用于switch语句的Java扩展枚举

Question

I have the following code

public interface EnumInterface{
   public String getTitle();
}


public enum Enum1 extends EnumInterface{
  private String title;
  Enum1(String title){
    this.title = title;
  }

  A("Apple"),B("Ball");
  @Override
  public String getTitle(){
    return title;
  }
}

public enum Enum2 extends EnumInterface{
  private String title;
  Enum1(String title){
    this.title = title;
  }

  C("Cat"),D("Doll");
  @Override
  public String getTitle(){
    return title;
  }
}

and I am using it in other class as follows

private EnumInterface[] enumList;//declared globally.

if(flagTrue){
  enumList = Enum1.values();
}else{
  enumList = Enum2.values();
}
....
....
private method1(int position){
  switch(enumList[postion]){
    case A:....
           break;
    case B:....
           break;
    case C:....
           break;
    case D:....
           break;
  }
}

I am getting the following compilation time error

Cannot switch on a value of type EnumInterface. Only convertible int values or enum variables are permitted.

I did my research and found that if I do it this way, the 'switch' case is not possible.

Answer 1

The Switch Statement is absolutely not what you want in this case. Even if the above example worked, and it does not for a reason, your results would be completely wrong. The switch on an enum is using the enum ordinal, which is 0 for Enum1.A , 1 for Enum1.B , 0 for Enum2.C , and 1 for Enum2.D using your classes above. You can see why this would be a really bad idea.

Your EnumInterface class is not tied of the Java type enum in any way, it just defines any class that implements it and provides your getTitle() method. This could be Enum1 , Enum2 , or any other class that might not even be an Enum. So when you want to switch based on the EnumInterface , you need to ask yourself what you actually want to switch on. Is it the title you want to be using as your conditional, or do the enums you defined bring something else to the table?

Now, I'm going to give you the benefit of the doubt and assume that whatever you are trying to do, it needs to be keyed off of an Enum. I will also assume that you cannot combine Enum1 or Enum2 for whatever reason. Below is my completely over-engineered solution for you:

public interface EnumInterface {

    public String getTitle();

    public void processEvent(SwitchLogicClass e);
}


public enum Enum1 implements EnumInterface{

    A("Apple"){
        public void processEvent(SwitchLogicClass e){
            //Any A specific Logic
            e.doSomethingA();
        }
    },
    B("Ball"){
        public void processEvent(SwitchLogicClass e){
            //Any B specific Logic
            e.doSomethingB();
        }
    };

    private String title;
    Enum1(String title){
        this.title = title;
    }


    @Override
    public String getTitle(){
        return title;
    }
}

Repeat for Enum2. Assume that the next class is called SwitchLogicClass

private EnumInterface[] enumList;//declared globally.

if(flagTrue){
    enumList = Enum1.values();
}else{
    enumList = Enum2.values();
}
....
....
private method1(int position){
    EnumInterface[position].processEvent(this);
}


public void doSomethingA(){
    //Whatever you needed to switch on A for
}

public void doSomethingB(){
    //Whatever you needed to switch on B for
}

....
....

You will almost certainly need to refactor based on whatever abstraction patterns you need to use, but the above is the best I can do with what I know of your code.

Answer 2

What did the extending accomplish that can't be done using a simple java enum ? You can just use if else blocks and and invoke equals method.