Map源码解读

一、Map

key不允许重复，和HashSet一样。 可以为null，但是只能包含一个null
value允许重复，可以包含多个null

二、HashMap

• 具体结构在前面HashSet已经介绍完毕
• 是线程不安全的

1. 数据结构

 private static void method01() {
     HashMap<String, String> map = new HashMap<>();
     map.put("name", "erick");

     /*是为了更好的遍历，底层是
     * 1. 内部类Map.Entry<String, String>
        static class Node<K,V> implements Map.Entry<K,V>
       2. 将entry放在set中
       3. 将node节点的引用放在set中*/
     Set<Map.Entry<String, String>> entries = map.entrySet();
     for (Map.Entry<String, String> entry : entries) {
         String key = entry.getKey();
         String value = entry.getValue();
     }
 }

static final float DEFAULT_LOAD_FACTOR = 0.75f;

public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

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

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

# 存放的元素是node
- Node<K,V> implements Map.Entry<K,V> 

- k为存入的key, v为对应的value

- 某个节点的元素，如果是之前是树，后续当树的节点减少到6个，就会
  重新变成链表

- 重复元素时候，替换对应的value，key不变

2. 遍历方式

package com.erick.feature.collection.d02;

import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class Demo05 {
    public static void main(String[] args) {
        HashMap<String, String> map = new HashMap<>();
        map.put("name", "erick");
        map.put("age", "11");
        method03(map);
    }

    private static void method01(Map<String, String> map) {
        Set<Map.Entry<String, String>> entries = map.entrySet();
        /*1. 通过获取到entry集合，增强for*/
        for (Map.Entry<String, String> entry : entries) {
            String key = entry.getKey();
            String value = entry.getValue();
        }
    }

    private static void method02(Map<String, String> map) {
        Set<Map.Entry<String, String>> entries = map.entrySet();
        /*2. 通过获取到entry集合，迭代器*/
        Iterator<Map.Entry<String, String>> iterator = entries.iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, String> next = iterator.next();
            String key = next.getKey();
            String value = next.getValue();
        }
    }

    private static void method03(Map<String, String> map) {
        /*3. 获取到所有的value，然后来遍历*/
        Collection<String> values = map.values();
        for (String value : values) {
            System.out.println(value);
        }
    }

    private static void method04(Map<String, String> map) {
        /*4. 获取到所有的value，然后来遍历*/
        Collection<String> values = map.values();
        Iterator<String> iterator = values.iterator();
        while (iterator.hasNext()){
            System.out.println(iterator.next());
        }
    }
}