Question

我正在用java编写一个由瓷砖网格组成的游戏。我不希望能够直观地定义瓷砖的边缘以及它们之间的相互关系，例如：要获得图块的相反边缘，我希望能够只键入TOP.opposite()。但是，当使用枚举来定义这些边时，我最终必须在contstructor中至少引用其中两个：

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;
   }
}

有没有办法使用枚举来解决这个问题，这很简单？

Answer 1

你可以这样做，这不是那么直观。

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;
    }
}

Answer 2

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();
}

Answer 3

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;
    }
}

Answer 4

这是另一种方式

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);
    }

}

然而，Peter Lawrey的解决方案更有效，编译时更安全。

Answer 5

您还可以在枚举中使用 static innerclass ：

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;     
}
}

从here被盗}

Answer 6

您可以创建一个静态Map，其中key是原始枚举，值是相反的边缘。在静态块中初始化它，并从opposite()方法返回映射。

private static Map<Edge, Edge> oppostiteMapping;

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

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

编辑：在评论中建议更好地使用EnumMap，所以我相应升级了

顺便说一下。当你创建类似静态fromString()方法等的东西时，这种方法通常很有用。

Answer 7

您可以定义一个类似于下面的方法。

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

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

Answer 8

您可以使用内部Map来定义这些关联。如果在初始化Map时，您已经创建了所有枚举值：

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);
  }
}

Answer 9

我的方法是使用序数。这是一个简单的例子，但是对于更复杂的例子，请参见下文。

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()];
    }
}

虽然不鼓励使用序数是脆弱的，但在其定义的同一枚举中使用序数不那么脆弱，并且在这里通过评论进一步减轻了它。虽然上面的例子非常简单，但下一个例子却不那么简单。比较原始方式和使用序数的方式：

从98行开始：

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);
        }
    }
}

到19行：

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()];
    }
}

Answer 10

我更喜欢这个：

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;
   }
}

Answer 11

使用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();
  }
}

非法转发参考和枚举

11 个答案: