Practice›Java Fundamentals›Module 8. Interfaces & Abstract Classes›8.1 Interfaces

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8.1 Interfaces — many classes, one contract

Lesson

Unrelated classes, same promise

Many classes can implement the same interface, each in its own way. They need not be related by inheritance at all — a Circle and a Rect share no superclass, yet both are Shapes because both fulfill the contract. This is the key difference from extends: an interface groups classes by what they can do, not by what they are.

Worked example

class Circle implements Shape {
    public double area() { return 3.14 * r * r; }
    public String name() { return "Circle"; }
}
class Rect implements Shape {
    public double area() { return w * h; }   // different formula
    public String name() { return "Rect"; }
}

Both classes satisfy Shape, but each computes area() its own way.

▶ Try it: run this — two unrelated classes, both honoring the same contract.

interface Shape {
    double area();
    String name();
}
class Circle implements Shape {
    private double r;
    public Circle(double r) { this.r = r; }
    public double area() { return 3.14 * r * r; }
    public String name() { return "Circle"; }
}
class Rect implements Shape {
    private double w, h;
    public Rect(double w, double h) { this.w = w; this.h = h; }
    public double area() { return w * h; }
    public String name() { return "Rect"; }
}
public class Main {
    public static void main(String[] args) {
        Circle c = new Circle(1);
        Rect r = new Rect(3, 4);
        System.out.println(c.name() + " area " + c.area());
        System.out.println(r.name() + " area " + r.area());
    }
}

Can two classes with no common superclass implement the same interface?

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Two classes implementing area() can...

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Type exactly what this program prints.

interface Shape { double area(); }
class Sq implements Shape { public double area(){ return 4.0; } }
class Tri implements Shape { public double area(){ return 6.0; } }
public class Main { public static void main(String[] a){
    System.out.println(new Sq().area());
    System.out.println(new Tri().area());
}}

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Match each keyword to its meaning.

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True or false?

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