🔹Interface abstract class difference

1. Basic Concept

• Abstract Class
  • A class that cannot be instantiated and may contain both abstract (no body) and concrete (with body) methods.
• Interface
  • A completely abstract contract (before Java 8) that defines what a class must do, not how.

🔹 2. Methods

• Abstract Class
  • Can have:
    • Abstract methods
    • Concrete methods
• Interface
  • Traditionally only abstract methods
  • Since Java 8:
    • default methods (with body)
    • static methods
  • Since Java 9:
    • private methods (for internal use)

🔹 3. Variables

• Abstract Class
  • Can have instance variables (fields)
• Interface
  • Variables are:
    • public
    • static
    • final (constants only)

🔹 4. Constructors

• Abstract Class
  • Can have constructors
• Interface
  • Cannot have constructors

🔹 5. Inheritance

• Abstract Class
  • A class can extend only one abstract (or any) class
    → supports single inheritance
• Interface
  • A class can implement multiple interfaces
    → supports multiple inheritance

🔹 6. Access Modifiers

• Abstract Class
  • Methods and variables can have any access modifier (private, protected, public)
• Interface
  • Methods are implicitly public (unless private helper methods)
  • Fields are always public static final

🔹 7. When to Use

• Abstract Class
  • Use when:
    • You want to share code (implementation) among related classes
    • There is an “is-a” relationship (e.g., Animal → Dog)
• Interface
  • Use when:
    • You want to define a contract
    • You need multiple inheritance
    • Classes are unrelated but share behavior (e.g., Flyable, Runnable)

🔹 8. Example

Abstract Class

abstract class Animal {
    abstract void sound();

    void sleep() {
        System.out.println("Sleeping...");
    }
}

Interface

interface Animal {
    void sound();  // implicitly public and abstract
}

🔹Difference between Abstract Class and Interface

The abstract class achieves partial abstraction (0 to 100%), whereas the interface achieves full abstraction (100%). The following table shows the key differences between difference between abstract class and interface: