Java枚举和切换语句 - 默认情况？ [英] Java Enums and Switch Statements - the default case?

问题描述

对于建议抛出异常的人：

抛出异常并不会给我一个编译时错误，它给我一个运行时错误。我知道我可以抛出一个异常，我宁愿在编译期间比运行时死亡。

For people suggesting throwing an exception:
Throwing an exception doesn't give me a compile-time error, it gives me a runtime error. I know I can throw an exception, I'd rather die during compilation than during runtime.

首先，我使用eclipse 3.4。

First-off, I am using eclipse 3.4.

我有一个模式属性为枚举的数据模型。

I have a data model that has a mode property that is an Enum.

enum Mode {on(...), off(...), standby(...); ...}

我正在写这个模型的视图，我有代码

I am currently writing a view of this model and I have the code

...
switch(model.getMode()) {
case on:
   return getOnColor();
case off:
   return getOffColor();
case standby:
   return getStandbyColor();
}
...

我收到错误此方法必须返回类型为java.awt.Color的结果，因为我没有默认情况，在函数结尾没有返回xxx。
我希望在有人向枚举添加其他类型（例如关闭）的情况下的编译错误，因此我不想放置抛出AssertionError的默认情况，因为这将使用修改的模式进行编译，直到运行时才会被视为错误。

I am getting an error "This method must return a result of type java.awt.Color" because I have no default case and no return xxx at the end of the function. I want a compilation error in the case where someone adds another type to the enum (e.g. shuttingdown) so I don't want to put a default case that throws an AssertionError, as this will compile with a modified Mode and not be seen as an error until runtime.

我的问题是这样的：

为什么EclipseBuilder（和javac）是否不认识到此开关涵盖所有可能性（或覆盖它们？），并停止警告我需要返回类型。有没有办法，我可以做我想要的，而不添加方式到模式？

My question is this:
Why does EclipseBuilder (and javac) not recognize that this switch covers all possibilities (or does it cover them?) and stop warning me about needing a return type. Is there a way I can do what I want without adding methods to Mode?

没有，是否有一个选项来警告/错误的switch语句不覆盖所有Enum的可能值？

Failing that, is there an option to warn/error on switch statements that don't cover all of the Enum's possible values?

编辑：
Rob：这是一个编译错误。我只是试图用javac编译它，并且我得到一个针对最后一个方法的缺少的返回语句错误。 Eclispe只是将错误放在方法的顶部。

Rob: It is a compile error. I just tried compiling it with javac and I get a "missing return statement" error targeting the last } of the method. Eclispe just places the error at the top of the method.

推荐答案

您可以随时使用枚举与访问者模式：

You could always use the Enum with Visitor pattern:

enum Mode {
  on {
      public <E> E accept( ModeVisitor<E> visitor ) {
         return visitor.visitOn();
      }
  },
  off {
      public <E> E accept( ModeVisitor<E> visitor ) {
         return visitor.visitOff();
      }
  },
  standby {
      public <E> E accept( ModeVisitor<E> visitor ) {
         return visitor.visitStandby();
      }
  }

  public abstract <E> E accept( ModeVisitor<E> visitor );

  public interface ModeVisitor<E> {
      E visitOn();
      E visitOff();
      E visitStandby();
  }
}

然后你会实现如下所示： p>

Then you would implement something like the following:

public final class ModeColorVisitor implements ModeVisitor<Color> {
    public Color visitOn() {
       return getOnColor();
    }

    public Color visitOff() {
       return getOffColor();
    }

    public Color visitStandby() {
       return getStandbyColor();
    }

}

您将使用如下：

return model.getMode().accept( new ModeColorVisitor() );

这是一个更详细的，但是如果一个新的枚举被声明，你会立即得到一个编译错误。

This is a lot more verbose but you'd immediately get a compile error if a new enum was declared.

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