关于继承和泛型的Java泛型的有用性扩展了自我

Question

我找到了泛型params扩展自己的泛型(这里).我不太了解.我怀疑一开始是错的,但没有人提出来.我对此有一些疑问:

1. 如何使用Variant泛型,你能举个例子吗？
2. 这种仿制药的好处或效果是什么.

这是从(这里)挑选的泛型样式代码.

abstract class Base<T extends Base<T>> {

}

class Variant<T extends Variant<T>> extends Base<T> {

}

谢谢!

Answer 1

我想你在谈论F-bounded类型.我发现它们在两个层次结构直接相关的情况下非常有用.想到的最明显的案例是构建器模式,它针对类的层次结构.在这种情况下,也可以使用构建器的层次结构.

一个例子可能会有所启发.考虑以下(非常刻板的)层次结构:

public abstract class Human {

    protected String name;

    protected int age;
}

public class Woman extends Human {

    protected String loveMovie;
}

public class Man extends Human {

    protected String soccerTeam;
}

现在,我们想为Man和创建构建器Woman.我们可以为每个构建器实现构建器,复制设置name和age属性的方法.然而,由于Man和Woman继承Human,我们可以有一个抽象HumanBuilder,使我们WomanBuilder和ManBuilder从它继承.这就是F-bound类型派上用场的地方.

该Human课程及其HumanBuilder内容如下:

public abstract class Human {

    protected String name;

    protected int age;

    public static abstract class HumanBuilder<H extends Human, 
                                              T extends HumanBuilder<H, T>> {
        protected String name;

        protected int age;

        @SuppressWarnings("unchecked")
        public T name(String name) {
            this.name = name;
            return (T) this;
        }

        @SuppressWarnings("unchecked")
        public T age(int age) {
            this.age = age;
            return (T) this;
        }

        protected void fill(H human) {
            human.name = this.name;
            human.age = this.age;
        }

        protected abstract H create();

        public final H build() {
            H human = this.create();
            this.fill(human);
            return human;
        }
    }
}

这将是Woman班级,以及它WomanBuilder:

public class Woman extends Human {

    protected String loveMovie;

    public static class WomanBuilder extends HumanBuilder<Woman, WomanBuilder> {

        protected String loveMovie;

        public WomanBuilder loveMovie(String loveMovie) {
            this.loveMovie = loveMovie;
            return this;
        }

        @Override
        protected void fill(Woman woman) {
            super.fill(woman);
            woman.loveMovie = this.loveMovie;
        }

        @Override
        protected Woman create() {
            return new Woman();
        }
    }
}

最后,这是Man班级,以及它的ManBuilder:

public class Man extends Human {

    protected String soccerTeam;

    public static class ManBuilder extends HumanBuilder<Man, ManBuilder> {

        protected String soccerTeam;

        public ManBuilder soccerTeam(String soccerTeam) {
            this.soccerTeam = soccerTeam;
            return this;
        }

        @Override
        protected void fill(Man man) {
            super.fill(man);
            man.soccerTeam = this.soccerTeam;
        }

        @Override
        protected Man create() {
            return new Man();
        }
    }
}

这种方法可以节省大量代码,特别是在实际用例中.

正如预期的那样,使用构建器不需要任何铸造:

Man man = new Man.ManBuilder()
    .name("Joe")
    .age(29)
    .soccerTeam("Los Angeles Galaxy")
    .build();

Woman woman = new Woman.WomanBuilder()
    .name("Jane")
    .age(25)
    .loveMovie("Meet Joe Black")
    .build();