Unique enum member values

Question

My problem: I want to specify a 1 to 1 connection between two enums. basically:

enum MyEnum{

    ENUM_VALUE_1,
    ENUM_VALUE_2,
    ENUM_VALUE_3,

}

enum          


        

Answer 1

You can use a static block to test your linkage:

import java.util.HashSet;
import java.util.Set;

public class EnumTrick {
  enum MyEnum {
    ENUM_VALUE_1, ENUM_VALUE_2, ENUM_VALUE_3,
  }

  enum MyOtherEnum {

    OTHER_ENUM_VALUE_1(MyEnum.ENUM_VALUE_3), 
    OTHER_ENUM_VALUE_2(MyEnum.ENUM_VALUE_1),
    OTHER_ENUM_VALUE_3(MyEnum.ENUM_VALUE_2), 
    OTHER_ENUM_VALUE_4(MyEnum.ENUM_VALUE_2);


    private MyEnum pair;

    MyOtherEnum(MyEnum pair) {
      this.pair = pair;

    }

    static {
      Set usedEnums = new HashSet();
      for(MyOtherEnum moe : MyOtherEnum.values()){
        if(! usedEnums.add(moe.pair) ){
          throw new IllegalArgumentException(moe.pair + " is already used!");
        }
      }
    }
  }

  public static void main(String [] args){
    MyOtherEnum moe = MyOtherEnum.OTHER_ENUM_VALUE_1;
  }
}

Will throw an exception:

Exception in thread "main" java.lang.ExceptionInInitializerError
  at EnumTrick.main(EnumTrick.java:35)
Caused by: java.lang.IllegalArgumentException: ENUM_VALUE_2 is already used!
  at EnumTrick$MyOtherEnum.(EnumTrick.java:28)
  ... 1 more