类集框架(三)

 1 import java.util.Map;
 2 import java.util.HashMap;
 3 
 4 public class Test{
 5     public static void main(String args []){
 6         HashMap<String,String> hashMap = new HashMap<String,String>();
 7         Map<String,String> map = hashMap;  //以后经常会这样向上转型为接口
 8         
 9         map.put("1","a");
10         map.put("2","b");
11         map.put("3","c");
12         map.put("3","c");
13         map.put("4","d");
14         map.put("3","e");  //键相同, 后面会把前面的覆盖掉
15         
16         int i = map.size();
17         System.out.println(i);
18         
19         String s = map.get("4");
20         System.out.println(s);
21     }
22 }