Java之HashMap.values()方法的误用解读

package collections;
 
import java.util.HashMap;
import java.util.List;
import java.util.Map;
 
public class Test {
 
    /**
     * @param args
     */
    public static void main(String[] args) {
          
        Map<String,String> map = new HashMap<String,String>();
        map.put("A", "A");
        map.put("B", "B");
        map.put("C", "C");
        map.put("D", "D");
        map.put("E", "E");
        List<String> valuesList = (List<String>) map.values();
        for(String str:valuesList){
            System.out.println(str);
        }
    }
 
}　　

public Collection<V> values() {
      Collection<V> vs = values;
      return (vs != null ? vs : (values = new Values()));
  }

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    size = elementData.length;
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
        elementData = Arrays.copyOf(elementData, size, Object[].class);
}　　

package collections;
 
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
 
public class Test {
 
    /**
     * @param args
     */
    public static void main(String[] args) {
          
        Map<String,String> map = new HashMap<String,String>();
        map.put("A", "A");
        map.put("B", "B");
        map.put("C", "C");
        map.put("D", "D");
        map.put("E", "E");
        //List<String> valuesList = (List<String>) map.values();
        List<String> valuesList = new ArrayList<String>(map.values());
        for(String str:valuesList){
            System.out.println(str);
        }
    }
}

package a;
import java.util.HashMap;

import java.util.Map.Entry;
public class Main{
    public static void main(String[] args){
    	HashMap<String,Integer> mp = new HashMap<String,Integer>();
    	mp.put("one",1);	//存放键值对
    	System.out.println(mp.get("one"));	//通过键取值，输出 1
    	System.out.println(mp.get("1"));	//通过键取值，不存在，输出 null
    	System.out.println("====================");
    	
    	System.out.println(mp.containsKey("one"));	//HashMap中是否包含该键，输出true
    	System.out.println(mp.containsKey("two"));	//不包含该键，输出false
    	System.out.println("====================");
    	
    	System.out.println(mp.containsValue(1));	//HashMap中是否包含该值，输出true
    	System.out.println(mp.containsValue(2));	//不包含该值，输出false
    	System.out.println("====================");
    	
    	System.out.println(mp.isEmpty());	//判断是否为空，输出false
    	System.out.println(mp.size()); 		//输出 HasMap 的长度，1
    	System.out.println("====================");
    	
    	mp.remove("one");	//从HashMap中删除该键，值也会被删除
    	System.out.println(mp.get("one"));	//输出null
    	System.out.println(mp.containsKey("one"));	//输出false
    	System.out.println(mp.containsValue(1));	//输出false
    	//也可以通过 mp.remove("one",1); 把键和值一起删掉
    	System.out.println("====================");
    	
    	mp.put("one", 1);
    	mp.put("two", 2);
    	mp.put("three", 3);
    	System.out.println(mp.values());//输出所有值，[1, 2, 3]
    	System.out.println(mp.keySet());//输出所有键，[one, two, three]
    	System.out.println(mp.entrySet());//输出所有键和值，[one=1, two=2, three=3]，中括号
    	System.out.println("====================");
    	
    	HashMap<String,Integer> mp2 = new HashMap<String,Integer>();
    	mp2.put("four", 4);
    	mp.putAll(mp2);	//添加同类型另一个HashMap，放进头部
    	System.out.println(mp);	//输出整个HashMap的键和值，{four=4, one=1, two=2, three=3}，大括号
    	System.out.println("====================");
    	
    	mp.replace("one", 5);	//替换键的值，java8才有
    	mp.replace("two", 2 , 6);	//替换键的旧值为新值
    	System.out.println(mp);	//输出{four=4, one=5, two=6, three=3}
    	System.out.println("====================");
    	
    	Object mp3 = mp.clone();	//克隆一个，顺序随机
    	System.out.println(mp3);	//输出{two=6, three=3, four=4, one=5}
    	System.out.println("====================");
    	
    	for(String key:mp.keySet())	//遍历整个HashMap的键
    		System.out.print(key+' ');//输出four one two three 
    	System.out.println();
    	for(Integer values:mp.values())	//遍历整个HashMap的值
    		System.out.print(values+' ');//输出36373835，并不是4 5 6 3 ，说明该方法不能输出值
    	System.out.println();
    	for(Entry<String,Integer> entry:mp.entrySet()) {	//遍历整个HashMap，输出键值
    		String key = entry.getKey();
    		Integer value = entry.getValue();
    		System.out.print(key+'='+value+' ');	//输出four=4 one=5 two=6 three=3 
    	}
    	System.out.println();
    	System.out.println("====================");
    	
    	mp.clear();	//清空数组
    	System.out.println(mp); 	//输出{}
    	System.out.println("====================");
    }
}