Map.Entry

import java.util.HashMap;
import java.util.Map;

public class TestMap {
    public static void main(String[] args) {
        Map<Integer, Integer> map = new HashMap<>();

        map.put(0, 0);
        map.put(1, 2);
        map.put(2, 4);
        map.put(3, 6);

        //entrySet() 方法返回映射中包含的映射的 Set 视图。
        System.out.println(map.keySet());//[0, 1, 2, 3]
        System.out.println(map.values());//[0, 2, 4, 6]

        System.out.println(map.entrySet());//[0=0, 1=2, 2=4, 3=6]

        for(Map.Entry<Integer, Integer> entry : map.entrySet()){
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
            System.out.println(entry.getClass());
        }
    }
}

public static void main(String[] args) {
 
    Map<String, String> map = new HashMap<String, String>();
    map.put("1", "value1");
    map.put("2", "value2");
    map.put("3", "value3");
  
    //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= "+ key + " and value= " + map.get(key));
    }
  
    //第二种
    System.out.println("通过Map.entrySet使用iterator遍历key和value：");
    Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
    while (it.hasNext()) {
        Map.Entry<String, String> entry = it.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
    System.out.println("通过Map.values()遍历所有的value，但不能遍历key");
    for (String v : map.values()) {
        System.out.println("value= " + v);
    }
 }