HashMap的遍歷

總的來講, 有5種遍歷, 其中1種是僅遍歷了values, 4種徹底遍歷了整個的map, 分類和原理介紹以下

劃分

Map的遍歷:
遍歷方式: 分爲foreach和iterator兩種, foreach效率好一點. 這是方式上面的區別, 還有在使用的遍歷因子的不一樣;
遍歷因子: 按照遍歷因子來劃分, 則有keySet()和entrySet()兩種, map的內部功能有Map.Entry接口實現, keySet()是直接取到keys, entrySet是首先取到每個entry, 而後使用entry的getKey() 和 getValue()來完整的實現遍歷, 速度要更好.

CreateTraversal.java(使用例子)

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("天海翼", "36C");
        map.put("小澤瑪利亞", "42F");
        map.put("", "");//能夠的, 這樣定義也能夠
        map.put(null, null);/*key能夠爲null, 可是隻能有一個, 只有一個起做用*/
        map.put(null, null);
        map.put("波多野結衣", "36C");
        map.put("蒼井空", "28B");
        map.put(" ", " ");//能夠的, 這樣定義也能夠
        map.put("小倉優子", "26A");
        map.put("深田恭子", null);
        map.put("雨宮琴音", "36C");
        map.put("北條麻妃", null);//value爲null, 資料暫空, value爲null能夠有多個
        map.put("青山葵", "43F");

        return map;
    }

    public static void traversalMap(Map<String, Object> map, int type) {
        if (type < 0 || type > 4) {
            log.warn("錯誤的遍歷類型, 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("推薦map.entrySet()+iterator");
                iteratorEntrySetTraversal(map);
                break;
            default:
                iteratorEntrySetTraversal(map);

        }
    }

    /*只是遍歷了values, 沒有遍歷keys*/
    private static void partForEachTraversal(Map<String, Object> map) {
        System.out.println("\n使用values來循環, 只能遍歷values, 沒法遍歷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的大小n: " + map.values().size());
        System.out.println("運行時間爲: " + time + "ms");
        System.out.println("");

    }

    /*map.keySet()+foreach*/
    private static void forEachKeySetTraversal(Map<String, Object> map) {
        System.out.println("map.keySet()+foreach, 完成遍歷, 速度慢!");
        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的大小n: " + map.values().size());
        System.out.println("運行時間爲: " + time + "ms");
        System.out.println("");
    }

    /*map.entrySet()+foreach*/
    private static void forEachEntrySetTraversal(Map<String, Object> map) {
        System.out.println("map.entrySet()+foreach, 完成遍歷, 速度快!");
        System.out.println("Map.Entry是Map類內部的一個接口,提供了Map類的主體方法和功能");
        long start = System.currentTimeMillis();
        Set<Map.Entry<String, Object>> entrySet = map.entrySet();//把這個set取出來
        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的大小n: " + map.values().size());
        System.out.println("運行時間爲: " + time + "ms");
        System.out.println("");
    }


    /*map.keySet()+iterator*/
    private static void iteratorKeySetTraversal(Map<String, Object> map) {
        System.out.println("map.keySet()+iterator, 完成遍歷");
        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的大小n: " + map.values().size());
        System.out.println("運行時間爲: " + time + "ms");
        System.out.println("");
    }

    /*map.entrySet()+iterator*/
    private static void iteratorEntrySetTraversal(Map<String, Object> map) {
        System.out.println("map.entrySet()+iterator, 完成遍歷, 推薦的作法!");
        long start = System.currentTimeMillis();
        //Iterator it = map.entrySet().iterator();/*---*/
        Iterator<Map.Entry<String, Object>> it = map.entrySet().iterator();//上下相同, 這個清晰一些
        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的大小n: " + map.values().size());
        System.out.println("運行時間爲: " + time + "ms");
        System.out.println("");
    }

    public static void main(String[] args) {
        System.out.println("因爲map使用key-value形式的數據結構,因此沒有使用數字形式的位置,不可以使用for(int i=0;i<n;i++形式)");
        Map<String, Object> mp = createMap();
        traversalMap(mp, 0);
        traversalMap(mp, 1);
        traversalMap(mp, 2);
        traversalMap(mp, 3);
        traversalMap(mp, 4);
    }
}

ref:

1.遍歷HashMap的四種方法, 2.HashMap遍歷和使用, 3.Java中如何遍歷Map對象的4種方法, 4.Java中遍歷Map的各類方式, 5.HashMap遍歷的兩種方式，推薦使用entrySet(), 6.HashMap循環遍歷方式及其性能對比, 7.JCFInternals