Top 60 Java Interview Questions (2025 Updated)

When it comes to technical interviews for software development roles, Java is one of the most commonly used programming languages. To help you prepare, we’ve curated a comprehensive list of Top 60 Java Interview Questions along with detailed answers to reinforce your concepts and coding logic.

Core Java Interview Questions

1. What is Java and why is it so popular?

Java is a versatile, class-based, object-oriented programming language developed by Sun Microsystems and now owned by Oracle. Its popularity stems from being platform-independent—“write once, run anywhere” via the Java Virtual Machine (JVM). Java’s stability, security, and wide adoption in enterprise applications, mobile apps (Android), web servers, and embedded systems contribute to its global usage. It also offers a vast ecosystem of libraries and tools, making it ideal for scalable application development. Java is also backward-compatible and has a strong developer community, making it one of the most reliable programming languages for both beginners and experts.

2. What is the difference between JDK, JRE, and JVM?

• JDK (Java Development Kit): Contains development tools like the compiler (javac) and debugger along with the JRE.
• JRE (Java Runtime Environment): Provides the environment to run Java applications; it includes JVM and standard libraries.
• JVM (Java Virtual Machine): Executes the bytecode generated by the compiler. It’s platform-dependent but ensures Java programs are platform-independent.

These components together form the core of Java’s architecture and enable cross-platform support.

3. What are the main features of Java?

Java offers several powerful features:

• Object-Oriented
• Platform Independent
• Secure and Robust
• Multithreaded
• Distributed
• Architecture-neutral

Its automatic garbage collection, memory management, and vast API support make it one of the most efficient programming languages for large-scale application development.

4. Explain the concept of OOP in Java.

Java is based on Object-Oriented Programming (OOP), which revolves around:

• Encapsulation: Wrapping data and code.
• Abstraction: Hiding complex details.
• Inheritance: Reusing code.
• Polymorphism: Multiple forms of a method.

These pillars enable Java to build modular, maintainable, and scalable code, making it ideal for enterprise-level applications.

5. What is the difference between overloading and overriding?

• Overloading: Defining multiple methods with the same name but different parameters within the same class.
• Overriding: Subclass provides a specific implementation of a method already defined in the parent class.

Overloading happens at compile-time, while overriding occurs at runtime.

6. What are constructors in Java?

A constructor initializes an object when it is created. It has the same name as the class and no return type. Java supports:

• Default Constructors
• Parameterized Constructors
• Constructor Overloading

If no constructor is defined, Java automatically provides a default one.

7. What is the difference between ‘==’ and .equals() in Java?

• ==: Compares references (memory locations).
• .equals(): Compares the contents (values) of objects.

For example, with strings:

String a = new String("hello");
String b = new String("hello");
System.out.println(a == b); // false
System.out.println(a.equals(b)); // true

8. Explain the concept of inheritance in Java.

Inheritance allows one class to inherit fields and methods from another, promoting code reuse and method overriding.

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

Java supports single, multilevel, and hierarchical inheritance but not multiple inheritance using classes.

9. What is an interface in Java?

An interface is a reference type in Java, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. Interfaces allow multiple inheritance and abstraction.

javaCopyEditinterface Drawable {
   void draw();
}
class Circle implements Drawable {
   public void draw() { System.out.println("drawing circle"); }
}

try {
   int a = 1 / 0;
} catch (ArithmeticException e) {
   System.out.println("Error: " + e);
}

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

• Checked: Checked at compile-time (e.g., IOException).
• Unchecked: Checked at runtime (e.g., NullPointerException).

Checked exceptions must be either caught or declared using throws.

14. What is a String Pool in Java?

String Pool is a special memory region where Java stores literal strings to optimize memory usage. When two string literals have the same content, they point to the same memory location.

String a = "Java";
String b = "Java";
System.out.println(a == b); // true

15. Difference between String, StringBuilder, and StringBuffer?

Use StringBuilder for non-threaded environments.

16. What are wrapper classes in Java?

Wrapper classes convert primitives into objects. Each primitive type has a corresponding wrapper class (e.g., int → Integer, char → Character). They’re used in collections, type conversions, and autoboxing.

17. What is autoboxing and unboxing?

• Autoboxing: Converting a primitive to its wrapper object automatically.
• Unboxing: Extracting the primitive value from a wrapper.

Integer i = 5; // autoboxing
int j = i;     // unboxing

18. What are static blocks and static methods?

• Static block: Executes when the class is loaded. Initializes static variables.
• Static method: Belongs to the class, not to instances.

They’re useful for configuration and utility purposes.

19. What is the difference between Heap and Stack memory in Java?

• Stack: Stores method calls, local variables. Faster.
• Heap: Stores objects and class variables. Larger and shared.

Understanding memory allocation helps avoid issues like memory leaks and OutOfMemoryError.

Top 60 Java Interview Questions

20. What is garbage collection in Java?

Garbage Collection (GC) is an automatic process in Java that deallocates memory used by unreachable objects. It’s done by JVM to prevent memory leaks and improve efficiency.

The GC process involves:

• Mark and sweep
• Generational GC
• Finalization

Manual calls to System.gc() can request garbage collection, but it’s not guaranteed to execute immediately.

21. What is the Collection Framework in Java?

The Java Collection Framework is a set of classes and interfaces that implement commonly reusable data structures. It provides both interfaces (like List, Set, Queue, and Map) and concrete classes (like ArrayList, HashSet, LinkedList, HashMap, etc.). Collections help in storing, retrieving, and manipulating data efficiently. All classes are found under the java.util package. It also includes algorithms for sorting and searching. Collections support generic types, making them type-safe and versatile. This framework simplifies programming by providing pre-built data structures and utility methods for working with groups of objects.

22. What is the difference between List and Set in Java?

• List: Allows duplicate elements and maintains insertion order. Common implementations include ArrayList, LinkedList, and Vector.
• Set: Does not allow duplicates and does not maintain order (except LinkedHashSet or sorted collections like TreeSet).

List list = new ArrayList<>();
Set set = new HashSet<>();

Use List when duplicates are allowed, and Set when uniqueness is required.

23. What is HashMap and how does it work?

HashMap is part of the Java Collection Framework and stores data in key-value pairs. It uses a hashing technique to compute an index into an array of buckets. When collisions occur (two keys with the same hash), it uses linked lists or balanced trees to store entries. HashMap allows one null key and multiple null values.

Key features:

• No order is guaranteed
• Not synchronized (use ConcurrentHashMap for thread safety)
• Fast access due to O(1) average lookup time

24. What is the difference between HashMap and Hashtable?

Use HashMap for non-threaded applications and ConcurrentHashMap for thread-safe operations.

25. Explain fail-fast and fail-safe iterators.

• Fail-fast iterators throw ConcurrentModificationException if the collection is modified while iterating (e.g., ArrayList, HashMap).
• Fail-safe iterators do not throw exceptions because they operate on a cloned copy of the collection (e.g., ConcurrentHashMap, CopyOnWriteArrayList).

Fail-fast improves error detection during iteration, while fail-safe ensures consistency in concurrent environments.

26. What are Java Generics?

Generics provide compile-time type safety by allowing classes, interfaces, and methods to operate on specific types without explicit typecasting.

List list = new ArrayList<>();
list.add("Java");

Benefits:

• Type safety
• Code reusability
• Eliminates the need for casting

Generics are extensively used in collections and utility classes.

27. What is the difference between ArrayList and LinkedList?

Use ArrayList for frequent access, and LinkedList for frequent insertions/deletions.

28. What is multithreading in Java?

Multithreading is Java’s capability to run multiple threads (lightweight processes) concurrently. Java supports multithreading using the Thread class and Runnable interface.

Benefits:

• Better resource utilization
• Improved application performance
• Smooth user experiences in GUIs

Threads can be started by calling .start() method, which invokes the run() method internally.

29. What is synchronization in Java?

Synchronization is used to control access to shared resources in a multi-threaded environment. It prevents thread interference and data inconsistency.

Use synchronized keyword on methods or blocks:

synchronized void print() {
   // critical section
}

Java also offers ReentrantLock for more flexible control over locking.

Top 60 Java Interview Questions

30. What is the difference between process and thread?

• Process: An independent executing program with its own memory.
• Thread: A lightweight subprocess sharing the same memory with other threads of the same process.

Threads are more efficient and quicker to start compared to processes.

31. What are Lambda Expressions in Java 8?

Lambda expressions allow writing anonymous methods (functional interfaces) in a concise way. Learn More on Lambda Expression in Java 8.

(int a, int b) -> a + b;

@FunctionalInterface
interface Calculator {
    int calculate(int a, int b);
}

Used primarily with lambda expressions and method references.

33. What is the Stream API in Java?

Introduced in Java 8, the Stream API allows functional-style operations on collections. Learn more on Stream API in Java 8.

Key operations:

• map()
• filter()
• forEach()
• collect()

Example:

list.stream()
    .filter(name -> name.startsWith("A"))
    .forEach(System.out::println);

Streams support parallel processing and lazy evaluation.

34. What is Optional in Java 8?

Optional is a container object used to avoid NullPointerException. It represents a value that may or may not be present.

Optional name = Optional.of("Java");
name.ifPresent(System.out::println);

It forces you to handle null cases more safely and clearly.

35. What is method reference in Java 8?

Method references are a shorthand notation for calling methods using ::.

Types:

• Static method: ClassName::methodName
• Instance method: object::methodName
• Constructor: ClassName::new

Example:

list.forEach(System.out::println);

They work with functional interfaces and improve readability.

36. What is the diamond operator (<>) in Java?

Introduced in Java 7, the diamond operator lets you infer type parameters for generics.

List list = new ArrayList<>();

It reduces verbosity and improves readability without compromising type safety.

37. What is a Singleton class in Java?

A Singleton class allows only one instance of itself to be created.

public class Singleton {
    private static Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) instance = new Singleton();
        return instance;
    }
}

It’s commonly used for shared resources like logging, caching, or DB connections.

38. What is the volatile keyword in Java?

The volatile keyword ensures that updates to a variable are always visible to other threads.

volatile boolean flag = true;

It prevents thread-local caching of variables and is often used for flags or control variables in multi-threaded programs.

39. What is the transient keyword in Java?

transient prevents a variable from being serialized. When an object is serialized, transient fields are skipped.

transient int temp;

Useful for sensitive data like passwords or session-related details.

Top 60 Java Interview Questions

40. What is the difference between final, finally, and finalize in Java?

They serve completely different purposes despite similar names.

41. What is method overloading in Java?

Method overloading allows multiple methods in the same class to share the same name but differ by parameter list (number, type, or order). It’s a form of compile-time polymorphism.

Example:

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

Benefits:

• Enhances readability
• Allows flexible method definitions
• Helps in code reusability

Overloading does not depend on return type alone.

42. What is method overriding in Java?

Method overriding occurs when a subclass provides a specific implementation of a method declared in its superclass.

Example:

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

Rules:

• Method signature must match
• Only inherited methods can be overridden
• Access modifier cannot be more restrictive

It’s used to achieve runtime polymorphism.

43. What is a constructor and how is it different from a method?

A constructor is a special block of code used to initialize objects. It has the same name as the class and no return type.

Differences from methods:

• Called automatically during object creation
• Cannot have return types
• Cannot be inherited but can be overloaded

public class Car {
    Car() { System.out.println("Car created"); }
}

Constructors help in setting up default values or resource allocation.

44. What is the use of ‘super’ keyword in Java?

The super keyword is used to:

• Access parent class methods or variables
• Call a parent class constructor from a child class constructor

class Animal {
    void eat() { System.out.println("eating"); }
}
class Dog extends Animal {
    void eat() {
        super.eat(); // calls parent method
        System.out.println("dog eating");
    }
}

This ensures that overridden methods or shadowed variables can still be accessed when needed.

45. What are static variables and methods in Java?

Static Variables:

• Shared among all instances of a class
• Memory is allocated once at class loading time

Static Methods:

• Belong to the class, not to any object
• Cannot access non-static data directly

Useful for utility or helper methods that don’t require instance state.

static int count = 0;
static void printCount() { System.out.println(count); }

46. What is a package in Java?

A package is a namespace that organizes related classes and interfaces. Java has built-in packages like java.util, java.io, etc., and you can create your own.

package com.example.myapp;

Benefits:

• Prevents name conflicts
• Helps organize code logically
• Supports access control

Packages improve maintainability and modularity.

47. What is classloader in Java?

The ClassLoader is a part of the JVM that dynamically loads classes into memory. Types:

• Bootstrap ClassLoader: Loads core Java classes.
• Extension ClassLoader: Loads classes from ext directory.
• System/Application ClassLoader: Loads classes from the classpath.

Understanding class loaders is crucial for frameworks, custom loading, and memory management in large applications.

48. What is the ‘this’ keyword in Java?

this refers to the current class instance. It’s used to:

• Differentiate between instance variables and parameters
• Call one constructor from another
• Return the current object

Example:

class Person {
    String name;
    Person(String name) {
        this.name = name;
    }
}

49. What are enums in Java?

An enum is a special class representing a group of constants.

enum Day { MONDAY, TUESDAY, WEDNESDAY }

Features:

• Type-safe
• Can have fields, constructors, and methods
• Useful in switch statements and control flow

Enums improve readability and reduce the risk of invalid values.

50. What is the difference between break and continue?

• break: Terminates the current loop or switch.
• continue: Skips the current iteration and proceeds to the next.

Example:

for(int i=0; i<5; i++) {
    if(i == 2) continue;
    System.out.println(i);
}

These keywords control the loop flow and are critical in logic-heavy applications.

Top 60 Java Interview Questions

51. What is instanceof keyword in Java?

The instanceof keyword checks whether an object is an instance of a specific class or subclass.

if (obj instanceof String) {
    System.out.println("It's a string!");
}

Returns true or false and is often used for type checking before casting.

52. What is the final keyword used for?

The final keyword can be applied to:

• Variables: Makes them constants
• Methods: Prevents overriding
• Classes: Prevents inheritance

Example:

final int MAX = 100;

It enforces immutability and strict structure in code.

53. What are default and static methods in interfaces?

Java 8 introduced:

• Default methods: Provide a default implementation in interfaces.
• Static methods: Utility methods in interfaces.

Example:

interface Vehicle {
    default void start() { System.out.println("Starting..."); }
    static void check() { System.out.println("Checking vehicle"); }
}

This supports backward compatibility and flexibility in design.

54. What is reflection in Java?

Reflection allows inspection and modification of classes, methods, and fields at runtime.

Class> clazz = Class.forName("com.example.MyClass");
Method[] methods = clazz.getDeclaredMethods();

Used in:

• IDEs
• Frameworks (Spring, Hibernate)
• Debuggers

It’s powerful but should be used carefully due to performance and security concerns.

55. What is the difference between compile-time and runtime polymorphism?

• Compile-time (Static): Achieved via method overloading.
• Runtime (Dynamic): Achieved via method overriding.

Polymorphism enables flexible code and allows the same interface to behave differently based on context.

56. What is the Java Memory Model (JMM)?

The Java Memory Model defines how threads interact through memory and what behaviors are allowed in multithreaded programs.

Key elements:

• Atomicity
• Visibility
• Ordering

It ensures consistent memory access across different threads and CPU cores.

57. What is the purpose of the transient keyword?

transient prevents serialization of a field, which means that the variable is not stored when the object is serialized.

Useful for:

• Passwords
• Temporary variables
• Session-related information

transient String password;

58. How is memory managed in Java?

Memory in Java is managed via:

• Heap: Stores objects and class variables.
• Stack: Stores local variables and method calls.
• Garbage Collector: Automatically deallocates memory for unreachable objects.

You can suggest GC with System.gc(), but JVM decides execution.

59. What is the difference between throw and throws?

• throw: Used to explicitly throw an exception.
• throws: Declares the exception a method might throw.

void method() throws IOException {
    throw new IOException("Error");
}

60. What are annotations in Java?

Annotations provide metadata about the program and are used for:

• Documentation (@Override)
• Compilation (@SuppressWarnings)
• Runtime processing (@Entity, @Autowired)

Custom annotations can also be created and processed using reflection or frameworks like Spring.

✅ Conclusion

These Top 60 Java Interview Questions cover all major topics essential for beginners and experienced developers preparing for technical interviews. From core Java concepts to advanced features introduced in Java 8 and beyond, these questions ensure you’re fully equipped to ace any Java interview. Download Java

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