HashMap的使用

参考的文档

阅读JDK1.8 HashMap源码分析
阅读源码

jdk1.8 HashMap的数据结构

数组+链表+红黑树
HashMap使用hash()方法来决定元素在数组位置，因为hash算法会出现过个元素的的hash()返回的值是一样的，它用链表存放这些hash值相同的元素，jdk1.6是用数组和链表实现的，在1.8里面当链表的长度（阀值）超过8，就会转为红黑树。

一些方法的使用

package com.fang.javalearn.javalearn.collection.map;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Random;
import java.util.Map.Entry;
import java.util.concurrent.CopyOnWriteArrayList;

import lombok.extern.slf4j.Slf4j;

@Slf4j
public class HashMapTest {
    public static void main(String[] args) {
        testHashMap();
    }

    private static void testHashMap() {
        Random ran = new Random();
        HashMap<String, Integer> map = new HashMap<>();
        map.put("one", ran.nextInt(100));
        map.put("two", ran.nextInt(100));
        map.put("three", ran.nextInt(100));

        // one iterator
        /*
         * Iterator it = map.entrySet().iterator(); while (it.hasNext()) { Entry entry =
         * (Entry) it.next(); log.info("key:" + entry.getKey() + ",value:" +
         * entry.getValue()); }
         */

        // two iterator 泛型
        Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, Integer> entry = iterator.next();
            log.info("key:" + entry.getKey() + ",value:" + entry.getValue());
        }
        //
        log.info("map size {}", map.size());

        // contains key
        log.info("map contains key " + "one:" + map.containsKey("two"));
        log.info("map contains key" + "nine:" + map.containsKey("nine"));

        // contains value
        log.info("contain value  0:" + map.containsValue(new Integer(0)));

        // remove
        map.remove("one");
        log.info("map contains key " + "one:" + map.containsKey("two"));

        // clear（）
        map.clear();
        while (iterator.hasNext()) {
            Map.Entry<String, Integer> entry = iterator.next();
            log.info("key:" + entry.getKey() + ",value:" + entry.getValue());
        }
        log.info("map size {}", map.size());

    }
}