Java Interfaces

An interface is a promise: any class that implements it must provide its methods. A class can implement many interfaces.

ExampleJava

public class Main {
  interface Shape {
    double area();
  }

  static class Circle implements Shape {
    double r;
    Circle(double r) { this.r = r; }
    public double area() { return Math.PI * r * r; }
  }

  static class Rect implements Shape {
    double w, h;
    Rect(double w, double h) { this.w = w; this.h = h; }
    public double area() { return w * h; }
  }

  public static void main(String[] args) {
    Shape[] shapes = { new Circle(1), new Rect(2, 3) };
    for (Shape s : shapes) {
      System.out.printf("%.2f%n", s.area());
    }
  }
}

Output

3.14
6.00

Quick check

What must a class do when it implements an interface?

Provide all its methodsExtend itMake a copy of it

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

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