Interface

Interface defines a contract: a set of abstract methods that a class must implement. It’s used to achieve full abstraction and multiple inheritance in Java.

Example

interface Animal {
    void makeSound(); // abstract by default
}
 
class Dog implements Animal {
    public void makeSound() {
        System.out.println("Bark");
    }
}
 
Animal animal = new Dog();
animal.makeSound(); // "Bark" 

Implementing multiple interfaces

interface A { void show(); }
interface B { void display(); }
 
class Test implements A, B {
    public void show() {}
    public void display() {}
}

Features

• All methods are abstract + public by default (until Java 8+).
• Fields are public static final (constants).
• A class can implement multiple interfaces.
• Can have:
  • default methods (with body),
  • static methods,
  • private methods (since Java 9).

Comparison

Interface vs Abstract Class

Feature	Abstract Class	Interface
Method Types	Abstract + Concrete	Abstract (default), default, static
Instantiation	✗	✗
Constructors	✅	✗
Multiple Inheritance	✗	✅
Access Modifiers	Any	public only (before Java 8)
Fields	Any	public static final only

Tip

Use interfaces for flexibility and when multiple inheritance is needed.
Prefer abstract classes when you want to share a common code.

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Parent: _OOP Advanced Tools