Map集合的四种遍历方式

package com.shenfeng;

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map;

public class Map_demo {
public static void main(String args[]){
Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "a");
map.put(2, "b");
map.put(3, "c");
System.out.println(map.size());
//第一种：通过Map.keySet遍历key和value
for (Integer in :map.keySet()) {
String str = map.get(in);
System.out.println(in +"     "+str);

}
//第二种：通过Map.entrySet 使用iterator遍历key和value
Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<Integer, String> entry = it.next();
System.out.println("key: "+entry.getKey() + "value: "+entry.getValue());

}
//第三种：用于数据量大时,通过Map.entrySet遍历key和value
for (Map.Entry<Integer,String> entry : map.entrySet()) {
System.out.println(entry.getKey()+"   "+entry.getValue());

}
//第四种：通过Map.values()遍历所有的value，但不能遍历key
for(String v :map.values()){
System.out.println(v);

}

//使用keySet()迭代
Iterator iterator =map.keySet().iterator();
while (iterator.hasNext()) {
String key;
String value;
key = iterator.next().toString();
value = map.get(key);
System.out.println(key+"     "+value);

}

}

}