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];
}
}