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问题描述

我之前了解到抽象类可以扩展具体类。虽然我没有从JAVA设计师那里看到它的原因,但它没问题。
我还了解到扩展具体类的抽象类可以使重写方法成为抽象。为什么?你能提供有用的用例吗?我正在努力学习设计模式,我不想错过任何东西。

I earlier learned that abstract class can extend concrete class. Though I don't see the reason for it from JAVA designers, but it is ok.I also learned that abstract class that extends concrete class can make overriden methods abstract. Why? Can you provide with use case where it is useful? I am trying to learn design patterns and I do not want to miss anything.

以下是示例:

public class Foo
{
    public void test()
    {
    }
}

public abstract class Bar extends Foo
{
   @Override
   public abstract void test();
}


推荐答案

如果我有我希望默认实现 test()的一组类(因此它们可以从 Foo 扩展),以及我想强制提供自己的实现的那些类的子集(在这种情况下,在子类中将其抽象化将强制执行此操作。)

This becomes useful if I have a set of classes that I want a default implementation of test() for (so they can extend from Foo), and a subset of those classes that I want to force to provide their own implementation (in which case making it abstract in the subclass would enforce this.)

当然,在这个例子中的替代方法是在顶级类而不是在子类中声明 test() abstract,这就是你通常所做的 - 但是有些情况满足is-a继承关系的地方意味着从设计的角度来看它偶尔会更有意义。这种情况很少见,但有时你会看到它。

Of course, the alternative way in this example would be to declare test() abstract in the top level class rather than in the subclass, and that's what you'd usually do - but there are cases where satisfying the is-a relationship of inheritance means that it occasionally makes more sense from a design perspective doing it this way around. It's rare, but you do sometimes see it.

顺便说一下,虽然是特殊情况,但请记住所有类都隐式扩展 Object 除非另有说明。因此,如果你包含这种情况,那么扩展具体类的抽象类毕竟不是那么罕见!

As an aside, though a special case, remember that all classes implicitly extend Object unless otherwise specified. So if you include this case, abstract classes extending concrete classes isn't so unusual after all!

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08-04 07:07
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