两种HashMap遍历方式

一般来说，有5种遍历，其中一种只遍历值，4种完全遍历整个地图。分类及原理介绍如下

划分

Map遍历：

遍历方法：分为foreach和iterator两种，foreach效率更高。这就是使用的方法和遍历因子的区别;

遍历因子：按照遍历因子划分，那么有keySet()和entrySet()两种，map的内部函数是通过Map.Entry接口实现的，keySet()是直接获取key，entrySet是获取每个条目先，然后使用条目的 getKey() 和 getValue() 完成遍历，速度会更好。

CreateTraversal.java(使用示例)

import lombok.extern.slf4j.Slf4j;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
/**
 * @Author: renjiaxin
 * @Despcription:
 * @Date: Created in 2018/8/17 9:52
 * @Modified by:
 */
@Slf4j
public class CreateTraversal {
    public static Map<String, Object> createMap() {
        Map<String, Object> map = new HashMap<>();
        map.put("Sky Sea Wing", "36C");
                 map.put("Ozawa Maria", "42F");
                 map.put("", "");//Yes, it can be defined like this
                 map.put(null, null);/*key can be null, but there can only be one, and only one works*/
        map.put(null, null);
                 map.put("Hatano Yui", "36C");
                 map.put("Aoi Sola", "28B");
                 map.put(" ", "");//Yes, it can be defined like this
                 map.put("Ogura Yuko", "26A");
                 map.put("Fukada Kyoko", null);
                 map.put("Amamiya piano sound", "36C");
                 map.put("Hojo Mafei", null);//value is null, data is temporarily empty, value is null and there can be multiple
                 map.put("Green Wasabi", "43F");
        return map;
    }
    public static void traversalMap(Map<String, Object> map, int type) {
        if (type < 0 || type > 4) {
                         log.warn("Wrong traversal type, type:{}", type);
        }
        switch (type) {
            case 0:
                partForEachTraversal(map);
                break;
            case 1:
                forEachKeySetTraversal(map);
                break;
            case 2:
                forEachEntrySetTraversal(map);
                break;
            case 3:
                iteratorKeySetTraversal(map);
                break;
            case 4:
                                 System.out.println("Recommended map.entrySet()+iterator");
                iteratorEntrySetTraversal(map);
                break;
            default:
                iteratorEntrySetTraversal(map);
        }
    }
         /*It just traversed the values, not the keys*/
    private static void partForEachTraversal(Map<String, Object> map) {
                 System.out.println("\nUse values ​​to loop, can only traverse values, not keys");
        long start = System.currentTimeMillis();
        for (Object value : map.values()) {
            System.out.print(value + " ");
        }
        long end = System.currentTimeMillis();
        long time = start - end;
                 System.out.println("\nmap size n: "+ map.values().size());
                 System.out.println("The running time is: "+ time + "ms");
        System.out.println("");
    }
    /*map.keySet()+foreach*/
    private static void forEachKeySetTraversal(Map<String, Object> map) {
                 System.out.println("map.keySet()+foreach, complete traversal, slow speed!");
        long start = System.currentTimeMillis();
        for (String key : map.keySet()) {
            System.out.print("key: " + key + ", values: " + map.get(key) + "; ");
        }
        long end = System.currentTimeMillis();
        long time = start - end;
                 System.out.println("\nmap size n: "+ map.values().size());
                 System.out.println("The running time is: "+ time + "ms");
        System.out.println("");
    }
    /*map.entrySet()+foreach*/
    private static void forEachEntrySetTraversal(Map<String, Object> map) {
                 System.out.println("map.entrySet()+foreach, complete traversal, fast!");
                 System.out.println("Map.Entry is an interface inside the Map class, which provides the main methods and functions of the Map class");
        long start = System.currentTimeMillis();
                 Set<Map.Entry<String, Object>> entrySet = map.entrySet();//Take out this set
        for (Map.Entry<String, Object> entry : entrySet) {/*---*/
            System.out.print("key= " + entry.getKey() + " and value= " + entry.getValue() + "; ");
        }
        long end = System.currentTimeMillis();
        long time = start - end;
                 System.out.println("\nmap size n: "+ map.values().size());
                 System.out.println("The running time is: "+ time + "ms");
        System.out.println("");
    }
    /*map.keySet()+iterator*/
    private static void iteratorKeySetTraversal(Map<String, Object> map) {
                 System.out.println("map.keySet()+iterator, complete traversal");
        long start = System.currentTimeMillis();
        Iterator<String> it = map.keySet().iterator();/**/
        while (it.hasNext()) {
            System.out.print("key :" + it.next() + " , value: " + map.get(it.next()));
        }
        long end = System.currentTimeMillis();
        long time = start - end;
                 System.out.println("\nmap size n: "+ map.values().size());
                 System.out.println("The running time is: "+ time + "ms");
        System.out.println("");
    }
    /*map.entrySet()+iterator*/
    private static void iteratorEntrySetTraversal(Map<String, Object> map) {
                 System.out.println("map.entrySet()+iterator, complete traversal, the recommended approach!");
        long start = System.currentTimeMillis();
        //Iterator it = map.entrySet().iterator();/*---*/
                 Iterator<Map.Entry<String, Object>> it = map.entrySet().iterator();//Same up and down, this one is clearer
        while (it.hasNext()) {
            System.out.print("key :" + it.next() + " , value: " + map.get(it.next()));
        }
        long end = System.currentTimeMillis();
        long time = start - end;
                 System.out.println("\nmap size n: "+ map.values().size());
                 System.out.println("The running time is: "+ time + "ms");
        System.out.println("");
    }
    public static void main(String[] args) {
                 System.out.println("Because map uses a key-value form of data structure, there is no position in digital form, so for(int i=0;i<n;i++ form)");
        Map<String, Object> mp = createMap();
        traversalMap(mp, 0);
        traversalMap(mp, 1);
        traversalMap(mp, 2);
        traversalMap(mp, 3);
        traversalMap(mp, 4);
    }
}