import java.util.*;

import java.util.concurrent.ConcurrentHashMap;

public class MapDemo {

public static void main(String[] args) {

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

map.put("1", "Value1");

map.put("2", "Value2");

map.put("3", "Value3");

map.put(null, "11");

//第一种：普遍使用，二次取值

System.out.println("通过Map.keySet遍历key和value：");

for (String key :

map.keySet()) {

System.out.println(key);

}

for (String value :

map.values()) {

System.out.println(value);

}

//第二种

System.out.println("通过Map.entrySet使用iterator遍历key和value：");

Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();

while (iterator.hasNext()) {

Map.Entry<String, String> entry = iterator.next();

System.out.println("key=" + entry.getKey() + " and value=" + entry.getValue());

}

//第三种：推荐，尤其是容量大时

System.out.println("通过Map.entrySet遍历key和value");

for (Map.Entry<String, String> entry : map.entrySet()

) {

System.out.println("key=" + entry.getKey() + " and value=" + entry.getValue());

}

//第四种：尤其推荐

map.forEach((k, v) -> System.out.println("Key=" + k + ",Value=" + v));

Vector vector = new Vector();

vector.add("11");

// Map<String,Integer> stringIntegerMap=new ConcurrentHashMap<>();

// stringIntegerMap.put(null,11);

HashSet hashSet = new HashSet();

hashSet.add(11);

}

}