当有现有类时，接口如何取代多重继承的需要 [英] How can interfaces replace the need for multiple inheritance when have existing classes

问题描述

首先......对不起这篇文章。我知道stackoverflow上有很多帖子正在讨论多重继承。但我已经知道Java不支持多重继承，我知道使用接口应该是另一种选择。但我不明白并且看到了我的困境：

First of all... Sorry for this post. I know that there are many many posts on stackoverflow which are discussing multiple inheritance. But I already know that Java does not support multiple inheritance and I know that using interfaces should be an alternative. But I don't get it and see my dilemma:

我必须对用Java编写的非常大而复杂的工具进行更改。在此工具中，有一个数据结构，它由许多不同的类对象构建，并具有链接的成员层次结构。无论如何......

I have to make changes on a very very large and complex tool written in Java. In this tool there is a data structure built with many different class objects with a linked member hierarchy. Anyway...

• 我有一个类标记，它有多个方法并返回一个对象标签，取决于对象的类。它需要成员和静态变量。


• 第二个名为 XMLElement 的类允许链接对象，最后生成XML文件。我还需要成员和静态变量。


• 最后，我有这么多的数据类几乎都应该扩展 XMLElement 和一些他们标记。

• I have one class Tagged which has multiple methods and returns an object tag depending on the object's class. It needs members and static variables.
• And a second class called XMLElement allows to link objects and in the end generate a XML file. I also need member and static variables here.
• Finally, I have these many many data classes which nearly all should extend XMLElement and some of them Tagged.

好的，这不行，因为它是只能扩展一个类。我经常读到Java的所有内容都没问题，也没有必要进行多重继承。我相信，但我不知道接口应如何取代继承。

Ok ok, this won't work since it's only possible to extend just one class. I read very often that everything with Java is ok and there is no need for having multiple inheritance. I believe, but I don't see how an interface should replace inheritance.

1. 将实际实现放在所有数据类中是没有意义的，因为它每次都是相同的但是这对接口来说是必要的（我认为）。


2. 我看不出如何将一个继承类更改为接口。我在这里有变量，它们必须完全在那里。

我真的不明白所以请有人解释我怎么样处理这个？

I really don't get it so please can somebody explain me how to handle this?

推荐答案

你可能更喜欢组合（和委托）而不是继承：

You should probably favor composition (and delegation) over inheritance :

public interface TaggedInterface {
    void foo();
}

public interface XMLElementInterface {
    void bar();
}

public class Tagged implements TaggedInterface {
    // ...
}

public class XMLElement implements XMLElementInterface {
    // ...
}

public class TaggedXmlElement implements TaggedInterface, XMLElementInterface {
    private TaggedInterface tagged;
    private XMLElementInterface xmlElement;

    public TaggedXmlElement(TaggedInterface tagged, XMLElementInterface xmlElement) {
        this.tagged = tagged;
        this.xmlElement = xmlElement;
    }

    public void foo() {
        this.tagged.foo();
    }

    public void bar() {
        this.xmlElement.bar();
    }

    public static void main(String[] args) {
        TaggedXmlElement t = new TaggedXmlElement(new Tagged(), new XMLElement());
        t.foo();
        t.bar();
    }
}

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