枚举，静态方法和实现接口方法的用途是什么？

Question

I am having some small doubts regarding Enums . I have the following confusions :-

1. What is the use of static methods in Enums ? Mostly we use Enums for declaring constants. What is the use of this feature?
2. I see, an Enum can implement an interface. What would be the use? I am just confused about practical life usage.

3. Another confusion is, see the code snippet below,

    enum Position { FIRST, SECOND, THIRD; public static void main(String[] args) { //line 1 Position p = Position.SECOND; //line 2 switch(Position.THIRD) { //line 3 case FIRST: //line 4 System.out.println("Congrats, You are the winner");//line 5 break; //line 6 case SECOND : //line 7 System.out.println("Congrats, You are the runner");//line 8 break; //line 9 default : //line 10 System.out.println("Better luck next time"); //line 11 } } } 

   If use in case statement like Position . First,it gives a compile time error. I understand JLS does not allow this. Reason written is with a class format, it can't make a symbolic linkage. But reverse is true. Like in line-3, i am using the enum itself with fully qualified name. So, my point is, what is the meaning of symbolic linkage and why line-3 is allowed also?

Note It was basically a typo mistake. My main question was, we are using syntax Position.THIRD in switch condition(which is allowed by compiler) but when we use same syntax in case, it is not allowed. Why is that?

Quote From JLS

(One reason for requiring inlining of constants is that switch statements require constants on each case, and no two such constant values may be the same. The compiler checks for duplicate constant values in a switch statement at compile time; the class file format does not do symbolic linkage of case values.)

Here it mentions about Symbolic linkage. But when i wrote something like this in switch condition switch(Position.THIRD) , it was allowed where as in CASE statement it was.

Answer 1

What is the use of static methods in Enums?

Same as in any other type. For enums, they're often "given this string value, return the equivalent enum" or something similar. (In cases where valueOf isn't appropriate, eg where there are multiple representations.)

I see, an Enum can implement an interface. What would be the use?

Same as with any interface - to decouple implementation from usage. For example, I've made enums implement a localization-related interface before now.

My main question was, we are using syntax Position.THIRD in switch condition(which is allowed by compiler) but when we use same syntax in case, it is not allowed. Why is that?

It's just a language decision - the enum type has to be the same as the type of the switch value itself, so it's redundant information. It has nothing to do with the section of the JLS you quoted.

Answer 2

枚举可以使单身人士变得非常漂亮……基本上可以回答您所有的问题:)

Answer 3

Enums are a lot like normal classes, except they have certain restrictions on instantiation and they can't extend classes, etc.

Static methods are typically factory methods that return instances of the enum, often looking up the instance whose field matches a parameter. See this example .

Enums can have fields, getters and setters and other methods and can thus implement interfaces - including generic ones. See this example .

You use an enum instead of a class when you want a finite set if named instances.