Illegal Forward Reference and Enums

Java Problem Overview

I'm programming a game in java which is made up of a grid of tiles. I wan't to be able to inuitively define the edges of the tiles and how they relate to each other, e.g. to get the opposite edge of a tile, I want to be able to just type TOP.opposite(). However, when using enums to define these edges I end up having to forward reference at least two of them in the contstructor:

public enum Edge {

   TOP(Edge.BOTTOM), //illegal forward reference
   BOTTOM(Edge.TOP),
   LEFT(Edge.RIGHT), //illegal forward reference
   RIGHT(Edge.LEFT);

   private Edge opposite;

   private Edge(Edge opp){
      this.opposite = opp;
   }

   public Edge opposite(){
      return this.opposite;
   }
}

Is there any way of getting round this problem using enums which is just as simple?

Java Solutions

Solution 1 - Java

You can do this which is not as intuitive.

public enum Edge {
    TOP, BOTTOM, LEFT, RIGHT;
    private Edge opposite;

    static {
        TOP.opposite = BOTTOM;
        BOTTOM.opposite = TOP;
        LEFT.opposite = RIGHT;
        RIGHT.opposite = LEFT;
    }
    public Edge opposite(){
        return this.opposite;
    }
}

Solution 2 - Java

enum Edge {
    TOP {
        @Override
        public Edge opposite() {
            return BOTTOM;
        }
    },
    BOTTOM {
        @Override
        public Edge opposite() {
            return TOP;
        }
    },
    LEFT {
        @Override
        public Edge opposite() {
            return RIGHT;
        }
    },
    RIGHT {
        @Override
        public Edge opposite() {
            return LEFT;
        }
    };

    public abstract Edge opposite();
}

Solution 3 - Java

public enum Edge {
    
    TOP,
    BOTTOM(Edge.TOP),
    LEFT,
    RIGHT(Edge.LEFT);

    private Edge opposite;

    private Edge() {
        
    }
    private Edge(Edge opp) {
        this.opposite = opp;
        opp.opposite = this;
    }

    public Edge opposite() {
        return this.opposite;
    }
}

Solution 4 - Java

You can also make use of an static innerclass inside the enum:

public enum EnumTest     
{     
NORTH( Orientation.VERTICAL ),     
SOUTH( Orientation.VERTICAL ),     
EAST( Orientation.HORIZONTAL ),     
WEST( Orientation.HORIZONTAL );     

private static class Orientation  
{  
private static final String VERTICAL = null;     
private static final String HORIZONTAL = null;     
}
}

Stolen from here :)

Solution 5 - Java

Here's another way

public enum Edge {

    TOP("BOTTOM"),
    BOTTOM("TOP"),
    LEFT("RIGHT"),
    RIGHT("LEFT");

    private String opposite;

    private Edge(String opposite){
        this.opposite = opposite;
    }

    public Edge opposite(){
        return valueOf(opposite);
    }

}

Peter Lawrey's solution is however more efficient and compiletime safe.

Solution 6 - Java

Add a method `opposite` to return enum object

You could just define a method, opposite().

In modern Java, switch expression

In modern Java, we can use a switch expression. The compiler ensures that we have covered all possible cases.

enum Edge
{
    TOP, BOTTOM, LEFT, RIGHT;

    public Edge opposite ( )
    {
        return switch ( this )
                {
                    case TOP -> BOTTOM;
                    case BOTTOM -> TOP;
                    case LEFT -> RIGHT;
                    case RIGHT -> LEFT;
                };
    }
}

Usage:

System.out.println( Edge.TOP.opposite() );

>BOTTOM

In earlier Java, `switch`

In older Java, use syntax seen in the following code.

Notice the need for a default case, in case you ever add an element to the enum or you inadvertently delete a case from the switch.

public enum Edge {
    TOP,
    BOTTOM,
    LEFT,
    RIGHT;

    public Edge opposite() {
        switch (this) {
            case TOP:
                return BOTTOM;
            case BOTTOM:
                return TOP;
            case LEFT:
                return RIGHT;
            case RIGHT:
                return LEFT;
            default:
                throw new RuntimeException("Oh dear");
        }
    }
}

Solution 7 - Java

You can create a static Map where key is the original enum and the value the opposite edge. Initialize it in a static block and the return the mapping from the opposite() method.

private static Map<Edge, Edge> oppostiteMapping;

static {
  oppositeMapping = new EnumMap<Edge, Edge>();
  oppositeMapping.put(TOP, BOTTOM);
  ...
}

public Edge opposite() {
    return oppositeMapping.get(this);
}

EDIT: as proposed in comment better to use EnumMap, so I upgraded accordingly

Btw. this approach is generally useful when you create something like static fromString() method etc.

Solution 8 - Java

You could use an internal Map instead to define these associations. This works if at the point of initializing the Map, you already have all enum values created:

public enum Edge {

  TOP,
  BOTTOM,
  LEFT,
  RIGHT;

  private static final Map<Edge, Edge> opposites = 
        new EnumMap<Edge, Edge>(Edge.class);
  static {
    opposites.put(TOP, BOTTOM);
    opposites.put(BOTTOM, TOP);
    opposites.put(LEFT, RIGHT);
    opposites.put(RIGHT, LEFT);
  }

  public Edge opposite(){
    return opposites.get(this);
  }
}

Solution 9 - Java

My method is by using ordinal. This is a simple example, but for a much more complex example see below.

public enum Edge {
	// Don't change the order! This class uses ordinal() in an arithmetic context.
	TOP,    // = 0
	LEFT,   // = 1
	RIGHT,  // = 2
	BOTTOM; // = 3

	public Edge other() {
		return values()[3 - ordinal()];
	}
}

Although using ordinal is discouraged for being fragile, using ordinal in the same enum as it's defined in is less fragile, and it's further mitigated here with a comment. Though the example above is quite trivial, the next example is less so. Compare the original way and the way using ordinal:

From 98 lines:

public enum Axes {
	NONE,
	HORIZONTAL,
	VERTICAL,
	BOTH;

	public Axes add(Axes axes) {
		switch (axes) {
			case HORIZONTAL:
				if (this == NONE)
					return HORIZONTAL;
				if (this == VERTICAL)
					return BOTH;
				break;
			case VERTICAL:
				if (this == NONE)
					return VERTICAL;
				if (this == HORIZONTAL)
					return BOTH;
				break;
			case BOTH:
				return BOTH;
			default:
				throw new AssertionError(axes);
		}
		return this;
	}

	public Axes remove(Axes axes) {
		switch (axes) {
			case HORIZONTAL:
				if (this == HORIZONTAL)
					return NONE;
				if (this == BOTH)
					return VERTICAL;
				break;
			case VERTICAL:
				if (this == VERTICAL)
					return NONE;
				if (this == BOTH)
					return HORIZONTAL;
				break;
			case BOTH:
				return NONE;
			default:
				throw new AssertionError(axes);
		}
		return this;
	}

	public Axes toggle(Axes axes) {
		switch (axes) {
			case NONE:
				return this;
			case HORIZONTAL:
				switch (this) {
					case NONE:
						return HORIZONTAL;
					case HORIZONTAL:
						return NONE;
					case VERTICAL:
						return BOTH;
					case BOTH:
						return VERTICAL;
					default:
						throw new AssertionError(axes);
				}
			case VERTICAL:
				switch (this) {
					case NONE:
						return VERTICAL;
					case HORIZONTAL:
						return BOTH;
					case VERTICAL:
						return NONE;
					case BOTH:
						return HORIZONTAL;
					default:
						throw new AssertionError(axes);
				}
			case BOTH:
				switch (this) {
					case NONE:
						return BOTH;
					case HORIZONTAL:
						return VERTICAL;
					case VERTICAL:
						return HORIZONTAL;
					case BOTH:
						return NONE;
					default:
						throw new AssertionError(axes);
				}
			default:
				throw new AssertionError(axes);
		}
	}
}

to 19 lines:

public enum Axes {
	// Don't change the order! This class uses ordinal() as a 2-bit bitmask.
	NONE,       // = 0 = 0b00
	HORIZONTAL, // = 1 = 0b01
	VERTICAL,   // = 2 = 0b10
	BOTH;       // = 3 = 0b11

	public Axes add(Axes axes) {
		return values()[ordinal() | axes.ordinal()];
	}

	public Axes remove(Axes axes) {
		return values()[ordinal() & ~axes.ordinal()];
	}

	public Axes toggle(Axes axes) {
		return values()[ordinal() ^ axes.ordinal()];
	}
}

Solution 10 - Java

I preferred this:

public enum Edge {
   TOP,
   BOTTOM,
   LEFT,
   RIGHT;

   private Link link;

   private Link getLink() {
	 if (link == null) {
		link = Link.valueOf(name());
	 }
     return link;
   }

   public Edge opposite() {
      return getLink().opposite();
   }
}

public enum Link {
   TOP(Edge.BOTTOM),
   BOTTOM(Edge.TOP),
   LEFT(Edge.RIGHT),
   RIGHT(Edge.LEFT);

   private Edge opposite;

   private Link(Edge opp) {
      this.opposite = opp;
   }

   public Edge opposite() {
      return this.opposite;
   }
}

Solution 11 - Java

With Java 8 lambdas:

public enum Edge {
  TOP(() -> Edge.BOTTOM),
  BOTTOM(() -> Edge.TOP),
  LEFT(() -> Edge.RIGHT),
  RIGHT(() -> Edge.LEFT);

  private Supplier<Edge> opposite;

  private Edge(Supplier<Edge> opposite) {
    this.opposite = opposite;
  }

  public Edge opposite() {
    return opposite.get();
  }
}

Content Type	Original Author	Original Content on Stackoverflow
Question	MattLBeck	View Question on Stackoverflow
Solution 1 - Java	Peter Lawrey	View Answer on Stackoverflow
Solution 2 - Java	Vitaliy Oliynyk	View Answer on Stackoverflow
Solution 3 - Java	eyelash	View Answer on Stackoverflow
Solution 4 - Java	jacktrades	View Answer on Stackoverflow
Solution 5 - Java	BalusC	View Answer on Stackoverflow
Solution 6 - Java	Jeff Foster	View Answer on Stackoverflow
Solution 7 - Java	Jan Zyka	View Answer on Stackoverflow
Solution 8 - Java	Péter Török	View Answer on Stackoverflow
Solution 9 - Java	Mark Jeronimus	View Answer on Stackoverflow
Solution 10 - Java	vitali_y	View Answer on Stackoverflow
Solution 11 - Java	Greg Dennis	View Answer on Stackoverflow

Illegal Forward Reference and Enums

Java Problem Overview

Java Solutions

Solution 1 - Java

Solution 2 - Java

Solution 3 - Java

Solution 4 - Java

Solution 5 - Java

Solution 6 - Java

Add a method `opposite` to return enum object

In modern Java, switch expression

In earlier Java, `switch`

Solution 7 - Java

Solution 8 - Java

Solution 9 - Java

Solution 10 - Java

Solution 11 - Java

How to enable loglevel debug on Apache2 server

How to create custom tags for HTML

Attributions

Java Problem Overview

Java Solutions

Solution 1 - Java

Solution 2 - Java

Solution 3 - Java

Solution 4 - Java

Solution 5 - Java

Solution 6 - Java

Add a method opposite to return enum object

In modern Java, switch expression

In earlier Java, switch

Solution 7 - Java

Solution 8 - Java

Solution 9 - Java

Solution 10 - Java

Solution 11 - Java

How to enable loglevel debug on Apache2 server

How to create custom tags for HTML

Attributions

Add a method `opposite` to return enum object

In earlier Java, `switch`