Java中的抽象方法 [英] Abstract methods in Java

问题描述

我想写一个抽象方法，但是编译器一直报错:

I want to write an abstract method but the compiler persistently gives this error:

抽象方法不能有主体

我有一个这样的方法:

public abstract boolean isChanged() {
    return smt else...
}

这里出了什么问题?

推荐答案

抽象方法意味着它没有默认实现，一个实现类将提供细节.

Abstract methods means there is no default implementation for it and an implementing class will provide the details.

本质上，你会有

abstract class AbstractObject {
   public abstract void method();
}

class ImplementingObject extends AbstractObject {
  public void method() {
    doSomething();
  }
}

因此，正如错误所述:您的抽象方法不能有主体.

So, it's exactly as the error states: your abstract method can not have a body.

Oracle 网站上有完整的教程:http://download.oracle.com/javase/tutorial/java/IandI/abstract.html

There's a full tutorial on Oracle's site at: http://download.oracle.com/javase/tutorial/java/IandI/abstract.html

您会这样做的原因是多个对象可以共享某些行为，但不是所有行为.

The reason you would do something like this is if multiple objects can share some behavior, but not all behavior.

一个非常简单的例子是形状:

A very simple example would be shapes:

您可以拥有一个通用图形对象，它知道如何重新定位自身，但实现类实际上会绘制自己.

You can have a generic graphic object, which knows how to reposition itself, but the implementing classes will actually draw themselves.

(这是从我上面链接的网站上截取的)

(This is taken from the site I linked above)

abstract class GraphicObject {
    int x, y;
    ...
    void moveTo(int newX, int newY) {
        ...
    }
    abstract void draw();
    abstract void resize();
}

class Circle extends GraphicObject {
    void draw() {
        ...
    }
    void resize() {
        ...
    }
}
class Rectangle extends GraphicObject {
    void draw() {
        ...
    }
    void resize() {
        ...
    }
}

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