Core Java Interview Questions

Object-Oriented Programming

1. What are the four pillars of OOP?

• Encapsulation: Wrapping data (variables) and code (methods) together as a single unit. Data hiding using private.
• Inheritance: Mechanism where a new class inherits properties and behavior from an existing class.
• Polymorphism: Ability to perform a single action in different ways (Overloading & Overriding).
• Abstraction: Hiding internal details and showing only functionality.

2. Difference between Abstract Class and Interface?

String & Immutability

3. Why is String immutable in Java?

• Security: Strings are used for passwords, URLs, db connections.
• Synchronization: Immutable objects are thread-safe.
• String Pool: Saves heap memory by reusing literals.
• Caching HashCode: Since it doesn't change, hashcode is cached (great for HashMap keys).

4. String vs StringBuilder vs StringBuffer?

• String: Immutable. Slow for concatenation.
• StringBuilder: Mutable. Not thread-safe. Fast.
• StringBuffer: Mutable. Thread-safe (synchronized). Slower than StringBuilder.

5. Difference between == and .equals()?

• == compares references (memory address).
• .equals() compares content (values).

Exceptions & JVM

6. Checked vs Unchecked Exceptions?

• Checked: Checked at compile-time (e.g., IOException, SQLException). Must be handled.
• Unchecked: Runtime exceptions (e.g., NullPointerException, ArrayIndexOutOfBoundsException). Not enforced.

7. Does finally block always execute?

Yes, except when:

• System.exit() is called.
• Power failure / JVM crash.
• Infinite loop in try/catch.

8. Use of static keyword?

• Variable: Shared copy for all instances.
• Method: Called without instance. Can't access instance members.
• Block: Runs once when class loaded.
• Class: Only nested classes can be static.

Quiz