# 实现接口

> 原文： [https://docs.oracle.com/javase/tutorial/java/IandI/usinginterface.html](https://docs.oracle.com/javase/tutorial/java/IandI/usinginterface.html)

要声明实现接口的类，请在类声明中包含`implements`子句。您的类可以实现多个接口，因此`implements`关键字后跟一个逗号分隔的类实现的接口列表。按照惯例，`implements`子句遵循`extends`子句，如果有的话。

## 样本接口，可重复

考虑一个定义如何比较对象大小的接口。

```
public interface Relatable {

    // this (object calling isLargerThan)
    // and other must be instances of 
    // the same class returns 1, 0, -1 
    // if this is greater than, 
    // equal to, or less than other
    public int isLargerThan(Relatable other);
}

```

如果您希望能够比较类似对象的大小，无论它们是什么，实例化它们的类都应该实现`Relatable`。

如果有某种方法来比较从类中实例化的对象的相对“大小”，任何类都可以实现`Relatable`。对于字符串，它可以是字符数;对于书籍，它可以是页数;对于学生来说，它可能是重量;等等。对于平面几何对象，区域将是一个不错的选择（参见后面的`RectanglePlus`类），而体积适用于三维几何对象。所有这些类都可以实现`isLargerThan()`方法。

如果您知道某个类实现了`Relatable`，那么您就知道可以比较从该类实例化的对象的大小。

## 实现可复制接口

这是[创建对象](../javaOO/objectcreation.html)部分中提供的`Rectangle`类，为实现`Relatable`而重写。

```
public class RectanglePlus 
    implements Relatable {
    public int width = 0;
    public int height = 0;
    public Point origin;

    // four constructors
    public RectanglePlus() {
        origin = new Point(0, 0);
    }
    public RectanglePlus(Point p) {
        origin = p;
    }
    public RectanglePlus(int w, int h) {
        origin = new Point(0, 0);
        width = w;
        height = h;
    }
    public RectanglePlus(Point p, int w, int h) {
        origin = p;
        width = w;
        height = h;
    }

    // a method for moving the rectangle
    public void move(int x, int y) {
        origin.x = x;
        origin.y = y;
    }

    // a method for computing
    // the area of the rectangle
    public int getArea() {
        return width * height;
    }

    // a method required to implement
    // the Relatable interface
    public int isLargerThan(Relatable other) {
        RectanglePlus otherRect 
            = (RectanglePlus)other;
        if (this.getArea() < otherRect.getArea())
            return -1;
        else if (this.getArea() > otherRect.getArea())
            return 1;
        else
            return 0;               
    }
}

```

因为`RectanglePlus`实现`Relatable`，所以可以比较任何两个`RectanglePlus`对象的大小。

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**Note:** The `isLargerThan` method, as defined in the `Relatable` interface, takes an object of type `Relatable`. The line of code, shown in bold in the previous example, casts `other` to a `RectanglePlus` instance. Type casting tells the compiler what the object really is. Invoking `getArea` directly on the `other` instance (`other.getArea()`) would fail to compile because the compiler does not understand that `other` is actually an instance of `RectanglePlus`.

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