interface

Learn to define contracts with interface.

What is an interface?

An interface defines the contract of methods that classes will implement. It contains method signatures; no bodies. It starts with the interface keyword.

Example: interface Drawable { void draw(); }. Here Drawable is a contract; draw is declared with its signature only. A class that implements this contract must write the draw method. The interface says what to do; it leaves how to the class. This lets different classes share a common behavior.

Method signatures inside an interface

Methods inside an interface have no body. Only the signature is written: return type, name and parameters. It ends with a semicolon.

Example: interface Speaker { void speak(String msg); }. Here speak has return type void and parameter String msg. No body; only the contract. The implementing class writes the method with its own body. Even if you do not write public, interface methods are implicitly public. This means the contract is always open.

Exercise Define the Drawable interface

Define an interface Drawable. Add the void draw() method signature. Then write class Circle implements Drawable; write the public void draw() method with a body that runs System.out.println("Drawing a circle");. In main call new Circle().draw().

Show example solution

interface Drawable {
    void draw();
}
class Circle implements Drawable {
    public void draw() {
        System.out.println("Drawing a circle");
    }
}
class Main {
    public static void main(String[] args) {
        new Circle().draw();
    }
}

Test yourself

Which keyword defines an interface?
- interface
- class
- contract
- abstract
Show answer

interface — The interface keyword defines a contract.

Complete the interface declaration

interface ____ {
    void draw();
}

Show answer

interface Drawable {
    void draw();
}

Match the interface parts

interface

Contract keyword
void draw()

Method signature
Drawable

Interface name
;

Signature end

Next lesson: implements

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