通配符、通配符上限、通配符下限

通配符：List<?> 意思是未知类型元素的List

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public void test(List<?> c) {     for(int i=0;i<c.size();i++)     {         System.out.println(c.get(i));     } }

通配符上限：List<? extends Shape>    代表Shape或Shape的子类型

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public class RightTest {     static<T> void test(Collection<? extends T> from,Collection<T> to)     {         for(T ele:from)         {             to.add(ele);         }     }     public static void main(String[] args)     {         List<Object> ao=new ArrayList<>();         List<String> as=new ArrayList<>();         test(as,ao);     } }

通配符下限：List<? super Type>   表示Type本身或者Type的父类

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package test;   import java.util.*;   public class TreeSetTest {     public static void main(String[] args)     {         TreeSet<String> ts1=new TreeSet<>(                 new Comparator<Object>()                 {                     public int compare(Object fst,Object snd)                     {                         return fst.hashCode()>snd.hashCode()? 1                                 :fst.hashCode()<snd.hashCode()? -1:0;                     }                 });         ts1.add("hello");         ts1.add("dao");         TreeSet<String> ts2=new TreeSet<>(                 new Comparator<String>()                 {                     public int compare(String first,String second)                     {                         return first.length()>second.length()? -1                                 :first.length()<second.length()? 1:0;                     }                 });         ts2.add("hello");         ts2.add("dao");         System.out.println(ts1);         System.out.println(ts2);     } }

因为TreeSet(Comparator<? super E> c)

可以传入String或者String的父类Object