java 如何要求泛型参数是实现接口的枚举？
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How can I require a generic parameter to be an enum that implements an interface?
提问by Sbodd
I'm not 100% convinced that this is a good idea, but I bumped into some code today that's currently implemented as:
我不是 100% 相信这是一个好主意，但我今天遇到了一些目前实现的代码：
class MyWidget <T extends Enum<T> > {
MyWidget(Map<T, Integer> valueMap) {
mValueMap = valueMap;
}
Map<T, Integer> mValueMap;
}
where MyWidgetthen offers methods that use mValueMapto convert the passed-in Enumto/from an Integer.
where MyWidgetthen 提供mValueMap用于将传入的转换Enum为/从Integer.
What I was considering doing was trying to refactor this, so that I'd declare my enumeration:
我正在考虑做的是尝试重构它，以便我声明我的枚举：
interface MyInterface {
public Integer getValue();
}
enum MyEnum implements MyInterface {
foo, bar;
public Integer getValue() {
return ordinal();
}
}
And I'd then be able to rewrite MyWidgetinto something that looked vaguely like this:
然后我就可以改写MyWidget成看起来像这样的东西：
public class MyWidget<T extends Enum<T> extends MyInterface> {
...
}
and would then be able to call the getValue()method from MyInterfaceon T-type objects within MyWidget. The problem, of course, is that "<T extends Enum<T> extends MyInterface>" isn't valid syntax. Is there any way to pull this off?
然后将能够调用getValue()从方法MyInterface上T型内不提出异议MyWidget。当然，问题在于“ <T extends Enum<T> extends MyInterface>” 不是有效的语法。有什么办法可以解决这个问题吗？
I don't want to just have MyWidget<T extends MyInterface>, because it's also important that T be an enumeration.
我不想只拥有MyWidget<T extends MyInterface>，因为 T 是枚举也很重要。
Thanks in advance!
提前致谢！
回答by Michael Myers
Use an '&' instead:
改用“ &”：
public class MyWidget<T extends Enum<T> & MyInterface> {
...
}
The JLScalls this an "intersection type", but I can find no mention of it in the Java tutorials. I'll just say that it does exactly what you were wishing that "extends" would do.
该JLS称这是“交集型”，但我能找到在Java教程没有提到它。我只想说它完全符合您希望“ extends”会做的事情。
Also, I should mention that you can have as many types as you want in the intersection type. So if you wanted, you could do:
另外，我应该提到，您可以在交集类型中拥有任意数量的类型。所以如果你愿意，你可以这样做：
public class MyWidget<T extends Enum<T> & MyInterface & Serializable & Cloneable> {
...
}
[Note: this code sample should not be construed as an endorsement of the Cloneableinterface; it was merely handy at the time.]
[注意：此代码示例不应被解释为对Cloneable接口的认可；它当时只是方便。]
回答by Alex Miller
The JSR 203 (new new IO) stuff for JDK 7 is making a lot of use of enums that implement interfaces (for example: http://openjdk.java.net/projects/nio/javadoc/java/nio/file/FileVisitOption.html) to allow them some wiggle room in the future for future additional sets of enum options. So that is a feasible approach and obviously one that was chosen after a lot of thought in one large Sun project.
JDK 7 的 JSR 203（新的新 IO）内容大量使用了实现接口的枚举（例如：http: //openjdk.java.net/projects/nio/javadoc/java/nio/file/FileVisitOption .html) 以允许他们在将来为将来的其他枚举选项集留出一些回旋余地。所以这是一种可行的方法，显然是在一个大型 Sun 项目中经过深思熟虑后选择的。
