Whether you’re preparing for a Java developer role or just brushing up your knowledge, mastering common Java interview questions can give you a big edge.
Top 50 Java Interview Questions and Answers
1. What is Java?
Java is a high-level, object-oriented programming language used to build applications.
2. Who developed Java?
Java was developed by James Gosling at Sun Microsystems in 1995.
3. Why is Java platform-independent?
Because Java code is compiled to bytecode, which runs on the Java Virtual Machine (JVM). This works on any platform.
4. What is JVM?
JVM (Java Virtual Machine) executes Java bytecode and provides platform independence.
5. What is JDK, JRE, and JVM?
- JDK: Java Development Kit – for coding, compiling, and running Java.
- JRE: Java Runtime Environment – for running Java apps.
- JVM: Java Virtual Machine – runs bytecode.
6. What is a Class?
A class is a template or blueprint used to create objects.
7. What is an Object?
An object is an instance of a class that holds data and methods.
8. What is the main() method?
The main() method is the entry point of any Java program.
public static void main(String[] args)
9. What is a Constructor?
A constructor is a method used to initialize objects.
10. Can we overload constructors?
Yes. Java supports constructor overloading.
11. What is Inheritance?
Inheritance allows one class to inherit properties from another.
class Animal {}
class Dog extends Animal {}
12. What is Polymorphism?
Polymorphism means many forms — the same method can behave differently in different classes.
13. What is Encapsulation?
Encapsulation means wrapping data and code together inside a class.
14. What is Abstraction?
Abstraction means hiding complex details and showing only essential features.
15. What is an Interface?
An interface is a contract that a class can implement. It only contains method declarations.
16. What is the difference between ‘==’ and ‘equals()’?
==compares memory location.equals()compares values.
17. What is a static keyword?
The static keyword defines a shared variable or method for all objects.
18. What is the final keyword?
final means constant. It can’t be changed or overridden.
19. What is a package?
A package is a way to group related classes together.
20. What are access modifiers?
They define visibility: public, private, protected, and default (no modifier).
21. What is method overloading?
Same method name with different parameters.
22. What is method overriding?
A child class provides its own version of a method from the parent class.
23. What is the super keyword?
super refers to the parent class object or constructor.
24. What is this keyword?
this refers to the current object.
25. What is a constructor chain?
Calling one constructor from another using this() or super().
26. What is garbage collection?
It’s Java’s way of removing unused objects from memory.
27. What is the use of the ‘new’ keyword?
It’s used to create objects in Java.
28. What is the difference between Array and ArrayList?
- Array: Fixed size
- ArrayList: Dynamic size, part of Java Collections
29. What is the difference between String, StringBuilder, and StringBuffer?
- String: Immutable
- StringBuilder: Mutable, not thread-safe
- StringBuffer: Mutable, thread-safe
30. What are wrapper classes?
They wrap primitive types into objects. Example: Integer, Double, Boolean
31. What is Exception Handling?
It handles runtime errors using try-catch blocks.
32. What is the difference between checked and unchecked exceptions?
- Checked: At compile time (e.g., IOException)
- Unchecked: At runtime (e.g., NullPointerException)
33. What are try, catch, finally blocks?
try: Code that may throw errorcatch: Handles errorfinally: Always runs
34. What is throw and throws?
throw: Used to manually throw an exceptionthrows: Declares exceptions a method might throw
35. What is multithreading?
Multithreading allows multiple tasks (threads) to run at the same time.
36. What is the difference between process and thread?
- Process: Independent program
- Thread: Smaller part of a process
37. How to create a thread?
By extending Thread class or implementing Runnable interface.
38. What is synchronization?
It prevents multiple threads from accessing the same resource simultaneously.
39. What is the Java Collections Framework?
A group of classes to store and manipulate groups of data.
40. What are the main interfaces in Collections?
- List
- Set
- Map
- Queue
41. Difference between List and Set?
- List: Allows duplicates
- Set: No duplicates
42. What is HashMap?
A HashMap stores data in key-value pairs.
43. What is the difference between HashMap and TreeMap?
- HashMap: Unordered
- TreeMap: Sorted order
44. What is the difference between ArrayList and LinkedList?
- ArrayList: Faster search
- LinkedList: Faster insert/delete
45. What is an abstract class?
A class that cannot be instantiated and can contain abstract methods.
46. Can a class implement multiple interfaces?
Yes. Java supports multiple interface inheritance.
47. Can a class extend multiple classes?
No. Java does not support multiple inheritance with classes.
48. What is the default value of int, boolean, and object types?
- int: 0
- boolean: false
- Object: null
49. What is autoboxing and unboxing?
- Autoboxing: Primitive to object
- Unboxing: Object to primitive
50. What is a Lambda expression in Java?
Introduced in Java 8, it lets you write shorter code for functional interfaces.
(a, b) -> a + b
Conclusion
These 50 Java interview questions and answers are perfect for freshers, students, or anyone brushing up on basics. Focus on OOPs concepts, exception handling, multithreading, and collections. Understanding them will give you the confidence to answer most beginner-level Java interview questions with ease.
Top 100 DSA questions: Link