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

问题描述

我想编写一个抽象方法，但编译器会持续发出此错误：

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

抽象方法不能有正文

abstract methods cannot have a body

我有这样的方法：

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

这里有什么问题？

推荐答案

抽象方法意味着它没有默认实现，实现类将提供详细信息。

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

基本上，你会有

class AbstractObject {
   public abstract void method();
}

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

所以，它正如错误所述：你的摘要方法不能有一个正文。

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

有一个关于Oracle网站的完整教程： 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

你会做这样的事情的原因是，多个对象可以分享某些行为，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:

你可以拥有一个通用的图形对象，它知道如何重新定位自己，但是实现类实际上会自己绘制。

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

（这取自我上面链接的网站）

(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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