【九月打卡】第3天 HashMap的使用-原创手记-慕课网

【九月打卡】第3天 HashMap的使用

课程名称：Map从入门到性能分析

课程章节：第2章 HashMap的使用

主讲老师：大谷

课程内容：

HashMap基本用法
HashMap的Entry结构
HashMap遍历-keySet
HashMap遍历-values
HashMap遍历-entrySet
HashMap遍历-Iterator
HashMap遍历-性能分析

课程收获：

通过学习本章节掌握了HashMap的基本用法、Entry结构和几种遍历方法，通过性能分析掌握了每种遍历方法的优劣。

1.Map接口通用方法

V put(K key, V value): Map中存入一个key-value映射
V get(Object key): 返回到指定键所映射的值
int size(): 返回Map中键值映射的数量
V remove(Object key): 从该Map中删除一个键的映射
boolean containsKey(Object key): 是否包含指定键的key

2.HashMap基本用法

2.1HashMap的构造方法

HashMap()
HashMap(int initialCapacity)
HashMap(int initialCapacity, float loadFactor)

2.2创建HashMap对象

Map<String,Object> userMap = new HashMap<>();
userMap.put("zhangsan", new Integer(120));
userMap.get("zhangsan");

3.HashMap的Entry结构

图片描述

static class Entry<K,V> implements Map.Entry<K,V>{
    final K key;
    V value;
    Entry<K,V> next;
    final int hash;
}

Map<String,Object> userMap = new HashMap<>();
userMap.put("zhang1", new Integer(2));
userMap.put("zhang2", new Integer(4));
userMap.put("zhang3", new Integer(3));
userMap.put("zhang4", new Integer(1));
userMap.put("zhang5", new Integer(5));
System.out.println(userMap);
// 输出顺序并没有按照put顺序输出
// {zhang2=4, zhang3=3, zhang1=2, zhang4=1, zhang5=5}

4.利用map.keySet()遍历HashMap

for(String key: map.keySet()){
    System.out.println(key + "***" + map.get(key));
}

public static void main(String[] args) {
    Map map1 = inputMap();
    System.out.println(map1);
    // {zhang2=4, zhang3=3, zhang1=2, zhang4=1, zhang5=5}
    showMap1(map1);
    // zhang2***4
    // zhang3***3
    // zhang1***2
    // zhang4***1
    // zhang5***5

}

public static Map inputMap(){
    Map<String,Object> userMap = new HashMap<>();
    userMap.put("zhang1", new Integer(2));
    userMap.put("zhang2", new Integer(4));
    userMap.put("zhang3", new Integer(3));
    userMap.put("zhang4", new Integer(1));
    userMap.put("zhang5", new Integer(5));
    return userMap;
}

public static void showMap1(Map<String,Object> map){
    for (String key: map.keySet()){
        System.out.println(key+"***"+map.get(key));
    }
}

5.利用entrySet遍历HashMap

for (Map.Entry<String, Object> entry : map.entrySet()) {
    System.out.println(entry.getKey() + "***" + entry.getValue());
}

6.利用Iterator遍历HashMap

Iterator<Map.Entry<String, Object>> it = map.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<String, Object> nextEntry = it.next();
	System.out.println(nextEntry.getKey() + "***" + nextEntry.getValue());
}

7.HashMap遍历-性能分析

public static void main(String[] args) {
    Map map1 = testSpeedMap();
    showMap1(map1);
    showMap2(map1);
    showMap3(map1);
    showMap4(map1);
}

/**
* 利用map.keySet()遍历
*
* @param map
*/
public static void showMap1(Map<String, Object> map) {
    Long start = System.currentTimeMillis();
    for (String key : map.keySet()) {
//            System.out.println(key + "***" + map.get(key));
    }
    Long end = System.currentTimeMillis();
    System.out.println("map.keySet()=" + (end - start));
}

/**
* 利用map.values()遍历
*
* @param map
*/
public static void showMap2(Map<String, Object> map) {
    Long start = System.currentTimeMillis();
    for (Object val : map.values()) {
//            System.out.println(val + "***");
    }
    Long end = System.currentTimeMillis();
    System.out.println("map.values()=" + (end - start));
}

/**
* 利用map.entrySet()遍历
*
* @param map
*/
public static void showMap3(Map<String, Object> map) {
    Long start = System.currentTimeMillis();
    for (Map.Entry<String, Object> entry : map.entrySet()) {
//            System.out.println(entry.getKey() + "***" + entry.getValue());
    }
    Long end = System.currentTimeMillis();
    System.out.println("map.entrySet()=" + (end - start));
}

/**
* 利用Iterator遍历
*
* @param map
*/
public static void showMap4(Map<String, Object> map) {
    Long start = System.currentTimeMillis();
    Iterator<Map.Entry<String, Object>> it = map.entrySet().iterator();
    while (it.hasNext()) {
        Map.Entry<String, Object> nextEntry = it.next();
//            System.out.println(nextEntry.getKey() + "***" + nextEntry.getValue());
    }
    Long end = System.currentTimeMillis();
    System.out.println("Iterator=" + (end - start));
}

public static Map testSpeedMap() {
    Map<String, Object> userMap = new HashMap<>();
    String[] str = new String[]{"a", "s", "d", "f", "g", "h", "j", "k", "l", "q"};
    String key;
    String value;
    // 10W条
    for (int i = 0; i <= 100000; i++) {
        int m = (int) (Math.random() * str.length);
        key = str[m] + i;
        value = "y" + i;
        userMap.put(key, value);
    }
    return userMap;
}

10W条数据：

map.keySet()=20
map.values()=10
map.entrySet()=9
Iterator=10

50W条数据：

map.keySet()=27
map.values()=22
map.entrySet()=22
Iterator=23

100W条数据：

map.keySet()=40
map.values()=34
map.entrySet()=36
Iterator=38

500W条数据：

map.keySet()=172
map.values()=153
map.entrySet()=166
Iterator=160

1000W条数据：

map.keySet()=319
map.values()=294
map.entrySet()=309
Iterator=306

结论：
效率最低： map.keySet()
循环取值用: map.values()
循环遍历用: map.entrySet()或Iterator

图片描述