50 java interviews questions

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# 🔥 Top 50 Java Interview Questions and Answers [2025 Edition]

Whether you're a **beginner**, **intermediate**, or **experienced Java developer**, these **top Java interview questions** will help you crack coding rounds and technical interviews at companies like TCS, Infosys, Capgemini, and product-based firms.

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## ✅ Core Java – Basic Level

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

- **JDK**: Java Development Kit – for developing Java applications.

- **JRE**: Java Runtime Environment – for running Java programs.

- **JVM**: Java Virtual Machine – runs Java bytecode.

### 2. What are the main OOP concepts in Java?

- Inheritance

- Encapsulation

- Abstraction

- Polymorphism

### 3. What is the difference between `==` and `.equals()` in Java?

- `==` compares object references.

- `.equals()` compares values (if overridden).

### 4. What is the default value of a boolean variable?

- `false`

### 5. What is a constructor in Java?

- A special method used to initialize objects.

- Same name as the class.

- No return type.

---

## 🔄 Java – Intermediate Level

### 6. What is method overloading vs overriding?

- **Overloading**: Same method name, different parameters.

- **Overriding**: Redefining a method in the child class.

### 7. Difference between `ArrayList` and `LinkedList`?

- `ArrayList`: Fast for read, slow for insert/delete.

- `LinkedList`: Fast for insert/delete, slow for random access.

### 8. What is the use of `final` keyword?

- `final` class: can’t be extended.

- `final` method: can’t be overridden.

- `final` variable: value can't change.

### 9. What is a static block?

```java

static {

   System.out.println("Static block called");

}

10. What are checked and unchecked exceptions?

• Checked: Must be handled at compile time (e.g., IOException).
• Unchecked: Runtime exceptions (e.g., NullPointerException).

🔁 Collections Framework

11. Difference between HashMap and Hashtable?

• HashMap is non-synchronized.
• Hashtable is synchronized (thread-safe, but slower).

12. Difference between List and Set?

• List: allows duplicates, maintains insertion order.
• Set: no duplicates, no guaranteed order.

13. How does HashMap work internally?

• Uses hashCode() and equals() to store key-value pairs in buckets.

14. What is Comparable vs Comparator?

• Comparable: natural ordering (compareTo()).
• Comparator: custom ordering (compare()).

15. What is fail-fast vs fail-safe?

• Fail-fast: throws ConcurrentModificationException (e.g., ArrayList).
• Fail-safe: avoids errors (e.g., CopyOnWriteArrayList).

🔄 Multithreading

16. Ways to create threads in Java?

• Extend Thread class
• Implement Runnable
• Use ExecutorService

17. Difference between sleep() and wait()?

• sleep(): pauses thread without releasing lock.
• wait(): pauses and releases lock.

18. What is synchronized keyword?

• Prevents concurrent access to blocks/methods.

19. What is volatile keyword?

• Ensures visibility of variable changes across threads.

20. What is deadlock in Java?

• Two threads wait for each other’s lock indefinitely.

🆕 Java 8 Features

21. What is a lambda expression?

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

22. What is a functional interface?

@FunctionalInterface

interface MyFunction {

   void apply();

}

23. What is Stream API?

• A new abstraction to process collections using functional-style code.

24. map() vs flatMap()?

• map(): transforms elements.
• flatMap(): flattens nested elements.

25. What is Optional?

Optional.ofNullable(name).orElse("Default");

💾 Memory & Garbage Collection

26. Heap vs Stack memory?

• Heap: for objects.
• Stack: for method calls/local variables.

27. What is garbage collection?

• JVM’s way of cleaning unused objects automatically.

28. How to make an object eligible for GC?

• Remove all references.

29. What is finalize()?

• Called before GC (not recommended for use).

30. What is memory leak?

• Memory that’s no longer needed but not released.

🔐 Advanced Java Concepts

31. What is 

transient

 keyword?

• Prevents field from being serialized.

32. Difference between 

this

 and 

super

?

• this: refers to current class.
• super: refers to parent class.

33. What is Java Reflection?

• Allows runtime inspection/modification of classes, methods, etc.

34. Use of enums?

• Special class to define constants.

35. What is serialization and deserialization?

• Convert object to byte stream and vice versa.

📦 Design Patterns & Architecture

36. Singleton Design Pattern

private static final Singleton INSTANCE = new Singleton();

37. Factory Pattern

• Used to create objects without exposing instantiation logic.

38. Builder Pattern

• For constructing complex objects step-by-step.

39. Dependency Injection

• Letting frameworks inject required dependencies.

40. Inversion of Control (IoC)

• Shifts object creation control to the container (e.g., Spring).

💡 Miscellaneous

41. Why is Java platform-independent?

• Bytecode runs on any OS with JVM.

42. Access modifiers: public, private, protected?

• Control visibility of classes and methods.

43. What are annotations?

@Override

@Deprecated

44. Abstract class vs Interface?

• Interface (Java 7): only abstract methods.
• Abstract: can have method body.

45. What is method reference?

System.out::println

🚀 Bonus Coding Questions

46. Reverse a string:

new StringBuilder(str).reverse().toString();

47. First non-repeating character in string?

// Count frequency, then scan string for first char with freq = 1

48. HashSet vs TreeSet?

• HashSet: unordered, faster.
• TreeSet: ordered (sorted), slower.

49. Palindrome check:

boolean isPalindrome(String s) {

  return s.equals(new StringBuilder(s).reverse().toString());

}