Java Interview Questions & Answers – Logic NextGen Tutorials

Java Interview Questions & Answers

1. What is Java?

Java is a high-level, object-oriented, platform-independent programming language used for developing web, desktop, mobile, and automation applications.

2. What are the features of Java?

Platform independent
Object-oriented
Secure
Robust
Multithreaded
Portable

3. What are variables in Java?

Variables are containers used to store data values.

Example:

int age = 25;

4. What are data types in Java?

Two types:

Primitive → int, char, boolean
Non-primitive → String, Array, Class

5. What is the difference between primitive and non-primitive data types?

Primitive	Non-Primitive
Stores value	Stores reference
Fixed size	Dynamic size

6. What is type casting in Java?

Converting one datatype into another datatype.

Example:

double d = 10.5;
int i = (int)d;

7. What are operators in Java?

Operators perform operations on variables.

Examples:

Arithmetic (+)
Logical (&&)
Relational (>)

8. What is the difference between == and .equals()?

==	.equals()
Compares memory reference	Compares values

9. What are OOPs concepts in Java?

Encapsulation
Inheritance
Polymorphism
Abstraction

10. What is encapsulation?

Encapsulation means wrapping data and methods together in a single class and restricting direct access using private variables.

11. What is inheritance?

Inheritance allows one class to acquire properties and methods of another class using extends keyword.

12. What is polymorphism?

Polymorphism means one method can perform different actions based on the object.
Types:

Overloading
Overriding

13. What is abstraction?

Abstraction means hiding implementation details and showing only required functionality.

14. What is the difference between abstraction and encapsulation?

Abstraction	Encapsulation
Hides implementation	Hides data

15. What is method overloading?

Method overloading means creating multiple methods with same name but different parameters.

16. What is method overriding?

Method overriding means redefining parent class method in child class with same signature.

17. What is the difference between overloading and overriding?

Overloading	Overriding
Same class	Parent-child class
Different parameters	Same parameters
Compile-time polymorphism	Runtime polymorphism

18. What is an interface in Java?

An interface is a blueprint containing abstract methods that must be implemented by classes.

19. What is an abstract class?

An abstract class is a class declared using abstract keyword. It can contain abstract and non-abstract methods

20. What is the difference between interface and abstract class?

Interface	Abstract Class
Supports multiple inheritance	Partial abstraction
Mostly abstract methods	Can contain concrete methods

21. What is constructor in Java?

Constructor initializes object.

22. What is constructor overloading?

A constructor is a special method used to initialize objects.

23. What is constructor overloading?

Creating multiple constructors with different parameters is called constructor overloading

23. What is the difference between constructor and method?

Constructor	Method
Initializes object	Performs operations
Same name as class	Any valid name

24. What is this keyword?

Refers to current class object.

25. What is super keyword?

super refers to parent class object or constructor.

26. What is static keyword in Java?

static belongs to class instead of object.

27. What is final keyword in Java?

Used to:

Prevent inheritance
Prevent overriding
Declare constants

28. Can we override static or final methods?

No, static and final methods cannot be overridden.

29. What is String in Java?

String is a sequence of characters used to store text.

30. What is the difference between String, StringBuffer, and StringBuilder?

String	StringBuffer	StringBuilder
Immutable	Mutable	Mutable
Slow	Thread-safe	Faster

31. Why is String immutable in Java?

For security, thread safety, and performance.

32. What is string pooling?

Java stores string literals in common memory called String Pool.

33. What is the difference between equals() and equalsIgnoreCase()?

equals()	equalsIgnoreCase()
Case-sensitive	Ignores case

34. How do you reverse a string in Java?

Using StringBuilder:

String rev = new StringBuilder(str).reverse().toString();

35. How do you remove duplicates from a string?

Duplicates can be removed using loops, Set collection, or HashSet.

36. How do you count characters in a string?

Using length() method.

str.length();

37. What is an array in Java?

An array is a collection of elements of the same datatype stored in a single variable. It is used to store multiple values together using index positions.

38. What is ArrayList?

ArrayList is a class in Java Collection Framework used to store dynamic collections of elements. Unlike arrays, its size can grow or shrink automatically.

39. What is the difference between Array and ArrayList?

Array	ArrayList
Fixed size	Dynamic size
Stores primitives	Stores objects

40. What is the difference between List, Set, and Map?

41. What is HashMap in Java?

Stores key-value pairs

42. What is the difference between HashMap and Hashtable?

HashMap	Hashtable
Not synchronized	Synchronized

43. What is the difference between HashSet and TreeSet?

HashSet	TreeSet
Unordered	Sorted

44. What is Iterator in Java?

Used to traverse collection elements.

45. What is the difference between Iterator and ListIterator?

Iterator	ListIterator
Forward only	Forward & backward

46. What is Collection Framework in Java?

Framework containing classes/interfaces for storing data.

47. What is Comparable and Comparator?

Comparable	Comparator
Natural sorting	Custom sorting

48. What is the difference between ArrayList and LinkedList?

ArrayList	LinkedList
Faster retrieval	Faster insertion/deletion

49. What is exception handling in Java?

Mechanism to handle runtime errors.

50. What is the difference between checked and unchecked exceptions?

Checked	Unchecked
Compile-time	Runtime

51. What is try-catch block?

Used to handle exceptions.

52. What is finally block?

Always executes whether exception occurs or not.

53. What is throw and throws keyword?

throw	throws
Throws exception	Declares exception

54. Can finally block execute without catch block?

Yes.

55. What is NullPointerException?

Occurs when accessing null object.

56. What is exception propagation?

Passing exception from one method to another.

57. What is file handling in Java?

Reading and writing files using Java classes.

58. How do you read data from a file in Java?

Using:

BufferedReader
Scanner
FileReader

59. How do you write data into a file?

Using FileWriter or BufferedWriter.

60. What is BufferedReader?

Used to read text efficiently.

61. What is FileInputStream and FileOutputStream?

Used to read/write byte data.

62. What is multithreading in Java?

Executing multiple threads simultaneously.

63. What is thread in Java?

Smallest unit of execution.

64. What is the difference between process and thread?

Process	Thread
Independent execution	Part of process

65. How do you create a thread in Java?

Using:

Thread class
Runnable interface

66. What is synchronization?

Prevents multiple threads accessing shared resources simultaneously.

67. What is thread sleep?

Pauses thread execution temporarily.

68. What is the difference between start() and run()?

start()	run()
Creates new thread	Executes normally

1. What is a Constructor in Java?

A constructor in Java is a special block of code used to initialize an object when it is created.

The constructor name must be the same as the class name
It does not have any return type (not even void)
It is automatically called when an object is created
It is mainly used to assign initial values to object variables

If a class does not have any constructor, the Java compiler automatically provides a default constructor

class Student {
    int id;

    Student() {   // Constructor
        id = 100;
    }
}

Key Points to Remember (for interview):

Constructor ≠ Method
Used for object initialization
Can be overloaded (multiple constructors with different parameters)
Types:
- Default constructor
- Parameterized constructor

2. What is Inheritance in Java?

Inheritance in Java is an OOP concept where one class (child/subclass) acquires the properties and behaviors (variables and methods) of another class (parent/superclass).

It helps in code reusability and reduces duplication.

Key Points (Interview Focus):

Achieved using the extends keyword
Promotes code reuse
Supports method overriding
Java supports:
- Single inheritance
- Multilevel inheritance
- Hierarchical inheritance
Java does not support multiple inheritance with classes (but supports it using interfaces)

class Animal {
    void eat() {
        System.out.println("Eating...");
    }
}

class Dog extends Animal {
    void bark() {
        System.out.println("Barking...");
    }
}

Here, Dog inherits the eat() method from Animal.

3. What is Polymorphism in Java?

Polymorphism in Java means “many forms”—it allows a single method or object to behave in different ways depending on the context.

Types of Polymorphism:

1. Compile-time Polymorphism (Method Overloading)

Same method name
Different parameters
Decided at compile time

class Calculator {
    int add(int a, int b) {
        return a + b;
    }

    int add(int a, int b, int c) {
        return a + b + c;
    }
}

2. Runtime Polymorphism (Method Overriding)

Same method name
Same parameters
Different implementation in child class
Decided at runtime

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Bark");
    }
}

Key Points (Interview Focus):

Achieves flexibility and reusability
Overloading → compile-time
Overriding → runtime
Runtime polymorphism uses inheritance