此更改是否改进了我关于 Demeter 法则的设计？

Question

假设我需要为第 3 方编写包装器 class，我无法更改。

class的界面是这样的

class Rewriter {
    public List<Mapping> getMappings();
}

包装器看起来像这样

class RewriterSpec {
    private final Rewriter rewriter;

    public RewriterSpec(Rewriter rewriter) {
        this.rewriter = rewriter;
    }

    public addMapping(Mapping m) {
        rewriter.getMappings().add(m);
    }
}

所以根据我的理解，RewriterSpec 违反了 Demeter 法则，因为它需要有关重写器的结构知识。

现在的问题是，从设计/测试的角度来看，只传递映射列表会更好吗？

class Rewriter {
    public List<Mapping> getMappings();
}

包装器看起来像这样

class RewriterSpec {
    private final Rewriter rewriter;

    public RewriterSpec(List<Mapping> mappings) {
        this.mappings = mappings;
    }

    public addMapping(Mapping m) {
        mappings.add(m);
    }
}

只通过引用传递列表可以吗？

Answer 1

维基百科声明如下：

https://en.wikipedia.org/wiki/Law_of_Demeter

More formally, the Law of Demeter for functions requires that a method m of an object O may only invoke the methods of the following kinds of objects:[2]

O itself

m's parameters

Any objects created/instantiated within m

O's direct component objects

A global variable, accessible by O, in the scope of m

遵循这些原则，你可以在Rewriter接口中定义一个方法，直接将Mapping对象添加到它的列表中，满足原则（一个全局变量，O可访问，在m的范围内）。

class RewriterSpec {
    private final Rewriter rewriter;

    public RewriterSpec(Rewriter rewriter) {
        this.rewriter = rewriter;
    }

    public addMapping(Mapping m) {
        rewriter.addMapping(m);
    }
}

考虑到 Rewriter 无法修改，您可以选择以下选项：

class RewriterSpec {
    private final Rewriter rewriter;
    private final List<Mapping> mappings;

    public RewriterSpec(Rewriter rewriter) {
        this.rewriter = rewriter;
        this.mappings = rewriter.getMappings();
    }

    public addMapping(Mapping m) {
        mappings.addMapping(m);
    }
}

此更改是否改进了我关于 Demeter 法则的设计？

Does this change improve my design with regards to the Law Of Demeter?

java

law-of-demeter