具有非平凡子类树的 Java 构建器模式

Question

我熟悉将构建器模式与泛型和子类化结合使用，但我看不出如何使它与非平凡的子类树一起工作(即 C extends B extends A)。这是我正在尝试做的一个简单示例:

class A {
    private final int value;

    protected A(ABuilder builder) {
        this.value = builder.value;
    }

    public int getValue() { return value; }

    public static class ABuilder<T extends ABuilder<T>> {
        private int value;

        public T withValue(int value) {
            this.value = value;
            return (T) this;
        }

        public A build() {
            return new A(this);
        }
    }
}

class B extends A {
    private final String name;

    public static BBuilder builder() {
        return new BBuilder();
    }

    protected B(BBuilder builder) {
        super(builder);
        this.name = builder.name;
    }

    public String getName() { return name; }

    public static class BBuilder<U extends BBuilder<U>> extends ABuilder<BBuilder<U>> {
        private String name;

        public U withName(String name) {
            this.name = name;
            return (U) this;
        }

        public B build() {
            return new B(this);
        }
    }
}

如果我声明没有通用类型的 BBuilder，一切都很好:

public static class BBuilder extends ABuilder<BBuilder>

因为我希望 BBuilder 由 CBuilder 扩展，所以我尝试使用与 ABuilder 相同的 Curiously Recurring Template Pattern。但是像这样，编译器将 BBuilder.withValue() 视为返回 ABuilder，而不是我想要的 BBuilder。这:

    B b = builder.withValue(1)
    .withName("X")
    .build();

不编译。任何人都可以看到我在这里做错了什么，我一直在尝试不同的泛型模式，但无法让它发挥作用。

感谢任何有建议的人。

Answer 1

看来你的错误只是声明了正确的参数:

class A {

    private final int value;

    public static <T extends Builder<T>> T builderA() {
        return (T)new Builder<>();
    }

    protected A(Builder<? extends Builder<?>> builder) {
        value = builder.value;
    }

    public static class Builder<T extends Builder<T>> {

        private int value;

        public T withValue(int value) {
            this.value = value;
            return (T)this;
        }

        public A build() {
            return new A(this);
        }
    }
}

class B extends A {

    private final String name;

    public static <T extends Builder<T>> T builderB() {
        return (T)new Builder<>();
    }

    protected B(Builder<? extends Builder<?>> builder) {
        super(builder);
        name = builder.name;
    }

    public static class Builder<T extends Builder<T>> extends A.Builder<T> {

        private String name;

        public Builder<T> withName(String name) {
            this.name = name;
            return this;
        }

        public B build() {
            return new B(this);
        }
    }
}

客户端代码:

A a = A.builder().withValue(1).build();
B b = B.builder().withValue(2).withName("xx").build();