Using string representations of enum values in swi

问题:

Why is it not possible to use enum values as strings in a switch case? (Or what is wrong with this:)

String argument;
switch (argument) {
    case MyEnum.VALUE1.toString(): // Isn't this equal to "VALUE1" ?
    // something    
break;
    case MyEnum.VALUE2.toString():
    // something else
break;

回答1:

You can only use strings which are known at compile time. The compiler cannot determine the result of that expression.

Perhaps you can try

String argument = ...
switch(MyEnum.valueOf(argument)) {
   case VALUE1:

   case VALUE2:

回答2:

case MyEnum.VALUE1.toString(): // Isn't this equal to "VALUE1" ?

No, not necessarily: you are free to provide your own implementation of toString()

public enum MyType {
VALUE1 {
    public String toString() {
        return "this is my value one";
    }
},

VALUE2 {
    public String toString() {
        return "this is my value two";
    }
}

}

Moreover, someone who is maintaining your code could add this implementation after you leave the company. That is why you should not rely on String values, and stick to using numeric values (as represented by the constants MyEnum.VALUE1, MyEnum.VALUE2, etc.) of your enums instead.

回答3:

To add to the Peter Lawrey's comments, have a look at this post from last year which discusses Switching on String in Java before and after JDK7.

回答4:

EDIT: Apologies for a C# answer to a Java question. I don't know what went wrong there.

It is possible to use string values (including the string values of enums). However, you may only use compile-time constants. You are calling a method, ToString(), which needs to be evaluated at runtime.

As of C# 6, you can use this constant alternative: case nameof(SomeEnum.SomeValue):

Nameof() is evaluated at compile time, simply to a string that matches the (unqualified) name of the given variable, type, or member. Its value matches that of SomeEnum.ToString().