具有泛型类型边界的Java Builder模式

Question

我正在尝试使用Builder模式而不是telescoping构造函数创建一个包含许多参数的类.我是按照Joshua Bloch的Effective Java描述的方式做的,在封闭类上有私有构造函数,还有一个公共静态Builder类.Builder类在调用build()之前确保对象处于一致状态,此时它将封闭对象的构造委托给私有构造函数.从而

public class Foo {

    // Many variables

    private Foo(Builder b) {
        // Use all of b's variables to initialize self
    }

    public static final class Builder {

        public Builder(/* required variables */) {

        }

        public Builder var1(Var var) {
            // set it
            return this;
        }

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

    }

}

然后我想为某些变量添加类型边界,因此需要参数化类定义.我希望Foo类的边界与Builder类的边界相同.

public class Foo<Q extends Quantity> {

    private final Unit<Q> units;
    // Many variables

    private Foo(Builder<Q> b) {
        // Use all of b's variables to initialize self
    }

    public static final class Builder<Q extends Quantity> {
        private Unit<Q> units;

        public Builder(/* required variables */) {

        }

        public Builder units(Unit<Q> units) {
            this.units = units;
            return this;
        }

        public Foo build() {
            return new Foo<Q>(this);
        }

    }

}

这编译很好,但编译器允许我做我认为应该是编译器错误的事情.例如

public static final Foo.Builder<Acceleration> x_Body_AccelField =
        new Foo.Builder<Acceleration>()
        .units(SI.METER)
        .build();

这里单位的说法是没有Unit<Acceleration>,但Unit<Length>,但它仍然是由编译器所接受.

我在这做错了什么？我想在编译时确保单元类型正确匹配.

Answer 1

units应该回来Builder<Q>,而不是一个不合格的Builder.