Java 常用的三个集合类(9)

import java.util.HashMap;

import java.util.Map;

import java.util.Set;

public class Test {

public static void main(String[] args) {

Map<Integer,String> map = new HashMap<Integer,String>();

map.put(1, "白菜");

map.put(2, "萝卜");

map.put(3, "茄子");

map.put(4, null);

map.put(null, null);

map.put(null, null);

System.out.println("map.size()="+map.size());

System.out.println("map.containsKey(1)="+map.containsKey(2));

System.out.println("map.containsKey(null)="+map.containsKey(null));

System.out.println("map.get(null)="+map.get(null));

System.out.println("map.get(2)="+map.get(2));

map.put(null, "黄瓜");

System.out.println("map.get(null)="+map.get(null));

Set set = map.keySet();

System.out.println("set="+set);

Collection<String> c = map.values();