Practice›Java Fundamentals›Module 8. Interfaces & Abstract Classes›8.4 Choosing

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8.4 Choosing — implementing several interfaces

Lesson

Many capabilities, one class

Because interfaces carry no state, a class can implement several at once — just list them, separated by commas. The class must then provide every method of every interface. Each interface is a separate type, so the same object can be used as any of them.

Worked example

class Tag implements Printable, Sized {
    public String label() { return "<" + text + ">"; }   // from Printable
    public int size() { return text.length(); }         // from Sized
}

Tag t = new Tag("hello");
Printable p = t;   // a Tag is a Printable
System.out.println(p.label());   // <hello>

The same Tag object can be seen as a Printable, a Sized, or a Tag.

▶ Try it: run this — Tag fulfills both contracts and can be referenced through either.

interface Printable {
    String label();
}
interface Sized {
    int size();
}
abstract class Shape {
    private String name;
    public Shape(String name) { this.name = name; }
    public String getName() { return name; }
    public abstract double area();
}
class Rect extends Shape implements Printable {
    private double w, h;
    public Rect(double w, double h) { super("Rect"); this.w = w; this.h = h; }
    public double area() { return w * h; }
    public String label() { return "[" + getName() + "]"; }
}
class Tag implements Printable, Sized {
    private String text;
    public Tag(String text) { this.text = text; }
    public String label() { return "<" + text + ">"; }
    public int size() { return text.length(); }
}
public class Main {
    public static void main(String[] args) {
        Tag t = new Tag("hello");
        System.out.println(t.label());
        System.out.println(t.size());
        Printable p = t;
        System.out.println(p.label());
    }
}

To implement two interfaces, you write...

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A class implementing two interfaces must provide...

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Type exactly what this program prints.

interface Printable { String label(); }
interface Sized { int size(); }
class Tag implements Printable, Sized {
    public String label(){ return "tag"; }
    public int size(){ return 3; }
}
public class Main { public static void main(String[] a){
    Tag t = new Tag();
    System.out.println(t.label());
    System.out.println(t.size());
}}

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If Tag implements Printable, then Printable p = new Tag(...) is...

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True or false?

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