Interface vs Abstract Class (Java)

1. Overview

Feature	Interface	Abstract Class
Purpose	Define contract (what to do)	Provide partial implementation (how + what)
Keyword	interface	abstract class

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2. Methods

Feature	Interface	Abstract Class
Abstract methods	Yes (default)	Yes
Concrete methods	Yes (default/static)	Yes
Default methods	Yes (default keyword)	Not needed (normal methods)
Static methods	Yes	Yes
Private methods	Yes (Java 9+)	Yes

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3. Variables

Feature	Interface	Abstract Class
Fields	public static final (constants only)	Instance + static variables allowed
Default modifier	public static final	No default restriction

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4. Constructors

Feature	Interface	Abstract Class
Constructor	❌ Not allowed	✅ Allowed

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5. Initialization Blocks

Abstract Class	Interface
✅ Instance and static blocks allowed	❌ Not allowed

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6. Inheritance

Feature	Interface	Abstract Class
Multiple inheritance	✅ Yes (implements multiple)	❌ No (only one class)
Extends	Can extend multiple interfaces	Can extend one class
Implements	N/A	Can implement interfaces

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7. Object Creation

Feature	Interface	Abstract Class
Instantiation	❌ Cannot instantiate	❌ Cannot instantiate

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8. Access Modifiers

Feature	Interface	Abstract Class
Methods	public by default	Any (public, protected, private)
Variables	public static final only	Any

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9. When to Use

Use Case	Interface	Abstract Class
Define contract	✅ Yes	❌ Less suitable
Shared code	❌ Limited	✅ Yes
Multiple inheritance needed	✅ Yes	❌ No
Common base class	❌ No	✅ Yes

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