java - 枚举样式工厂作为 Java 中的内部枚举

Question

我尝试将枚举样式的工厂模式实现为内部枚举，但它没有用。有没有不将内部枚举分离到新文件中的解决方案？换句话说，内部枚举样式的工厂模式是否可能？

代码如下。

public class SampleParent {

    private class InnerChild { }
    private class InnerChildA extends InnerChild { }
    private class InnerChildB extends InnerChild { }
    private class InnerChildC extends InnerChild { }

    enum InnerChildEnum {
        CHILD_A {
            @Override
                public InnerChild getInstance() {
                    return new InnerChildA();  // compile error
                }
        },
        CHILD_B {
            @Override
                public InnerChild getInstance() {
                    return new SampleParent.InnerChildB();  // compile error
                }
        },
        CHILD_C {
            @Override
                public InnerChild getInstance() {
                    return SampleParent.new InnerChildC();  // compile error
                }
        },
        ;

        public abstract InnerChild getInstance();
    }

    private static class InnerChildFactoryEnumStyled {
        public static InnerChild getInnerChild(InnerChildEnum child) {
            return child.getInstance();
        }
    }


    public static void main(String[] args) {

        // I want to write this way
        InnerChild child = InnerChildFactoryEnumStyled.getInnerChild(InnerChildEnum.CHILD_A);
    }
}

编译错误信息如下

$ javac SampleParent.java 
SampleParent.java:12: error: non-static variable this cannot be referenced from a static context
                return new InnerChildA();
                       ^
SampleParent.java:18: error: non-static variable this cannot be referenced from a static context
                return new SampleParent.InnerChildB();
                       ^
SampleParent.java:24: error: cannot find symbol
                return SampleParent.new InnerChildC();
                       ^
  symbol: variable SampleParent
3 errors

Answer 1

您的内部类不是静态的，因此它们必须引用封闭类 SampleParent 的实例。将您的类声明更改为

private static class InnerChild { }
private static class InnerChildA extends InnerChild { }
private static class InnerChildB extends InnerChild { }
private static class InnerChildC extends InnerChild { }

你可以。 G。 return new InnerChildA(); 在你的枚举中。