有人可以推荐Java 8模式来代替switch语句吗? [英] Can somebody recommend a java 8 pattern to replace a switch statement?

问题描述

我有以下代码:

 public class A {
    private String type;
    String getType() { return type;}
 }

现在在很多代码位置，我都有这样的代码

Now in many code places I have code like this

 switch (a.geType()) {
  case "A" : return new Bla();
  case "B" : return new Cop();
 }

或其他地方

switch (a.geType()) {
  case "A" : return new Coda();
  case "B" : return new Man();
 }

(请注意，我知道我应该在生产代码中使用枚举).

(Note that I know I should use an Enumeration in production code).

我要实现的是，当将新类型添加到类A时，编译器应标记所有需要调整的switch语句?

What I want to achive is that when a new type is added to class A the compiler should flag all the switch statements that need to be adjusted?

是否有Java惯用的方式来做到这一点?

Is there a java idiomatic way to do this?

推荐答案

将新类型添加到class A时，编译器应标记所有需要调整的switch语句吗?

when a new type is added to class A the compiler should flag all the switch statements that need to be adjusted?

一种好的方法是将switch语句替换为 multiple dispatch 的更可靠实现，例如

A good approach to this would be replacing switch statements with a more robust implementation of multiple dispatch, such as the Visitor Pattern:

interface VisitorOfA {
    Object visitA(A a);
    Object visitB(B b);
}
class A {
    Object accept(VisitorOfA visitor) {
        return visitor.visitA(this);
    }
}
class B extends A {
    Object accept(VisitorOfA visitor) {
        return visitor.visitB(this);
    }
}

有了此基础架构，您可以删除您的switch语句，将其替换为访客的实现:

With this infrastructure in place, you can remove your switch statements, replacing them with implementations of the visitor:

Object res = a.accept(new VisitorOfA() {
    public Object visitA(A a) { return new Bla(); }
    public Object visitB(B b) { return new Cop(); }
});

在A中添加新的子类型(例如class C)时，您要做的就是在VisitorOfA中添加新方法:

When you add a new subtype to A, say, class C, all you need to do is adding a new method to VisitorOfA:

Object visitC(C c);

现在，编译器将发现所有尚未实现此新方法的地方，从而帮助您避免在运行时出现问题.

Now the compiler will spot all places where this new method has not been implemented, helping you avoid problems at runtime.

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