexible, fast alternative for implementing stacks and queues.
138. What is a Stack?
A Stack is a class in Java that represents a last-in, first-out (LIFO) data structure. It extends the Vector class and provides methods for adding, removing, and inspecting elements at the top of the stack. It is commonly used for tasks like expression evaluation, backtracking, and undo mechanisms in applications.
139. What is a Vector?
A vector is a type of dynamic array that allows flexible increment and decline of the array size. It is quite useful in implementing methods for accessing, adding, removing, and manipulating elements. This is why vectors are suitable for situations requiring concurrent access from multiple threads.
140. What is a Collection?
A Collection is an interface that represents a group of objects. It is used for performing various data manipulation tasks including data query, sorting, storing, updating, and deleting using its sub-interfaces and methods.
141. What is the difference between Collection and Collections?
A collection in Java is an interface, representing a group of objects. On the other hand, Collections is a utility class that provides various methods for working with collections. Generally, a collection deals with the interfaces for data structures, while Collections supplies utility methods for common tasks related to collection.
142. How do you iterate over a collection?
The way of iterating over a collection can be different depending on specific scenarios. For instance,
-When you want to traverse through all elements of an iterable without conditions:
for (iterable_type iterable_element : collection)
– If you have an iterable but need to traverse conditionally:
for (Iterator iterator = collection.iterator(); iterator.hasNext();)
– If the data structure doesn’t implement the iterable interface:
for (int i = 0; i < collection.length; i++)
143. What is the Iterator interface?
The iterator interface is a part of the java.lang package that is used to access elements of the collection. It enables methods for producing an iterator to iterate over the elements in the collection.
144. What is the ListIterator interface?
The Lisiterator interface extends the Iterator interface and provides additional functionality for iterating over lists. It allows bidirectional iteration of elements in a list. This interface is commonly used when you need to iterate over lists and perform operations like adding, removing, or replacing elements.
145. How do you convert an array to a list?
You can use the ‘Arrays.asList()’ method to convert an array to a list in Java. Simply insert the array as an argument to this method and it will wrap the array in the AbstractList class, providing a list containing all elements of the array.
146. How do you convert a list to an array?
You can simply call the ‘toArray()’ method to convert a list to an array in Java. Use the syntax, String[] newArray = listToConvert. toArray(new String[0]).
147. What is the Comparable interface?
The comparable interface is an interface in Java that is used to specify how the instances of a class should be compared to one another. The compareTo() method, defined in the interface, is used for comparison.
148. What is the Comparator interface?
The Comparator interface in Java is an interface used to define custom ordering for objects that do not implement the Comparable interface. It allows you to specify how objects should be compared based on specific criteria.
149. How do you sort a collection?
The simplest way to sort a collection is using the sort() method. Using this method will sort a list based on their natural ordering (if they implement the Comparable interface) or using a custom Comparator.
150. How do you search for an element in a collection?
You can use methods like contains() or indexOf() depending on the type of collection. These methods help to verify whether the collection contains a specific element or retrieve its index if it exists. You can also iterate through the collection manually and perform custom checks if needed.
151. What is the purpose of the assert statement in Java?
The main purpose of the assert statement in Java is to test your assumptions while programming your application. If an assertion fails, it typically indicates a bug or unexpected error in the code.
152. How do you handle concurrent modification exceptions in Java collections?
You can use an Iterator to iterate through the collection using the remove() method instead of directly modifying it. You can also use synchronized collections from the java.util.concurrent package or specialized concurrent collections like ConcurrentHashMap or CopyOnWriteArrayList. These can handle concurrent modifications safely without throwing exceptions.
Java Collections Framework
153. What is a string in Java?
A string in Java is a sequence of characters stored as an object. A string typically represents textual data and is used for manipulating and storing text in Java programs.
154. How do you create a string?
The simplest way to create a string in Java is using string literals, where you have to call characters enclosed within double quotes, for instance, “Hi, Sir”. You can also use the String class constructor.
155. What is the String class?
The string class is one of the fundamental classes in Java that represents sequences of characters.
156. What is the difference between String and StringBuilder?
The key difference between String and StringBuilder lies in their mutability. Strings are immutable, while StringBuilder is mutable. String is preferable when dealing with constant values or when immutability is desired for thread safety. On the other hand, StringBuilder is more efficient when dealing with frequent string modifications since it avoids unnecessary object creation.
157. What is the difference between String and StringBuffer?
Strings are immutable, which means their values cannot be changed once they’re created. Any operation that appears to modify a string actually creates a new string object. On the other hand, StringBuffer is mutable. Therefore, you can modify the contents of the string without creating a new object each time.
158. How do you concatenate strings in Java?
You can concatenate strings using the + or += operators. Insert the + operator within two string characters, such as “Hi” + ‘Sir”. You can also use the concat() method to concatenate one string to another. For this, you can use the “Hi”.concat(“Sir”).
158. What is the substring method?
The substring in Java is a widely used method which is a part of the java.lang package. This method allows you to extract a portion of a bigger string that creates a small string.
160. How do you compare strings in Java?
For equality comparison between strings, you can use the String.equals() method.
String str1 = "Hello";
String str2 = "World";
if (str1.equals(str2)) {
System.out.println("Strings are equal");
} else {
System.out.println("Strings are not equal");
}
You can also use the compareTo() method to compare strings lexicographically.
int result = str1.compareTo(str2);
if (result == 0) {
System.out.println("Strings are equal");
} else if (result < 0) {
System.out.println("str1 comes before str2");
} else {
System.out.println("str1 comes after str2");
}
161. What is the equals method?
The equals() method is used to test the equality of two strings. In case the strings have the same sequence of characters or they are equal, the method returns ‘True’, and ‘False’ if not.
162. What is the compareTo method?
The compareTo method is a method used in Java to compare two objects where one object is the current item and another is set as a parameter. Basically, this method compares characters of two strings based on their Unicode values.
163. What is the startsWith method?
The startsWith() is a convenience method in Java that is used to check if a string starts with a specific value or a given string.
164. What is the endsWith method?
The endsWith() method is used to check if a string ends with a specific suffix, where the suffix is usually represented by a string parameter. This method returns true if the given string ends with that suffix.
165. What is the indexOf method?
The indexOf() method is used to find the index of the first occurrence of a specified character or substring within a string. It returns the index of the first occurrence of the specified character or substring, or -1 if the character or substring is not found.
166. What is the lastIndexOf method?
The lastIndexOf() method is used to find the position of the last occurrence of a specified character or substring within a string. It returns the index of the last occurrence of the specified character or substring, or -1 if the character or substring is not found.
167. What is the length method?
The length() method is used to determine the number of characters in a string. It returns the length of the specified string including spaces and punctuation.
168. How do you convert a string to uppercase?
You can convert a string to uppercase using the toUpperCase() method. This method returns a new string with all characters converted to uppercase. For example, “banana”.toUpperCase() will return “BANANA”.
169. How do you convert a string to lowercase?
In java, using the ‘toLowerCase()’ method is the most convenient way to convert a string to lowercase. This method returns a new string with all characters converted to lowercase. For example, “HELLO”.toLowerCase() will return “hello”.
170. How do you trim whitespace from a string?
You can trim whitespace from a string using the trim() method, which removes leading and trailing spaces. Besides, using strip(), stripLeading(), and stripTrailing() methods are also effective for more specific whitespace removal.
171. How do you replace characters in a string?
You can replace characters in a string using the replace() method to replace all occurrences of a character, or replaceAll() for replacing patterns using regex. The replaceFirst() method is useful to replace the first occurrence.
172. How do you split a string?
You can split a string using the String.split() method, which allows you to divide the string based on a regular expression. It’s commonly used to split strings by spaces, commas, or other delimiters.
173. What is the toCharArray method?
The toCharArray method is a built-in function used in Java to convert a string into a character array. It’s often used when you need to access or manipulate individual characters within a string.
174. How do you format strings in Java?
The String.format() method is quite simple and common to format strings in Java. You can also use the Formatter class. These methods are commonly used to embed variables in a string with a specific format.
175. What is the valueOf method?
The valueOf method is a part of the String class in Java. This static method is used to convert different types of data such as numbers, characters, and objects into a string.
176. What is the intern method?
The intern method in Java is used to store a string in the string pool. Using this method saves memory as it doesn’t allow creating a new object when there is an identical string content existing in the string constant pool.
177. How do you check if a string is empty?
You can use the isEmpty() method to check empty strings, where the method returns True if there is an empty string. As an alternative, you can compare a string to an empty string with equals(“”). These are common ways to ensure a string has no characters.
178. What is the matches method?
The matches method is a part of the Element interface in Java that checks if a string matches a specified regular expression. The method returns true if the string conforms to the pattern, and false otherwise.
179. What is the replaceAll method?
The replaceAll() method replaces all parts of a string that match a given regex with a new string. It’s similar to the replace() method. However, replaceAll() uses a regex pattern for matching, while replace() simply looks for an exact string.
180. What is the replaceFirst method?
The replaceFirst() method in Java replaces the first substring of a string that matches a given regex with a specified replacement. It’s similar to replaceAll(), but only affects the first occurrence of the match.
181. How do you convert a string to a byte array?
You can use the getBytes() method to convert a string to a byte array in Java. This method encodes the string into a sequence of bytes using the default charset or a specified charset. Load the Java.util package and input the following codes:
String str = "example";
byte[] byteArray = str.getBytes();
You can also specify a charset:
byte[] byteArray = str.getBytes(StandardCharsets.UTF_8);
182. How do you convert a byte array to a string?
To convert a byte array to a string in Java, you use the String constructor that takes a byte array as an argument. Write the following code first:
byte[] byteArray = {65, 66, 67};
String str = new String(byteArray);
You can also specify a charset using:
String str = new String(byteArray, StandardCharsets.UTF_8);
Java Input and Output (I/O)
183. What is the Java I/O system?
The Java I/O system is a framework in Java for handling input and output operations, such as reading from and writing to files, streams, and other sources. It includes various classes like InputStream, OutputStream, Reader, and Writer. This system is part of the java.io package.
184. What is a stream in Java?
A stream is a wrapper consisting of a sequence of data elements that enable efficient reading or writing of data from a source to a destination. Streams assist input and output operations, which allows the transfer of data between different sources and destinations in a sequential manner.
185. What are the types of streams in Java?
There are several types of streams in Java based on directionality, data type, source, or destination. These include input streams and output streams, byte streams and character streams, file streams, and network streams. Each type serves specific purposes.
186. What is the difference between byte streams and character streams?
Byte streams deal with raw binary data, reading, and writing data byte by byte. On the other hand, character streams handle textual data to read and write characters and strings.
187. What is the InputStream class?
The InputStream class is an abstract class that serves as the superclass for all input stream classes. InputStream provides a common interface for reading data by byte or in larger chunks.
188. What is the OutputStream class?
The OutputStream class in Java is an abstract class that serves as a blueprint or a superclass for all output stream classes. It’s used for writing data bytes to different destinations, like files, network connections, or memory buffers.
189. What is the Reader class?
The Reader class is an abstract class in Java that is used to handle character-based input streams. It’s part of Java’s I/O library, allowing you to read text from files, streams, or other sources.
190. What is the Writer class?
The Writer class in Java is an abstract class that serves as a superclass and is used for writing character-based data to output destinations. You can use methods such as write() and flush() from the Writer class to get a convenient interface for writing characters efficiently and reliably.
191. What is the File class?
The File class is a core component in Java that represents files and directories in the file system to perform operations like creation, deletion, renaming, and checking existence.
192. How do you read data from a file?
For quick reading of an entire file at once, use the Files class. If you’re dealing with a large file and need to read it line by line, go for a BufferedReader. In case your file has data separated by a specific delimiter, you can use a Scanner class.
193. How do you write data to a file?
To write data to a file in Java, you can use the FileWriter or BufferedWriter class for simple text writing, FileOutputStream class for binary data, or PrintWriter class for formatted text output.
194. What is the BufferedReader class?
The BufferedReader class in Java is used to read text from a character-input stream. This class buffers characters to provide efficient reading of characters, arrays, and lines.
195. What is the BufferedWriter class?
The BufferWriter class is a Java Writer subclass that is used to write text to a character-output stream. It efficiently buffers characters that in turn, reduces system calls and improves performance.
196. What is the FileReader class?
The FileReader class is an extension of the InputSTreamReader class in Java, which is used to read the content of a file character by character.
197. What is the FileWriter class?
The FileWriter class in Java is used to write character data to files. It’s part of the Java I/O package and provides functionality to write characters to a file in a character-based manner.
198. What is the PrintWriter class?
The PrintWriter class in Java is a character-oriented class used for writing text to a file. It’s designed to write formatted representations of objects to a text-output stream.
199. How do you read data from the console?
You can use the Scanner class or the BufferedReader class in combination with the InputStreamReader class to read data from the console in Java. The Scanner class is suitable for reading inputs such as strings, integers, and floating-point numbers from the console. The BufferedReader class is preferable to read input from the console line by line when combined with InputStreamReader.
200. How do you write data to the console?
The System.out.println() method is quite a common and simple way to write data to the Java console. It prints the data to the console followed by a newline character. If you need more advanced formatting options, you can use the PrintWriter class.
201. What is the Scanner class?
The Scanner class is found in the java.util package, which is used to read input from various sources like the console or files. It provides methods to parse primitive data types and strings.
202. How do you use the Scanner class to read input?
Run the java.util package and import the Scanner class from. Then, create a Scanner object by passing the input source, like System.in for reading from the console. Finally, use methods like nextInt(), nextLine(), or nextDouble() to retrieve different types of input.
203. What is serialization in Java?
Serialization in Java is the process of converting an object into a stream of bytes for saving the object to a file or sent over a network. This stream of bytes can then be deserialized, or converted back into an object, at a later time.
204. What is the Serializable interface?
The Serializable interface in Java is a marker interface that is used to serialize objects. When a class implements Serializable, it indicates that its objects can be converted into a stream of bytes and written to an OutputStream.
205. How do you serialize an object?
Create an instance of ObjectOutputStream and pass an OutputStream object to its constructor. Use the writeObject() method of the ObjectOutputStream to serialize the object.
206. How do you deserialize an object?
When the object state is saved into a stream of byte data of files and databases, it is called Serialization. The opposite process, deserialization, is when the byte stream is converted to restore the object’s state. The ObjectInputStream class is required, which contains the method readObject() to deserialize an object.
207. What is the ObjectInputStream class?
ObjectInputStream class is used to deserialize the data and the objects that are written by the ObjectOutputStream class. To read the data using the ObjectInputStream, the class must implement the ‘Serializable’ interface.
208. What is the ObjectOutputStream class?
ObjectOutputStreams writes the primitive data types and objects into the ObjectStream. This process is called Serialization. In order to do this, the ObjectOutputStream package needs to be imported.
ObjectOutputStream obj= new ObjectOutputStream(file);
In the following example, the object ‘obj’ of the ObjectOutputStream is created to link to the file output named ‘file’.
209. What is the FileInputStream class?
FileInputStream class helps the user to read data from the file in the form of byte sequences. Extending the InputStream abstract class, its byte-oriented features make it more suitable for reading the image data than character data.
210. What is the FileOutputStream class?
To write data to the file taking the user input, the FileOutputStream class is used. Both byte and character-oriented data can be stored in the file using this class.
FileOutputStream f1 =new FileOutputStream("D:\\input1t.txt");
f1.write(“Hello”);
f1.close();
This simple example shows how an object ‘f1’ can be created and connected to the FileOutputStream. After that, the method .write() can write the data into the file, following .close() to close the file after the task.
211. What is the DataInputStream class?
The DataInputStream class is used to read the primitive data types from the user input stream. It is a machine-independent way to store data in no-byte orientation which makes it unsuitable for multi-threaded access.
212. What is the DataOutputStream class?
To write the primitive data types into binary files, the DataOutputStream class is required. This data can’t be read by the users as it is in binary form. For this reason, the input stream is needed to read the data from these files.
Java Concurrency
213. What is concurrency in Java?
In Java, concurrency allows the users to perform multiple tasks simultaneously and in parallel. The ‘Concurrency’ package covers the multithreading, parallelism, and concurrency platforms, enabling the development of concurrent applications. Using ‘ java.util.concurrent’, this package can be imported to any class to give a multithreaded feature.
214. What is a thread in Java?
The smallest unit of the process or the separate path that the program takes while execution is thread. Every program has at least one thread, or the ‘main’ thread.
However, Java allows multiple threads to perform many concurrent works at a time. For example, while writing a document, the user input is taken in one thread, while the formatting of the document is another.
215. What is the Runnable interface?
This interface of the runnable is implemented when the multiple objects of the class need to be executed by threads. Having only one method, the interface is simple. After the creation of the thread, the undefined, void method run() is invoked to execute the thread separately.
216. What is the Thread class?
Thread class is extended by Object class which implements the Runnable Interface. This class contains constructors and methods that are used to create a thread object. It is required to perform the operations of threading.
217. How do you create a thread in Java?
For creating a thread in Java two methods can be followed. Extending the Thread class or implementing the Runnable Interface can be used to perform multithreading.
Using the Thread class, the following approach is needed –
public class MultiThreading extends Thread{
public void run(){
System.out.println(“Run”);
}
public static void main(String args[]){
MultiThreading t1 = new MultiThreading();
start.t1();
}
}
However, if the Runnable Interface is implemented, an object of the Thread class needs to be created separately.
class Multithreading implements Runnable{
public void run(){
System.out.println("Run");
}
public static void main(String args[]){
MultiThreading t1 = new MultiThreading();
Thread t2 =new Thread(t1);
t2.start();
}
]
218. What is the Callable interface?
A callable interface is also used to perform multiple tasks in parallel with pinpoint precision. Actually, it is an improved version of Runnable Interface. Giving the users enhanced control over error handling, this interface contains the call() method which throws checked exceptions.
219. What is the Future interface?
This interface represents a result of an asynchronous computation. Under the concurrent package, it has the methods to check if the computation is completed or not. Apart from ensuring this, it also waits and retrieves the results by the get() method.
220. What is the difference between Runnable and Callable?
The main highlighting difference between the runnable and the callable interface is that one can throw exceptions handling the multithreading works efficiently, and the other doesn’t.
Runnable Interface
- It is under the java.lang package
- Works with the help of the run() method
- Can’t throw any exceptions or return the results of the calculations.
Callable Interface
- Located in the package of java.util.concurrent
- It uses the call() method to execute threads
- Can throw both exceptions and processed results of the calculations.
221. What is the Executor framework?
Located in the java.util.concurrent package, the executor framework is responsible for creating threading, managing it, and scheduling the tasks accordingly. The main feature of this is reusing the existing threads without creating it time and again. With the help of the factory tools, ThreadPool can be easily created.
222. What is the ExecutorService interface?
ExecutorService Interface is a subinterface of the Executive class. It is implemented in the executor class and is used to manage concurrent tasks. When the number of tasks is more than the available threads it also helps to queue the tasks until a free thread is available.
223. What is the ScheduledExecutorService interface?
Being a sub-interface of the executive service interface it is also located in java.util.concurrent package. It is used to schedule the tasks one after another periodically so that it becomes easier to manage the simultaneous work. For this, parameterized methods like schedule(), scheduleWithFixedDelay(), and scheduleAtFixedRate() are commonly used.
224. What is the ThreadPoolExecutor class?
ThreadPoolExecutor class is useful in creating the instance of it to specify the number of the threads that will remain alive after the creation. Also, it enables the user to limit the number of threads to handle the works that are too large to fit in the queue.
Methods like aliveCoreThreadTimeOut() can be used to terminate the core thread when no task arrives within the alive time.
225. What is the ForkJoinPool class?
The ForkJoinPool class is a type of Executor Class that divides the larger tasks into smaller divisions to handle them efficiently. Forks refer to the steps required to split the tasks whereas Join is used to join the subtasks into one result after their execution.
Extending the AbstractExecutorService class, it inherits its invokeAll() and invokeAny() methods to perform the tasks.
226. What is a thread pool?
A thread pool is a group of threads that are working for multiple tasks with several reuses. When a task is assigned, a thread is pulled from this pool. After the successful completion of the task, the thread is sent back to the pool again.
227. What is the synchronized keyword?
The ‘synchronized’ keyword is used to make the code multithread-safe by controlling the access of the threads from multiple sources. By using this keyword, the user can allow only one thread to use the resource. This flexibility resolves the inconsistencies and errors that may arise during simultaneous work.
228. What is a lock in Java?
The Lock is an interface of the java.util.concurrent.lock package having the similar features as the synchronized blocks. It is used to lock a single object or class for a specific period to prevent its accession by the other threads. Void methods like lock() and unlock () are used in this case.
229. What is the Lock interface?
Available in the java.util.concurrent.lock package, it is one of the most used interfaces in Java. This interface works as thread synchronization features by locking and unlocking the objects while multiple threads work simultaneously. Unlike the synchronized blocks, this enables a proper sequencing of the threads with its void pre-defined methods.
230. What is the ReentrantLock class?
When accessing the shared resources, the RentractLock is required. It also implements the Lock interface to use its lock() and unlock() methods to manipulate the synchronization of the tasks.
It allows one to re-enter the lock on a resource more than one time. Each time the thread enters the lock, its value is incremented by one, and with every unlock request it is decremented by one. When the value reaches zero, the resource is unlocked.
231. What is the Condition interface?
Condition interface is used to give the threads a condition. Until the conditions aren’t met, the execution of the thread is suspended.
232. What is the ReadWriteLock interface?
The interface of ReadWriteLock is a special type of interface that allows several threads to read from a source but allows only one thread to write at one time. This feature removes the concurrent errors that might occur when writing and reading occur simultaneously.
The methods of readLock() and writeLock() are needed to achieve this concurrency. However, an object of the Lock class needs to be created beforehand.
233. What is the ReentrantReadWriteLock class?
The ReentrantReadWriteLock class is also implemented by the ReadWriteLock interface. Its features are the same as the ReentrantLock class which ensures the resources can be entered more than once.
234. What is a semaphore?
Basically, a semaphore is like a counter that limits the number of threads that can access a shared resource. This non-negative variable gives access to the threads when the value exceeds zero. When it is not, the access is denied.
In Java, there is an entire class, Semaphore class, containing constructors and methods to complete this task and synchronize the multithreading.
235. What is the Semaphore class?
Belongs to the java.util.concurrent package, the Semaphore class provides all the mechanisms of semaphore. It implements the Serializable interface and uses the methods acquire() and release() to handle multiple threads at a time.
236. What is a deadlock?
Deadlock occurs when two or more threads are blocked. To avoid this issue, synchronized keyword is used when working with multiple threads at a time.
237. How do you prevent deadlock?
Deadlock is a natural phenomenon that occurs when working with multiple threads simultaneously. Some effective practices are maintained in order to avoid this. For instance, placing the locks in the right order, avoiding unnecessary locks, and invoking thread.join() methods can be used to prevent this.
238. What is a livelock?
Livelock is a similar concurrency issue to the deadlock. In contrast to the deadlock, here threads do not wait infinitely; instead, they change their states between one another. The outcome of this livelock is the threads ultimately cannot perform their tasks.
239. What is thread starvation?
Thread starvation is the process when the threads are starved from access to the shared resource and have to wait to execute their work. Though this waiting period is not infinite, these threads acquired the resources whatever is available to execute the run() method.
240. What is the ThreadLocal class?
The ThreadLocal class is used to provide the thread-local variables which are copied when the threads access it. As in this case, each thread has a distinct copy of the variable, there is no issue with the synchronization. All in all, it enhances the performance and the scalability of the program.
241. What is the volatile keyword?
The volatile keyword is used to modify the value of the variable by multiple threads. Applicable to both the primitive data types and objects, it makes the class thread-safe.
No caching of the volatile variables happens. Rather, they are read from the main memory. This informs the compiler to prevent it from any optimization.
242. What is the AtomicInteger class?
The AtomicInteger class mainly deals with the integer values with the atomic operations that are done with single, uninterrupted units. To ensure a multithread-safe environment, several methods are available.
Java Networking
243. What is Java networking?
Java networking allows the connection of two or more devices together so that they can share the resources. This feature also allows a centralized software management option.
244. What is the java.net package?
The package of java.net contains several classes and interfaces that help to achieve the networking features in Java. Classes ranging from Socket, URL, URL connections, and CookieManager along with several other interfaces, enable the user to do low and high-level API tasks.
245. What is the URL class?
URL or Uniform Resource Located class is generally used to refer to the address of the resource on the World Wide Web. As a URL consists of four components – protocol, server name, port number, and file – the constructors and methods of this class also provide this information.
However, the URL class can point to anything from simple file locations to complicated database queries in search engines.
246. What is the URLConnection class?
URLConnection class is an abstract class that provides the link between the URL and the application. With the help of this class, any resource from the URL object can be read and written. All these can be done by configuring the header field.
247. What is the HttpURLConnection class?
Being a subclass of the URLConnection class, the HttpURLConnection class is specifically used to read the HTTP protocols. From header information to the status code, everything can be retrieved by this. A range of methods and a constructor are available in the class for this function.
248. How do you send a GET request in Java?
To send a GET request, at first, an object of URL has to be created. Following it, the openConnection() method needs to be invoked. Then, setting the requestMethod() to default ‘GET’, setRequestProperty() is called to set the value to ‘ User-Agent’. Lastly, the Reader and InputStream are used to read and process the information.
249. How do you send a POST request in Java?
For sending a POST request, the same methods and parameters are followed as used in the GET request. The difference is that this time, you need to set the requestMethod() to ‘POST’ now and create the instance of OutputStream to write the required parameters.
250. What is a socket?
The socket is the endpoint of the communications between the devices. A specific class of Socket is provided in the java.net package to simplify the processes of the socket.
251. What is the Socket class?
The Socket class is used to create a socket. When the connection between the client application and the server is established, an object of this class is created.
The communication between the server and the client is done by reading and writing. Methods like getInputStream() and getOutputStream() are used to return the streams of the sockets.
252. What is the ServerSocket class?
By establishing the connection with the client, the ServerSocket class is mainly used to create a server socket. When the server can’t listen to the specified port, it throws an exception. Several methods are present in the class to ensure an efficient networking connection.
Java String Handling
253. What is the DatagramSocket class?
For connection-less socket programming using UDP, DatagramSocket is used. This class contains connectionless sockets for sending and receiving datagram packets. Along with this, the broadcast messages can also be sent and received.
254. What is the DatagramPacket class?
Without establishing the connection, the DatagramPacket class ensures the message is routed from one device to the other depending on the information. Therefore, this class is a way to provide a connectionless packet delivery service.
255. What is the InetAddress class?
The InetAddress class is used to get the IP address of the hostname. For example, for the URL of www.vivasoftltd.com, the hostname is Vivasoft and its IP address can be retrieved by this address.
256. How do you get the IP address of a host?
When any IP address needs to be found from a hostname, the InetAddress class is used. It has predefined methods and constructors that can easily access this in a few lines of code.
Here’s a simple example of which IP address can be accessed by just the URL.
import java.net;
public class example {
public static void main (Strings[]args){
InetAddress ip1 = InetAddress.getByName(“www.facebook.com”);
System.out.println(ip1.getHostAddress());
}
}
257. How do you get the hostname of an IP address?
To get the hostname, the InetAddress class has predefined methods. The getHostName() should be included after the name of the object. With this, by giving the URL simply, the hostname and the IP address can be retrieved.
258. What is the NetworkInterface class?
The NetworkInterface class has names and a list of IP addresses that are assigned. It can be both software and hardware. With the help of this, the local interface can also be identified.
259. How do you list all network interfaces?
The NetworkInterface class has predefined methods to find the list of the network interfaces. The method getInetAddress() is used to return an enumeration of the inetAddress of all the network interfaces.
Other than that, the getInterfaceAddress() void method can be used to retrieve the list of all the interface addresses.
260. What is the Proxy class?
Proxy classes are implemented at runtime to implement the interfaces. It contains certain methods and constructors to create dynamic proxy classes and objects. Every proxy class is a subclass of the Proxy class and has an invocation handler object with it.
261. What is the Authenticator class?
This class is used to provide authentication while visiting some URLs. This class itself extends the Object class which knows how to find the authentication of the network. Several accessor methods are present in the class to get the required information.
262. How do you handle cookies in Java?
To use and handle cookies, the Cookie class is used. This class enables to create the cookies using the constructor and adding and fetching cookies with the help of particular methods.
263. How do you handle HTTPS connections in Java?
URL, URLConnection, and HTTPURLConnection classes are used to handle the HTTP connections. Among them, the HTTPURLConnection class is specifically used for the HTTP protocols. The methods of these classes are suitable when working with this type of protocol.
264. What is the SSLContext class?
SSLContext class is used to secure the socket protocol. The implementation of this class acts as a factory of SSLEngines.
265. How do you create a secure server in Java?
To create a secure server in Java, the server should be set up first by creating a class for this. Then, a client also needs to be set up by the same process. Write the necessary coding that is required. After that, run the server class first, following the client class to encrypt the server.
266. What is the MulticastSocket class?
To send and receive the multicast datagrams, the MulticastSocket class is used. It is a subclass of the DatagramSocket.
From joining and leaving the multicast group, this class also ensures the proper sending and receiving of the multicast datagrams. For this, the multicast group address and the port number should be specified.
267. How do you join a multicast group in Java?
While joining a multicast group, the object of the MulticastSocket class needs to be created with the necessary port. Invoking the method of joinGroup() with InetAddress as parameter, a multicast group can be joined.
268. How do you leave a multicast group in Java?
To leave a multicast group, a method of leavegroup() is used which takes objects of SocketAddress and NetworkInterface. This feature is essential as the host receives datagrams from the group of ports which are not needed many times.
269. What is the URLStreamHandler class?
URLStreamHandler class is the superclass for all the stream protocol handler classes. This class ensures the connection between the particular protocol types. Starting from HTTP to gopher, everything can be handled by using this class.
270. How do you create a custom protocol handler in Java?
By implementing the URLStreamHandler class, custom protocol handlers can be created. The objects of the URLConnection need to be created and the getInputStream method() is needed to be called to take the data of the handler.
271. What is the SocketChannel class?
This class is used to create the objects of the socket channels. It is a stream-orienting of the connecting sockets. Using the open() method, a simple socket channel can be opened.
272. What is the ServerSocketChannel class?
Located in java.nio.channels package, the ServerSocketChannel class is used to listen to the TCP connections. Just like the SocketChannels, this class can also create server-socket channels with the open() method.
Java Memory Management
273. What is memory management in Java?
Memory management is a process of allocating and deallocating memory to the objects. However, in Java memory management is performed automatically and no logic is required while programming the application.
274. What is the Java memory model?
Java memory model (JMM) mainly deals with how threads use the memory. By providing a safe framework for multithreading, it gives the option to use synchronized blocks, volatile variables, and memory barriers.
275. What is the heap?
In Java, the heap is the memory that is shared by the threads while executing a program. Being a component of JVM, it dynamically allocates the memory and manages the object when the program is run.
276. What is the stack?
Simply put, the stack is a linear data structure to store the collection of data. Using the models of LIFO (Last-In-First-Out), it reorders the objects using classes and interfaces. Java provides the Stack class having push, pop, and search functions to work with the listings.
277. What is the garbage collector?
In Java, the garbage is the unnecessary objects that occupy the memory. Therefore, to get rid of this unused code and free up the space, the garbage collector is used.
278. What are the different garbage collection algorithms?
Garbage collection is the process of deallocating unused memory to free up the space. There are several different algorithms to collect the garbage with different approaches.
Serial garbage collection, parallel garbage collection, concurrent mark, and sweep collection are some of the commonly used ones.
279. What is the finalize method?
Before destroying the object by garbage collection, this method is called to clean up the memory by releasing the resources. From detaching event listeners to closing the Database and Network Connections, this method is quite effective.
Once the method is used, the garbage collector permanently destroys the object. This process is known as finalization.
280. What are strong references?
Strong references are not accessible to garbage collections. Following is an example of a strong reference.
MyRefClass obj = new MyRefClass();
However, when the value of the object is set to null, it is no longer considered a strong reference and is collected by the garbage collector.
281. What are soft references?
Soft references are the ones that are usually not collected. They are only collected when the JVM runs out of memory.
To create soft references the class of SoftReference must be used.
282. What are weak references?
Weak References are the ones that are marked by the garbage collector to be removed. To create this type of reference object the class of WeakReference should be used. Apart from that, the value also needs to be set to null.
283. What are phantom references?
Phantom references are collected by the collectors. The finalize() method is invoked on them and put in a reference queue where they are deleted one by one.
284. What is the ReferenceQueue class?
ReferenceQueue class is used to determine the type of reference. Only the null set of references is listed in this queue. According to this queue, the program decides what action it should take.
285. What is the PhantomReference class?
The phantomReference class is used to create phantom references. This explicitly declares the object to this reference so that it can be queued in the ReferenceQueue class. Following the list, these objects are deleted one by one.
286. What is the difference between heap memory and non-heap memory?
The unrequired objects are often stored in the heap memory before they are collected by the garbage collectors. On the other hand, the non-heap memory is used to store the information like loaded classes.
287. How do you analyze memory usage in Java applications?
Several methods can be followed to analyze memory usage in Java. Monitoring the CPU usage, analyzing the garbage collection, and checking the executable methods, classes, and threads are essential.
At first, the work might seem complex. Thus, often users hire Java developers who are specialized to effectively analyze the memory usage of the applications.
288. What is the MemoryMXBean class?
This class is an interface that is used to access memory-related properties. The instance of this class can be obtained by calling the ManagementFactory.
289. How do you perform garbage collection tuning?
Garbage collection tuning is required to obtain the desired results from the JVM. To perform this there are several simple to complex processes.
It can be done simply by changing the heap size. Again, garbage collection strategies and IAC solutions are some of the high-end processes that can be used.
290. What is the difference between the young generation and the old generation in JVM?
Young generations are the memory where the newly created objects are stored. As the objects are created in the Eden space, they are collected by the garbage collector when they are not reachable anymore.
Contrastingly, the old generation is used to store long-lived objects. The objects that are left in the garbage collection of the young generation, are prompted to the old generation’s memory. Here, they are deleted which are of no use and are occupying space for too long.
291. How do you monitor garbage collection activity?
To monitor the garbage collection activity, the option of JVM with the name ‘verbosegc’ is available. Another separate CUI application, ‘jstat’ is also there to monitor this activity.
292. What is a memory leak detector?
Memory leak is a situation where the memories are used up by the unwanted objects, yet cannot be removed from the allocated space. Therefore, a memory leak detector is used to identify and solve this.
However, a single detector is not enough for this. Multiple diagnosing is required for different cases to find all the memory leaks.
293. How do you use the -XX:+UseG1GC JVM option?
The -XX:+UseG1GC JVM option tells the JVM to perform G1 garbage collection. It operates similarly to the CMS collector.
To determine the liveness of the object, G1 knows the global marking phase. After the completion of the marking phase, G1 identifies the empty spaces which are garbage collected.
294. What is the purpose of the jmap tool?
Jmap tools are used to print the statistics to the memory of JVM. It is usually used in command-line options. Other than that, the use of this prints the list of the shared objects.
295. How do you perform a heap dump?
Traversing to the JDK bin file, the JMAP command is typed with the heap dump file name. Also by default, the heap dump is generated in the JDK file.
296. What is the jhat tool used for?
The jhat tool is used to debug the memory leaks. Examining all the reachable objects, it determines the references that are keeping the objects alive.
297. How do you analyze a heap dump using jhat?
To use the jhat, at first, the location of the JPS of the specific device needs to be found. Then the jhat command line is given to complete the analysis.
298. What is the purpose of the VisualVM tool?
This tool is used to see detailed information on the remote and local applications of JVM. it provides a visual interface to view this information.
299. How do you monitor JVM performance?
JVM performance can be monitored by analyzing the CPU performance, executing threads, and checking the Garbage Collection. At the same time, tuning of the JVM is also required for the diagnosis practices.
300. What is the jstack tool used for?
The jstack tool is used to obtain the stack traces and their respective thread information of the processes. It belongs to the bin folder of the OpenJDK.
301. How do you troubleshoot memory leaks?
The most common and easy way to troubleshoot memory leaks is to use the JFR (Java Flight Recorder) tool. By recording the runtime information, it can identify and solve the memory leaks.
302. What is the purpose of the jconsole tool?
The jconsole tool is used to provide the performance consumption-related information of the application that is currently running. It can graphically monitor the local and the remote programs.
Conclusion
Your intelligence and promptness in answering questions can be a big weapon to secure your position in reputed companies. To ace your interview, practice coding exercises, review key topics like data structures and algorithms, and demonstrate problem-solving skills. Vivasoft has the top 1% of talent in the world. We’ve done our rigorous testing and the questions above are just a part of that test. Contact us to complete your project or scale your team with the best professional Java developers.
