如何设计扩展 [英] Howto design for extension

问题描述

Checkstyle 规则 DesignForExtension 。它说：如果你有一个公共/受保护的方法，它不是抽象的，也不是最终的也不是空的，它不是为扩展而设计的。在Checkstyle页面上阅读此规则的说明了解原理。

There is a Checkstyle rule DesignForExtension. It says: if you have a public/protected method which is not abstract nor final nor empty it is not "designed for extension". Read the description for this rule on the Checkstyle page for the rationale.

想象一下这种情况。我有一个抽象类，它定义了一些字段和这些字段的验证方法：

Imagine this case. I have an abstract class which defines some fields and a validate method for those fields:

public abstract class Plant {
    private String roots;
    private String trunk;

    // setters go here

    protected void validate() {
        if (roots == null) throw new IllegalArgumentException("No roots!");
        if (trunk == null) throw new IllegalArgumentException("No trunk!");
    }

    public abstract void grow();
}

我还有Plant的子类：

I have also a subclass of Plant:

public class Tree extends Plant {
    private List<String> leaves;

    // setters go here

    @Overrides
    protected void validate() {
        super.validate();
        if (leaves == null) throw new IllegalArgumentException("No leaves!");
    }

    public void grow() {
        validate();
        // grow process
    }
}

继Checkstyle之后规则Plant.validate（）方法不是为扩展而设计的。但是在这种情况下如何设计扩展？

Following the Checkstyle rule the Plant.validate() method is not designed for extension. But how do I design for extension in this case?

推荐答案

该规则正在抱怨，因为派生（扩展）类可以在不告诉您的情况下完全替换您提供的功能关于它。这是一个强有力的迹象，表明您还没有充分考虑如何扩展类型。你想要做的是这样的事情：

The rule is complaining because it is possible for a deriving (extending) class to completely replace the functionality you provided without telling you about it. It's a strong indication that you haven't fully considered how the type might be extended. What it wants you to do instead is something like this:

public abstract class Plant {
    private String roots;
    private String trunk;

    // setters go here

    private void validate() {
        if (roots == null) throw new IllegalArgumentException("No roots!");
        if (trunk == null) throw new IllegalArgumentException("No trunk!");
        validateEx();
    }

    protected void validateEx() { }

    public abstract void grow();
}

请注意，现在有人仍然可以提供自己的验证码，但他们可以'替换您预先编写的代码。根据您打算如何使用验证方法，您也可以将其设为公开最终版。

Note that now someone can still supply their own validation code, but they can't replace your pre-written code. Depending on how you meant to use the validate method you could also make it public final instead.

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