blocks|key|340184|text|创建抛出异常的默认情况：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|340185|throw+new+RuntimeExeption("this+code+should+never+be+hit+unless+someone+updated+the+enum")+|code-block|syntax|javascript|340186|..。这很好地解释了Eclipse抱怨的原因:虽然您的切换今天可能涵盖所有枚举用例，但可能有人添加了一个用例，明天就不需要重新编译了。|340187|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Create a default case that throws an exception:

<pre><code>throw new RuntimeExeption("this code should never be hit unless someone updated the enum") 
</code></pre>

... and that pretty much describes why Eclipse is complaining: while your switch may cover all enum cases today, someone could add a case and not recompile tomorrow.

blocks|key|345097|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|345098|为什么EclipseBuilder没有意识到此开关涵盖了所有可能性(或者它涵盖了所有可能性？)不要再警告我需要一个返回类型。有没有办法在不向Mode添加方法的情况下做我想做的事情？|blockquote|345099|345100|345101|这不是Eclipse中的问题，而是编译器javac的问题。javac所看到的是在没有匹配的情况下没有返回值(即使您知道匹配的是所有的情况，这一事实也是不相关的)。在默认情况下你必须返回一些东西(或者抛出一个异常)。|offset|length|style|CODE|345102|就我个人而言，我只会抛出某种异常。|345103|entityMap^0|0|0|0|0|K|5|T|5|0|0^^$0|@$1|2|3|-4|4|5|6|P|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|Q|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|R|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|S|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|T|7|@$H|U|I|V|J|K]|$H|W|I|X|J|K]]|8|@]|9|$]]|$1|L|3|M|4|5|6|Y|7|@]|8|@]|9|$]]|$1|N|3|-4|4|5|6|Z|7|@]|8|@]|9|$]]]|O|$]]

<blockquote>
 Why does EclipseBuilder 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?
</blockquote>

It's not an issue in Eclipse, but rather the compiler, <code>javac</code>. All <code>javac</code> sees is that you don't have a return value in the case in which nothing is matched (the fact that you know you are matching all cases is irrelevant). You have to return something in the default case (or throw an exception).

Personally, I'd just throw some sort of exception.

blocks|key|3326012|text|我想说这可能是因为model.GetMode()+可以返回null。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3326013|entityMap^0|P|2|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

I'd say it's probably because model.GetMode() could return null.

blocks|key|342568|text|既然我不能评论..。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|342569|342570|总是，总是，总是有一个默认的大小写。你会惊讶于它被“频繁地”命中的频率(在Java中比在C中少，但仍然如此)。|ordered-list-item|342571|已经说过了，如果我只想在我的情况下只处理开/关该怎么办？由javac进行的语义处理会将其标记为问题。|342572|342573|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|L|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|M|8|@]|9|@]|A|$]]|$1|F|3|G|5|E|7|N|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|J|$]]

Since I can't just comment...

<ol>
<li>Always, Always, Always have a default case. You'd be surprised how "frequently" it would be hit (Less in Java than C, but still). </li>
<li>Having said that, what if I only want to handle only on/off in my case. Your semantic processing by javac would flag that as an issue.</li>
</ol>

blocks|key|345153|text|您的问题是您试图使用switch语句作为枚举已锁定的指示器。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|345154|事实是，'switch‘语句和java编译器无法识别您不想在枚举中允许其他选项。在枚举中只需要三个选项的事实与您的switch语句的设计是完全不同的，正如其他人所指出的，switch语句应该始终具有默认语句。(在您的情况下，它应该抛出一个异常，因为它是一个未处理的场景。)|345155|您应该在枚举中自由地散布注释，这样每个人都知道不要接触它，并且应该修复switch语句，以便在无法识别的情况下抛出错误。这样你就覆盖了所有的基础。|345156|编辑|offset|length|style|BOLD|345157|关于抛出编译器错误的问题。严格来说，这并不合理。您有一个具有三个选项的枚举和一个具有三个选项的开关。如果有人向枚举添加值，您希望它抛出编译器错误。但是枚举可以是任意大小的，所以如果有人更改了编译器错误，抛出它是没有意义的。此外，您正在基于switch语句定义枚举的大小，该语句可能位于完全不同的类中。|345158|Enum和Switch的内部工作方式是完全分开的，应该保持不耦合。|345159|entityMap^0|0|0|0|0|2|0|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|U|8|@$H|V|I|W|J|K]]|9|@]|A|$]]|$1|L|3|M|5|6|7|X|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|Y|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|Q|$]]

Your problem is that you are trying to use the switch statement as an indicator that your enum is locked down.

The fact is that the 'switch' statement and the java compiler cannot recognize that you do not want to allow other options in your enum. The fact that you only want three options in your enum is completely separate from your design of the switch statement, which as noted by others should ALWAYS have a default statement. (In your case it should throw an exception, because it is an unhandled scenario.) 

You should liberally sprinkle your enum with comments so that everyone knows not to touch it, and you should fix your switch statement to throw errors for unrecognized cases.
That way you've covered all the bases.

EDIT

On the matter of throwing compiler error. That does not strictly make sense. You have an enum with three options, and a switch with three options. You want it to throw a compiler error if someone adds a value to the enum. Except that enums can be of any size, so it doesn't make sense to throw a compiler error if someone changes it. Furthermore, you are defining the size of your enum based on a switch statement which could be located in a completely different class. 

The internal workings of Enum and Switch are completely separate and should remain uncoupled.

blocks|key|3113169|text|我不知道为什么会出现这个错误，但这里有一个建议，为什么不在枚举本身中定义颜色呢？这样你就不会意外忘记定义新的颜色了。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3113170|例如：|3113171|import+java.awt.Color;

public+class+Test+{

++++enum+Mode+
++++{
++++++++on+(Color.BLACK),+
++++++++off+(Color.RED),
++++++++standby+(Color.GREEN);

++++++++private+final+Color+color;+
++++++++Mode+(Color+aColor)+{+color+=+aColor;+}
++++++++Color+getColor()+{+return+color;+}
++++}

++++class+Model
++++{
++++++++private+Mode+mode;
++++++++public+Mode+getMode+()+{+return+mode;+}
++++}

++++private+Model+model;

++++public+Color+getColor()
++++{
++++++++return+model.getMode().getColor();
++++}+++
}|code-block|syntax|javascript|3113172|顺便说一句，为了比较，这里是原始的情况，有编译器错误。|3113173|import+java.awt.Color;
public+class+Test+{

++++enum+Mode+{on,+off,+standby;}

++++class+Model
++++{
++++++++private+Mode+mode;
++++++++public+Mode+getMode+()+{+return+mode;+}
++++}

++++private+Model+model;

++++public+Color+getColor()
++++{
++++++++switch(model.getMode())+{
++++++++case+on:
+++++++++++return+Color.BLACK;
++++++++case+off:
+++++++++++return+Color.RED;
++++++++case+standby:
+++++++++++return+Color.GREEN;
++++++++}
++++}+++
}|3113174|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I don't know why you get this error, but here is a suggestion, Why don't you define the color in the enum itself? Then you can't accidentally forget to define a new color.

For example:

<pre><code>import java.awt.Color;

public class Test {

 enum Mode 
 {
 on (Color.BLACK), 
 off (Color.RED),
 standby (Color.GREEN);

 private final Color color; 
 Mode (Color aColor) { color = aColor; }
 Color getColor() { return color; }
 }

 class Model
 {
 private Mode mode;
 public Mode getMode () { return mode; }
 }

 private Model model;

 public Color getColor()
 {
 return model.getMode().getColor();
 } 
}
</code></pre>

btw, for comparison here is the original case, with compiler error. 

<pre><code>import java.awt.Color;
public class Test {

 enum Mode {on, off, standby;}

 class Model
 {
 private Mode mode;
 public Mode getMode () { return mode; }
 }

 private Model model;

 public Color getColor()
 {
 switch(model.getMode()) {
 case on:
 return Color.BLACK;
 case off:
 return Color.RED;
 case standby:
 return Color.GREEN;
 }
 } 
}
</code></pre>

blocks|key|3326125|text|您必须在Eclipse+(window+->首选项)中启用“枚举类型常量未包含在开关中”的设置，并设置错误级别。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3326126|在方法的末尾抛出一个异常，但不要使用默认大小写。|3326127|public+String+method(Foo+foo)
++switch(foo)+{
++case+x:+return+"x";
++case+y:+return+"y";
++}

++throw+new+IllegalArgumentException();
}|code-block|syntax|javascript|3326128|现在，如果有人后来添加了新的案例，Eclipse会让他知道他遗漏了一个案例。所以不要使用默认，除非你有很好的理由这样做。|3326129|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

You have to enable in Eclipse (window -> preferences) settings "Enum type constant not covered in switch" with Error level.

Throw an exception at the end of the method, but don't use default case.

<pre><code>public String method(Foo foo)
 switch(foo) {
 case x: return "x";
 case y: return "y";
 }

 throw new IllegalArgumentException();
}
</code></pre>

Now if someone adds new case later, Eclipse will make him know he's missing a case. So don't ever use default unless you have really good reasons to do so.

blocks|key|342658|text|您可以始终使用带有访问者的枚举模式：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|342659|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();
++}
}|code-block|syntax|javascript|342660|然后，您将实现类似于以下内容：|342661|public+final+class+ModeColorVisitor+implements+ModeVisitor<Color>+{
++++public+Color+visitOn()+{
+++++++return+getOnColor();
++++}

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

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

}|342662|您可以按如下方式使用它：|342663|return+model.getMode().accept(+new+ModeColorVisitor()+);|342664|这要冗长得多，但是如果声明了一个新的枚举，就会立即得到一个编译错误。|342665|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

You could always use the Enum with Visitor pattern:

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

 public abstract &lt;E&gt; E accept( ModeVisitor&lt;E&gt; visitor );

 public interface ModeVisitor&lt;E&gt; {
 E visitOn();
 E visitOff();
 E visitStandby();
 }
}
</code></pre>

Then you would implement something like the following:

<pre><code>public final class ModeColorVisitor implements ModeVisitor&lt;Color&gt; {
 public Color visitOn() {
 return getOnColor();
 }

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

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

}
</code></pre>

You'd use it as follows:

<pre><code>return model.getMode().accept( new ModeColorVisitor() );
</code></pre>

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

blocks|key|345202|text|这样做的一个很好的方法是添加默认用例，以返回一些错误值或抛出异常，并对jUnit使用自动化测试，例如：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|345203|@Test
public+void+testEnum()+{
++for(Mode+m+:+Mode.values()+{
++++m.foobar();+//+The+switch+is+separated+to+a+method
++++//+If+you+want+to+check+the+return+value,+do+it+(or+if+there's+an+exception+in+the+
++++//+default+part,+that's+enough)
++}
}|code-block|syntax|javascript|345204|当您获得自动化测试时，这将处理为所有枚举定义的foobar。|345205|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

A nice way for this would be to add the default case to return some error value or throw exception and to use automated tests with jUnit, for example:

<pre><code>@Test
public void testEnum() {
 for(Mode m : Mode.values() {
 m.foobar(); // The switch is separated to a method
 // If you want to check the return value, do it (or if there's an exception in the 
 // default part, that's enough)
 }
}
</code></pre>

When you got automated tests, this will take care of that foobar is defined for all enumerations.

blocks|key|3113324|text|现在(这个答案是在最初的问题几年后写的)，eclipse允许在Window+->+Preferences+->+Java+->编译器->+Error/warnings+->潜在的编程问题中进行以下配置：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3113325|即使存在默认情况，|3113326|也不完全切换情况信号|blockquote|3113327|3113328|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|L|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

Nowadays (this answer is written several years after the original question), eclipse allows following configuration at Window -> Preferences -> Java -> Compiler -> Error/warnings -> Potential programming problems:

<blockquote>
 Incomplete switch cases 
 
 <blockquote>
 Signal even if default case exists
 </blockquote>
</blockquote>

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.

First-off, I am using eclipse 3.4.

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

<pre><code>enum Mode {on(...), off(...), standby(...); ...}
</code></pre>

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

<pre><code>...
switch(model.getMode()) {
case on:
 return getOnColor();
case off:
 return getOffColor();
case standby:
 return getStandbyColor();
}
...
</code></pre>

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.

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?

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

Edit: 
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.

Java Enums and Switch Statements - the default case?

编译器

Eclipse

Java

对于建议抛出异常的人：抛出异常不会给我带来编译时错误，它会给我带来运行时错误。我知道我可以抛出异常，我宁愿在编译期间死掉，也不愿在运行时死掉。首先，我使用的是eclipse 3.4。我有一个数据模型，它的mode属性是一个Enum。enum Mode {on(...), off(...), standby(...); ...

问Java Enums和Switch语句-默认情况？
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