java泛型通配符-协变与逆变

周末研究了下java泛型，关于泛型通配符的协变与逆变问题，题目如下：

•  题目要求：创建一个泛型类Generic1<T>,它只有一个方法，将接受一个T类型的参数。创建第二个泛型类Generic2<T>，它也只有一个方法，将返回类型T的参数。编写一个泛型方法，它具有一个调用第一个泛型类的方法的逆变参数。编写第二个泛型方法，它具有一个调用第二个泛型类的方法的协变参数。
• 实例代码如下：

 

package generics.exercises;

import java.util.ArrayList;
import java.util.List;

import typeinfo.pets.Dog;

public class ContraVarianceAndCovariant {
	static class Generic1Sup<T> {
	}

	static class Generic1<T> extends Generic1Sup<T> {
		void setT(T t) {
		}
	}

	static class Generic1Sub<T> extends Generic1<T> {
	}

	static class Generic2Sup<T> {
	}

	static class Generic2<T> extends Generic2Sup<T> {
		T getT(T t) {
			return t;
		}
	}

	static class Generic2Sub<T> extends Generic2<T> {
	}

	static <T> void writeWithWildcard(List<? super T> list, T item) {
		list.add(item);
	}

	static <T> T readCovariant(List<? extends T> list) {
		return list.get(0);
	}

	static List<Generic1<Dog>> dogs = new ArrayList<Generic1<Dog>>();
	static List<Generic1Sup<Dog>> dogsSup = new ArrayList<Generic1Sup<Dog>>();
	static List<Generic1Sub<Dog>> dogsSub = new ArrayList<Generic1Sub<Dog>>();

	static List<Generic2<Dog>> dogs2 = new ArrayList<Generic2<Dog>>();
	static List<Generic2Sup<Dog>> dogs2Sup = new ArrayList<Generic2Sup<Dog>>();
	static List<Generic2Sub<Dog>> dogs2Sub = new ArrayList<Generic2Sub<Dog>>();

	static void f1() {
		writeWithWildcard(dogs, new Generic1<Dog>());
		writeWithWildcard(dogsSup, new Generic1<Dog>());
		// ! writeWithWildcard(dogsSub, new Generic1<Dog>());
		Generic1<Dog> generic1 = dogs.get(0);
		generic1.setT(new Dog("dog1"));
		System.out.println(generic1);
	}

	static void f2() {
		Generic2<Dog> generic2 = readCovariant(dogs2);
		generic2 = (Generic2<Dog>) readCovariant(dogs2Sup);
		generic2 = readCovariant(dogs2Sub);
		generic2.getT(new Dog("dog2"));
		System.out.println(generic2);
	}

	static class CovariantReader<T> {
		T readCovariant(List<? extends T> list) {
			return list.get(0);
		}
	}

	static void f3() {
		CovariantReader<Generic2<Dog>> fruitReader = new CovariantReader<Generic2<Dog>>();
		Generic2<Dog> generic2 = fruitReader.readCovariant(dogs2);
		// ! generic2 = fruitReader.readCovariant(dogs2Sup);
		generic2 = fruitReader.readCovariant(dogs2Sub);
		generic2.getT(new Dog("dog2"));
	}
}

 感兴趣的读者可以研究下，为题目提供更好的解决方案。

 

评论

1 楼 xcv4javaeye 2010-09-18  

f1不叫协变吧？这只是正常的多态啊？逆变俺还没搞明白...