Questions

1. Why do we need OOP? What are some major OOP languages?

Object-Oriented Programming (OOP) helps in code reusability, maintainability, and organization using objects and classes.

Major OOP Languages:

• Java
• C++
• Python
• C#
• Ruby

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2. Interface vs Abstract Class

Feature	Interface	Abstract Class
Methods	Only method signatures (no implementation, except default methods in Java 8+)	Can have both abstract methods and concrete methods
Inheritance	Used for defining behavior across unrelated classes	Used for shared behavior in a class hierarchy

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3. Why do we need equals() and hashCode()? When to override?

• Used to compare object equality and enable correct hashing.
• Override when creating custom objects that need to be compared logically.

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4. Diamond Problem in Java & How to Fix It

• Occurs in multiple inheritance when a class inherits from two interfaces with the same method.
• Fix: Use default methods in interfaces or avoid multiple inheritance of implementations.

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5. Why do we need a Garbage Collector? How does it run?

• Frees memory by removing unused objects to prevent memory leaks.
• Runs automatically but can be triggered manually with System.gc().

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6. Java static Keyword Usage

• Used for class-level variables, methods, blocks, and nested classes.
• Belongs to the class, not instances.

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7. Immutability: Definition, Usage, and Implementation

• Definition: An immutable object cannot be modified after creation.
• Where to Use: String, Wrapper classes, and custom immutable objects.
• Why: Ensures thread safety and efficient caching.
• How: Use final fields and avoid setters.

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8. Composition vs Aggregation

Feature	Composition	Aggregation
Association	Strong (contained object cannot exist without the container)	Weak (contained object can exist independently)

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9. Cohesion vs Coupling

• Cohesion: How well a class’s methods relate to its purpose.
• Coupling: Dependency between classes.

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10. Heap vs Stack

Feature	Heap	Stack
Stores	Objects	Method calls and local variables
Scope	Shared among threads	Thread-specific
Management	Managed by Garbage Collector	Auto-cleared

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11. What is an Exception? Types of Exceptions

• Definition: An event disrupting program execution.
• Types:
  • Checked: Must be handled (e.g., IOException).
  • Unchecked: Runtime errors (e.g., NullPointerException).

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12. How to Summarize "Clean Code" in the Shortest Way?

• Simple, readable, maintainable, and follows best practices.

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13. What is Method Hiding in Java?

• Occurs when a subclass defines a static method with the same signature as a static method in the parent class.
• Unlike method overriding, the method call depends on the reference type.

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14. Abstraction vs Polymorphism in Java

Feature	Abstraction	Polymorphism
Definition	Hides implementation details using abstract classes or interfaces	Allows methods to behave differently based on the object type
Example	Abstract classes, Interfaces	Method overriding, Method overloading