Java：如何在其类型定义中修改当前枚举的值？
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Java: How to modify value of current enum inside of its type definition?
提问by zer0stimulus
I defined an enum type that implements an interface as follows:
我定义了一个实现接口的枚举类型，如下所示：
public enum MyEnum implements MyInterface
{
val1, val2, val3;
private MyEnum() {}
private MyEnum(Parcel in)
{
readFromParcel(in);
}
public void readFromParcel(Parcel in)
{
MyEnum val = MyEnum.values()[in.readInt()];
// ??? How to I assign val to my current enum?
}
}
How do I access the value of the current enum object so I can make the assignment inside of readFromParcel()? (Please see comment in code)
如何访问当前枚举对象的值，以便我可以在其中进行赋值readFromParcel()？（请参阅代码中的注释）
采纳答案by polygenelubricants
Inside an instance method, you can refer to the "current enumobject" as simply this. It works this way because enumconstants are actual objects, i.e. instances of a class-- a very special type of class, but a classnonetheless. (Note that I mentioned that these are enumconstants: it is the convention in Java to use all uppercase letters when naming constants.)
在实例方法中，您可以enum简单地将“当前对象”称为this. 它以这种方式工作，因为enum常量是实际对象，即 a 的实例class—— 一种非常特殊的 类型class，但class仍然是 a 。（请注意，我提到这些是enum常量：Java 中的约定是在命名常量时使用所有大写字母。）
Your usage of values()is also very peculiar (not to mention that it'll perform horribly since a new array must be constructed at each call). Perhaps you'd want to take a look at EnumMap, which is a special kind of Mapoptimized for enumconstants as keys.
您的使用values()也非常奇特（更不用说它的性能会很糟糕，因为每次调用都必须构造一个新数组）。也许您想看看EnumMap，这是一种Map针对enum常量作为键进行了优化的特殊类型。
If you're trying to mutate fields contained in these enumconstants, then you should seriously consider a redesign. You should generally minimize mutability anyway, but having these staticsingletons be mutable does not sound like a good design. Instead of having these mutable fields intrinsic within the enumconstants themselves, a Mapfrom the constants to these mutable values would be a much better design.
如果您试图改变包含在这些enum常量中的字段，那么您应该认真考虑重新设计。无论如何，您通常应该尽量减少可变性，但是让这些static单例可变听起来并不是一个好的设计。与其将这些可变字段固有地包含在enum常量本身中，Map不如从常量到这些可变值是一个更好的设计。
See also
也可以看看
- Java Tutorials/
enum - Effective Java 2nd Edition
- Java教程/
enum - 有效的 Java 第二版
Various questions on Java enum
关于Java的各种问题 enum
回答by Tom Hawtin - tackline
It's really not a good idea to make enums mutable. However, the thisfor an enumis just this.
使枚举可变真的不是一个好主意。但是，thisfor anenum只是this。
MyEnum val = this;
The actual fields val1, val2and val3(should be VAL1, VAL2and VAL3) are implicitly public static final.
实际的字段val1，val2以及val3（应该是VAL1，VAL2和VAL3）是含蓄public static final。
回答by Devon_C_Miller
Enums are immutable, so you cannot. Your best be is to make readFromParcelstatic and have it return an enum. So, something like this:
枚举是不可变的，所以你不能。你最好的办法是制作readFromParcel静态并让它返回一个枚举。所以，像这样：
public static MyEnum readFromParcel(Parcel in)
{
MyEnum val = MyEnum.values()[in.readInt()];
return val;
}
回答by Adrian Regan
Use this.
使用这个。
if this.equals(val) {
...
}
