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

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8.1 Interfaces — a contract of methods

Lesson

Promising what a class can do

An interface is a contract: a list of method signatures with no bodies. A class that says implements Shape promises to provide a body for every method in the interface. Unlike a superclass, an interface holds no fields and no code — it only states what must be done, never how.

Worked example

interface Shape {
    double area();    // no body — just the signature
    String name();
}

class Circle implements Shape {
    private double r;
    public Circle(double r) { this.r = r; }
    public double area() { return 3.14 * r * r; }   // must provide
    public String name() { return "Circle"; }        // must provide
}

If Circle left out either method, it would not compile — the contract is not fulfilled.

▶ Try it: run this — Circle implements both interface methods, so both can be called.

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(2);
        System.out.println(c.name());
        System.out.println(c.area());
    }
}

An interface contains...

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class Circle implements Shape means Circle must...

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Type exactly what this program prints.

interface Shape { String name(); }
class Circle implements Shape { public String name(){ return "Circle"; } }
public class Main { public static void main(String[] a){
    System.out.println(new Circle().name());
}}

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If a class implements an interface but omits one method, then...

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True or false?

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