java基础进阶篇:hashmap底层探究

JDK1.7版  底层使用 数组+链表实现

package com.company;

import java.util.HashMap;

/**
 * @author Vijay
 * @date 2020/4/28  -18:47
 */
public class MyHashMap<K,V> {


    private  Entry[]  table;  //定义数组,也就是哈希表
    private  static  final Integer CAPACITY=8;  //默认为数组长度为8  0-7
    private int size=0;  //map长度

        public MyHashMap()
        {
            this.table=new Entry[CAPACITY];//当map对象创建时,让数组为8个类型为Entry类型
        }
       public  Object get(Object key)
      {
        int i = key.hashCode();
        int index=i%8;
        for (Entry<K,V> entry=table[index] ;entry!=null;entry=entry.next) {
            if (entry.k.equals(key))
            {
                return  entry.v;
            }
        }
        return  null;
    }
      public  Object put(K key,V value)    //模仿map存值
      {
          int i = key.hashCode();  //用map的key获取hashcode  对8取余，获取数组下标位置
          int index=i%8;  
         //用for循环判断新存的值是否与存入的值相同，相同就不替换，

          for (Entry<K,V> entry=table[index] ;entry!=null;entry=entry.next) {
              if (entry.k.equals(key))
              {
                  V oldValue=entry.v;
                  entry.v=value;
                  return  oldValue;  //不替换
              }
          }
          addEntry(key, value, index);
          return  null;
      }
    //添加元素
    private void addEntry(K key, V value, int index) {
        Entry entry = new Entry<>(key, value, table[index]);
        table[index]=entry;
        size++;//元素加1
    }

    public int size()
      {
         //还回元素
          return  size;
      }

    public static void main(String[] args) {

       MyHashMap<String, String > map = new MyHashMap<>();
        map.put("张三","sad");
        map.put("李四","sads");
        map.put("王五","sad2");
        map.put("奥迪","sadd");
        map.put("大蘇打","sad3");

    }

      class Entry<K,V>
      {
                 private K k;
          private  V v;
          private  Entry<K,V> next;

          public K getK() {
              return k;
          }

          public V getV() {
              return v;
          }

          public Entry<K, V> getNext() {
              return next;
          }

          public Entry(K k, V v, Entry<K, V> next) {
              this.k = k;
              this.v = v;
              this.next = next;//存下一个
          }
      }

}