java - Enum 的参数始终设置为 null

Question

我有两个类(class)，FruitPlant和Fruit ，并且每个人都有自己的Type枚举:

public class FruitPlant extends Plant {
    public static enum Type {
        // Dry-hot 
        PEACH_TREE("peach", 3, 80, Fruit.Type.PEACH),
        CACTUS("cactus", 10000, 120, Fruit.Type.PRICKLYPEAR), 

        // Dry-cool
        BLUEBERRY_BUSH("blueberry", 3, 40, Fruit.Type.BLUEBERRY),
        CHERRY_TREE("cherry", 3, 60, Fruit.Type.CHERRY), 

        // Wet-hot
        ORANGE_TREE("orange", 2, 80, Fruit.Type.ORANGE),
        MELON_VINE("melon", 1.5, 80, Fruit.Type.MELON), 

        // Wet-cool
        APPLE_TREE("apple", 2, 60, Fruit.Type.APPLE), 
        STRAWBERRY_BUSH("strawberry", 1.5, 60, Fruit.Type.STRAWBERRY);

        public double waterHardiness;
        public int preferedTemp;
        public int[] babyImage;
        public int[] childImage;
        public int[] subAdultImage;
        public int[] adultImage;
        public int[] pregnantImage;
        public Fruit.Type fruitType;

        Type (String imagePath, double waterHardiness, int preferedTemp, Fruit.Type fruitType){
            this.waterHardiness = waterHardiness;
            this.preferedTemp = preferedTemp;
            babyImage = Image.loadImage("/res/plants/" + imagePath + "/baby.png");
            childImage = Image.loadImage("/res/plants/" + imagePath + "/child.png");
            subAdultImage = Image.loadImage("/res/plants/" + imagePath + "/subadult.png");
            adultImage = Image.loadImage("/res/plants/" + imagePath + "/adult.png");
            pregnantImage = Image.loadImage("/res/plants/" + imagePath + "/pregnant.png");
            this.fruitType = fruitType;
        }
    }

    Type type;
    public FruitPlant(Type type) {
        this.type = type;
        System.out.println(type.fruitType); // Prints whatever Fruit.Type the FruitPlant.Type has
    }
}

public class Fruit implements Element, Holdable, Plantable {
    public static enum Type {
        PRICKLYPEAR(FruitPlant.Type.CACTUS), 
        PEACH(FruitPlant.Type.PEACH_TREE),
        CHERRY(FruitPlant.Type.CHERRY_TREE), 
        BLUEBERRY(FruitPlant.Type.BLUEBERRY_BUSH),
        MELON(FruitPlant.Type.MELON_VINE), 
        ORANGE(FruitPlant.Type.ORANGE_TREE),
        APPLE(FruitPlant.Type.APPLE_TREE), 
        STRAWBERRY(FruitPlant.Type.STRAWBERRY_BUSH);


        public FruitPlant.Type plant;

        Type(FruitPlant.Type plant) {
            System.out.println(plant);
            this.plant = plant;
        }
    }

    Type type;
    public Fruit(Type type){
        this.type = type;
        System.out.println(type.plant); // Prints null
    }
}

以及一些所需的测试代码:

public class Main {
    public static void Main(String args[]){
        System.out.println("Next line printed should be BLUEBERRY:");
        FruitPlant blueberryBush = new FruitPlant(FruitPlant.Type.BLUEBERRY_BUSH);
        System.out.println("Next line printed should be BLUEBERRY_BUSH:");
        Fruit blueberry = new Fruit(Fruit.Type.BLUEBERRY);
        System.out.println("End test");

    }
}

每当我创建FruitPlant时对象并传递给它 FruitPlant.Type枚举 FruitPlant构造函数，fruit该枚举的字段返回给我 Fruit.Type枚举。然而，每当我制作 Fruit对象并给它一个 Fruit.Type枚举 Fruit构造函数，plant该枚举的字段的计算结果始终为 null .

这与系统的循环性有关吗？为什么 Fruit.Type.plant现场返回null而FruitPlant.Type.fruit返回 Fruit.Type enum ？

PS:这两个类还有更多内容，但我已经删除了大部分内容，因为它们不相关。我还使用了带有类型枚举的骨架类，因为大多数水果植物和水果的行为相似，并且创建 8 个基本相同的类感觉是错误的。

Answer 1

出现此问题的原因是您有两个枚举，Fruit.Type 和 FruitPlant.Type，它们都在常量声明中相互引用:

public class FruitPlant extends Plant {
    public static enum Type {
        // Dry-hot 
        PEACH_TREE("peach", 3, 80, Fruit.Type.PEACH),
        // ...
    }
    // ...
}

// ...
public class Fruit implements Element, Holdable, Plantable {
    public static enum Type {
        // ...
        PEACH(FruitPlant.Type.PEACH_TREE),
        // ...
    }
    // ...
}

枚举常量 Fruit.Type.PEACH 和 FruitPlant.Type.PEACH_TREE 必须在使用之前实例化。

假设首先实例化PEACH；那么 FruitPlant.Type.PEACH_TREE 尚未实例化，因此该名称的静态成员的默认初始值为 null。它要引用的对象尚不存在，因此无法引用它。另一方面，如果先实例化PEACH_TREE，则它无法获取对PEACH的引用，因为该对象尚不存在；当前引用它的枚举成员的默认初始值为 null。

解决方案是延迟这些枚举之间的循环引用，直到所有枚举常量都已初始化；您可以编写方法 getPlant 和 getFruit 返回正确的值，而不是将它们存储为字段。调用这些方法时，所有枚举常量都将被实例化，因此不会因未初始化的成员而产生默认 null 值。

public class FruitPlant extends Plant {
    public static enum Type {
        PEACH_TREE("peach", 3, 80),
        // ...

        public Fruit.Type getFruit() {
            switch(this) {
                case PEACH_TREE: return Fruit.Type.PEACH;
                // ...
                default: throw new IllegalStateException();
            }
        }

        // ...
    }
    // ...
}