Java Enum Methods - return opposite direction enum

Question

I would like to declare an enum Direction  that has a method that returns the opposite direction  the following is not syntactically correct  i e  enums cannot be instantiated  but it illustrates my point   Is this possible in Java   Here is the code    public enum Direction         NORTH 1        SOUTH -1        EAST -2        WEST 2         Direction int code             this code code              protected int code       public int getCode                return this code              static Direction getOppositeDirection Direction d              return new Direction d getCode     -1

User · Answer

Yes we do it all the time  You return a static instance rather than a new Object   static Direction getOppositeDirection Direction d          Direction result   null         if  d    null              int newCode   -d getCode               for  Direction direction   Direction values                    if  d getCode      newCode                      result   direction                                                return result

User · Answer

For those lured here by title  yes  you can define your own methods in your enum  If you are wondering how to invoke such non-static method  you do it same way as with any other non-static method - you invoke it on instance of type which defines or inherits that method  In case of enums such instances are simply ENUM CONSTANTs   So all you need is EnumType ENUM CONSTANT methodName arguments      Now lets go back to problem from question  One of solutions could be   public enum Direction        NORTH  SOUTH  EAST  WEST       private Direction opposite       static           NORTH opposite   SOUTH          SOUTH opposite   NORTH          EAST opposite   WEST          WEST opposite   EAST             public Direction getOppositeDirection             return opposite             Now Direction NORTH getOppositeDirection   will return Direction SOUTH     Here is little more  hacky  way to illustrate  jedwards comment but it doesn t feel as flexible as first approach since adding more fields or changing their order will break our code   public enum Direction       NORTH  EAST  SOUTH  WEST          cached values to avoid recreating such array each time method is called     private static final Direction   VALUES   values         public Direction getOppositeDirection             return VALUES  ordinal     2    4

User · Answer

Create an abstract method  and have each of your enumeration values override it   Since you know the opposite while you re creating it  there s no need to dynamically generate or create it   It doesn t read nicely though  perhaps a switch would be more manageable   public enum Direction       NORTH 1             Override         public Direction getOppositeDirection                 return Direction SOUTH                       SOUTH -1             Override         public Direction getOppositeDirection                 return Direction NORTH                       EAST -2             Override         public Direction getOppositeDirection                 return Direction WEST                       WEST 2             Override         public Direction getOppositeDirection                 return Direction EAST                        Direction int code           this code code            protected int code       public int getCode             return this code             public abstract Direction getOppositeDirection

User · Answer

This works   public enum Direction       NORTH  SOUTH  EAST  WEST       public Direction oppose             switch this                case NORTH  return SOUTH              case SOUTH  return NORTH              case EAST   return WEST              case WEST   return EAST                    throw new RuntimeException  Case not implemented

User · Answer

For a small enum like this  I find the most readable solution to be   public enum Direction        NORTH            Override         public Direction getOppositeDirection                 return SOUTH                        SOUTH            Override         public Direction getOppositeDirection                 return NORTH                       EAST            Override         public Direction getOppositeDirection                 return WEST                       WEST            Override         public Direction getOppositeDirection                 return EAST                         public abstract Direction getOppositeDirection

User · Answer

public enum Direction       NORTH  EAST  SOUTH  WEST       public Direction getOppositeDirection            return Direction values    this ordinal     2    4             Enums have a static values method that returns an array containing all of the values of the enum in the order they are declared  source  since NORTH gets 1  EAST gets 2  SOUTH gets 3  WEST gets 4  you can create a simple equation to get the opposite one    value   2    4

[java] Java Enum Methods - return opposite direction enum

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