最终私有枚举字段中的循环依赖导致 Null 个值

Question

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Consider these enum declarations:

enum Color {
    RED(Shape.CIRCLE),
    GREEN(Shape.TRIANGLE),
    BLUE(Shape.SQUARE);

    private final Shape shape;

    Color(Shape shape) {
        this.shape = shape;
    }

    Shape getShape() {
        return shape;
    }
}

enum Shape {
    CIRCLE(Color.RED),
    TRIANGLE(Color.GREEN),
    SQUARE(Color.BLUE);

    private final Color color;

    Shape(Color color) {
        this.color = color;
    }

    Color getColor() {
        return color;
    }
}

There are cyclic dependencies between the enum fields. There are no compiler warnings (using Java 8).

However, all these tests will fail in the second line:

@Test
public void testRedAndCircle() {
    assertThat(Color.RED.getShape()).isNotNull();
    assertThat(Shape.CIRCLE.getColor()).isNotNull(); // fails
}

@Test
public void testCircleAndRed() {
    assertThat(Shape.CIRCLE.getColor()).isNotNull();
    assertThat(Color.RED.getShape()).isNotNull(); // fails
}

@Test
public void testGreenAndTriangle() {
    assertThat(Color.GREEN.getShape()).isNotNull();
    assertThat(Shape.TRIANGLE.getColor()).isNotNull(); // fails
}

@Test
public void testBlueAndSquare() {
    assertThat(Color.BLUE.getShape()).isNotNull();
    assertThat(Shape.SQUARE.getColor()).isNotNull(); // fails
}

How can the null value in the enum field be explained?

It seems that the enum object in the private final fields is not yet completely instantiated.

Answer 1

Enum constants are initialized like static fields, so you get a similar outcome as described here . Basically, to initialize one enum correctly, the other already needs to be fully initialized, and vice versa. Obviously, this is a circular dependency and cannot work.

I would replace one of the getters, eg, the one in Shape with something like this:

Color getColor() {
    for (Color c : Color.values()) {
        if (c.getShape() == this) {
            return c;
        }
    }
    throw new AssertionError();
}

This way, you don't need the color field in Shape and the circular dependency is avoided.

An added benefit is that the two enums are guaranteed to be consistent. A change in the Color enum will automatically be reflected in Shape .

And if you worry about performance ( I wouldn't ), you could also keep the field in Shape and initialize it the first time getColor is called.