<? extends A> 和 <? super A> 的意思

/**
 * @author  Lee Boynton
 * @author  Arthur van Hoff
 * @author  Martin Buchholz
 * @author  Ulf Zibis
 * @see     java.lang.Object#toString()
 * @see     java.lang.StringBuffer
 * @see     java.lang.StringBuilder
 * @see     java.nio.charset.Charset
 * @since   JDK1.0
 */

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {

}

package com.jvm;

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

public class T20160727 {
    public static void main(String[] args) {
        List<String> list = new ArrayList<String>();
        list.add("ghi");
        list.add("def");
        list.add("abc");
        System.out.println(list);
        sort(list);
        System.out.println(list);
    }

    public static <T extends Comparable<? super T>> void sort(List<T> list){
        Collections.sort(list);
    }
}

输出：
[ghi, def, abc]
[abc, def, ghi]

<? extends A>表示类型必须是A或者A的子类
<? super A>表示类型必须是A或者A的超类

下面是JDK String的定义

public final class String  implements java.io.Serializable, Comparable<String>, CharSequence 
这里满足String是Comparable<String>的子类，而Comparable中的T就是String本身