基于Java快速实现一个简单版的HashMap详解

package com.zaevn.hashmap;

/**
 * @author: zae
 * @date: 2023/1/30
 * @time: 11:25
 */
public class Node {

    String key;
    String value;
    Node next;

    public Node(String key, String value, Node nextNode) {
        this.key = key;
        this.value = value;
        this.next = nextNode;
    }
}

package com.zaevn.hashmap;

/**
 * @author: zae
 * @date: 2023/1/30
 * @time: 11:27
 */
public class ListNode {
    // 头节点
    Node head;

    /**
     * 添加数据，挂载链表的节点
     * @param key
     * @param value
     */
    public void addNode(String key,String value){
        // 如果头节点是空,则结束
        if(head == null ){return;}

        // 如果头节点不为空，则往下挂载节点
        Node node = new Node(key,value,null);
        Node temp = head;
        while(true){
            // 遇到相同的key,覆盖数据
            if(key.equals(temp.key)){
                temp.value = value;
                return;
            }

            if(temp.next == null){
                break;
            }
            temp = temp.next;
        }
        // 循环结束后则挂上数据
        temp.next = node;
    }

    /**
     * 获取数据
     * @param key
     * @return
     */
    public String getNode(String key){
        if(head == null ){return null;}

        Node temp = head;
        while(true){
            if(key.equals(temp.key)){
                return temp.value;
            }
            if(temp.next == null){
                break;
            }
            temp = temp.next;
        }
        return null;
    }
}

package com.zaevn.hashmap;

/**
 * @author: zae
 * @date: 2023/1/30
 * @time: 11:27
 */
public class MyHashMap {
    // 数组初始化:2的n次方
    ListNode[] map = new ListNode[8];
    // ListNode的个数
    int size;

    // 由于扩容时是先创建一个新数组，因此先声明出来
    ListNode[] mapNew;
    int sizeNew;

    /**
     * put方法
     * @param key
     * @param value
     */
    public void put(String key,String value){
        if(size>map.length * 0.75){
            System.out.println("开始进行扩容，当前size="+size+",数组长度为："+map.length);
            doExtendMap();
            System.out.println("扩容结束，当前size="+size+",数组长度为："+map.length);
        }

        // 1.对key进行hash算法然后取模
        int index = Math.abs(key.hashCode())%map.length;

        ListNode listNode = map[index];
        // 如果索引位置的元素为空，则新加一个元素(创建头节点)
        if(listNode == null){
            ListNode listNodeNew = new ListNode();
            Node node = new Node(key,value,null);
            listNodeNew.head = node;
            map[index] = listNodeNew;
            size ++;
        }else{
            // 如果索引位置的元素不为空，则往链表中挂载数据
           listNode.addNode(key,value);
        }
    }

    public String get(String key){
        // 1.对key进行hash算法然后取模
        int index = Math.abs(key.hashCode())%map.length;

        if(map[index] == null){
            return null;
        }else{
            return map[index].getNode(key);
        }
    }

    /**
     * 达到阈值后开始进行扩容
     */
    public void doExtendMap(){
        sizeNew = 0;
        // 1.先创建一个新的数组，长度为原来的二倍
        mapNew = new ListNode[map.length * 2];

        // 2.将旧数据映射到新的数组上(因为数组长度变化，因此hash规则变化，所有的值需要重新计算hash值)
        for(int i = 0;i<map.length;i++){
            ListNode listNode = map[i];
            if(listNode == null){
                continue;
            }
            Node temp = listNode.head;
            while (true){
                doPutData(mapNew,temp.key,temp.value);
                if(temp.next == null){
                    break;
                }
                temp = temp.next;
            }
        }

        // 3.将新的数组替换旧的数组
        map = mapNew;
        this.size = sizeNew;
    }

    private void doPutData(ListNode[] mapParam,String key,String value){
        int index = Math.abs(key.hashCode())%mapParam.length;
        ListNode listNode = mapParam[index];
        if(listNode == null){
            ListNode listNodeNew = new ListNode();
            Node node = new Node(key,value,null);
            listNodeNew.head = node;
            mapParam[index] = listNodeNew;
            sizeNew ++;
        }else{
            listNode.addNode(key,value);
        }
    }

    public static void main(String[] args) {
        // 1、一般校验
        MyHashMap hashMap0=new MyHashMap();
        hashMap0.put("key1","value1");
        System.out.println("一般校验："+hashMap0.get("key1"));
        System.out.println("--------------------------------------------");


        // 2、同key覆盖校验
        MyHashMap hashMap1=new MyHashMap();
        hashMap1.put("key2","value00");
        hashMap1.put("key2","value01");
        System.out.println("同key覆盖校验："+hashMap1.get("key2"));
        System.out.println("--------------------------------------------");

        // 3、哈希碰撞校验(k1和k9的经过哈希计算后得到的索引都是6)
        MyHashMap hashMap2=new MyHashMap();
        hashMap2.put("k1","value_k1");
        hashMap2.put("k9","value_k9");
        System.out.println("哈希碰撞校验：k1:"+hashMap2.get("k1")+"  k9:"+hashMap2.get("k9"));
        System.out.println("--------------------------------------------");


        // 4、扩容校验
        MyHashMap hashMap3=new MyHashMap();
        hashMap3.put("m3","cccccc");
        hashMap3.put("c1","kkkkkk");
        hashMap3.put("c2","mmmmmmm");
        hashMap3.put("b1","bbbbbbb");
        hashMap3.put("m1","cccccc");
        hashMap3.put("c3","kkkkkk");
        hashMap3.put("c4","mmmmmmm");
        hashMap3.put("b2","bbbbbbb");
        hashMap3.put("m2","cccccc");
        hashMap3.put("c5","kkkkkk");
        hashMap3.put("c6","mmmmmmm");
        hashMap3.put("b3","bbbbbbb");
        System.out.println("扩容后的c4:"+hashMap3.get("c4"));
        System.out.println("扩容后的b3:"+hashMap3.get("b3"));
    }

}