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

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8.1 Interfaces — the interface as a type

Lesson

Polymorphism through a contract

An interface is a type: you can declare Shape s = new Circle(2);, build a Shape[] holding any implementers, and write methods that take a Shape parameter. Calling s.area() runs the actual object's version — the same dynamic dispatch as with a superclass, but the objects need share nothing except the contract.

Worked example

Shape[] shapes = { new Circle(1), new Rect(2, 3) };
for (int i = 0; i < shapes.length; i++) {
    System.out.println(shapes[i].name() + ": " + shapes[i].area());
}
// Circle: 3.14
// Rect: 6.0

One loop, one contract, two very different area formulas.

▶ Try it: run this — a Shape[] holds both classes and each answers with its own area.

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) {
        Shape[] shapes = { new Circle(1), new Rect(2, 3) };
        for (int i = 0; i < shapes.length; i++) {
            System.out.println(shapes[i].name() + ": " + shapes[i].area());
        }
    }
}

Can you declare a variable of an interface type?

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For Shape s = new Circle(2);, s.area() runs...

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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){
    Shape[] arr = { new Sq(), new Tri() };
    for (int i = 0; i < arr.length; i++) System.out.println(arr[i].area());
}}

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A method with a Shape parameter accepts...

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True or false?

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