我认为你是在谈论F-bounded types。我发现它们在两个层次直接相关的情况下非常有用。想到的最明显的例子是构建器模式,针对类的层次结构。在这种情况下,也可以使用建造者的层次结构。
一个例子可能提供一些线索。考虑下面的(非常刻板)层次:
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,我们要为Man
和Woman
创造的建设者。我们可以为每个实现一个构建器,复制设置name
和age
属性的方法。但是,由于Man
和Woman
继承自Human
,我们可以有一个抽象HumanBuilder
,并使我们的WomanBuilder
和ManBuilder
继承它。这是F-bound类型派上用场的地方。
的Human
类及其HumanBuilder
一起,情况如下:
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
类,沿其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,这里的Man
类,连同其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();
}
}
}
这种方法节约了相当多行代码,特别是在现实世界使用情况。
正如预期的那样,使用的建设者不需要任何铸造:
采取有关的递归定义看[这个问题](http://stackoverflow.com/q/211143/4125191) 'Enum'。如果'getNewInstance'声明'T'作为它的返回类型 – RealSkeptic
http://stackoverflow.com/q/17099185/2158288 – ZhongYu