WeakedHashMap会在key对象已经没有引用时，如果调用System.gc()会尝试remove这个key

import java.util.*;public class WeakedHashMap {    public static void main(String args[]){    	Map
    
      map = new WeakHashMap
     
      ();    	//Map
      
        map = new HashMap
       
        ();      	String alibaba = new String("TEST");        map.put(alibaba, "TEST");           while (map.containsKey("TEST")) {               try {                   Thread.sleep(500);                   alibaba = null;             } catch (InterruptedException ignored) {                }                System.out.println("Checking for empty");                System.gc();           }      }}