Java中 <? super T>，<? extends T>，和<T>的区别

在看Collections源代码中，看到如下代码：

public static <T extends Comparable<? super T>> void sort(List<T> list) {
Object[] a = list.toArray();
Arrays.sort(a);
ListIterator<T> i = list.listIterator();
for (int j=0; j<a.length; j++) {
i.next();
i.set((T)a[j]);
}
}

<T extends Comparable<? super T>>

/*** Created by CSH on 12/7/2015.*///测试泛型public class TestGeneric {@Testpublic void test01(){new A<If<Father>>();    //不报错new B<If<Father>>();    //不报错new C<If<Father>>();    //不报错new A<If<Child>>();     //报错new B<If<Child>>();     //报错new C<If<Child>>();     //不报错new A<If<GrandFather>>();   //不报错new B<If<GrandFather>>();   //报错new C<If<GrandFather>>();   //报错}}class GrandFather {}class Father extends GrandFather{}class Child extends Father {}interface If<T>{void doSomething();}class A  <T extends If<? super Father>> {}class B <T extends If<Father>> {}class C <T extends If<? extends Father>>{}

这例子可以区分super和extends这2个关键字的区别super：<? super Father> 指的是Father是上限，传进来的对象必须是Father，或者是Father的父类，因此 new A<If<Child>>()会报错，因为Child是Father的子类extends：<? extends Father> 指的是Father是下限，传进来的对象必须是Father，或者是Father的子类，因此 new C<If<GrandFather>>()会报错，因为GrandFather是Father的父类<Father> 指的是只能是Father，所以new B<If<Child>>()和 new B<If<GrandFather>>()都报错

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