十二、自己動手實現------------“ HashTable 哈希表 ”

參考文章：

https://www.cnblogs.com/ysocean/p/8032656.html                     Java數據結構和算法（十三）——哈希表（強推）

https://www.cnblogs.com/skywang12345/p/3310835.html           Java 集合系列10之 HashMap詳細介紹(源碼解析)和使用示例

https://github.com/liuyubobobo/Play-with-Algorithms  

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       哈希表也是一種重要的數據結構，其核心思想是 “用空間換時間” ，其查詢操作基本上能達到 O(1)  級別。哈希表的原理，上面兩篇博客應該能夠將的明明白白，代碼部分參考 波波老師  的代碼部分，寫的是非常的優雅。

 

下面不講原理，直接上代碼主體部分：

import java.util.Map;
import java.util.TreeMap;

public class HashTable<K, V> {

    private static final int upperTol = 10;
    private static final int lowerTol = 2;
    private static final int initCapacity = 7;

    private TreeMap<K, V>[] hashtable;
    private int size;
    private int M;

    public HashTable(int M){
        this.M = M;
        size = 0;
        hashtable = new TreeMap[M];
        for(int i = 0 ; i < M ; i ++)
            hashtable[i] = new TreeMap<>();
    }

    public HashTable(){
        this(initCapacity);
    }

    private int hash(K key){
        return (key.hashCode() & 0x7fffffff) % M;
    }

    public int getSize(){
        return size;
    }

    public void add(K key, V value){
        TreeMap<K, V> map = hashtable[hash(key)];
        if(map.containsKey(key))
            map.put(key, value);
        else{
            map.put(key, value);
            size ++;

            if(size >= upperTol * M)
                resize(2 * M);
        }
    }

    public V remove(K key){
        V ret = null;
        TreeMap<K, V> map = hashtable[hash(key)];
        if(map.containsKey(key)){
            ret = map.remove(key);
            size --;

            if(size < lowerTol * M && M / 2 >= initCapacity)
                resize(M / 2);
        }
        return ret;
    }

    public void set(K key, V value){
        TreeMap<K, V> map = hashtable[hash(key)];
        if(!map.containsKey(key))
            throw new IllegalArgumentException(key + " doesn't exist!");

        map.put(key, value);
    }

    public boolean contains(K key){
        return hashtable[hash(key)].containsKey(key);
    }

    public V get(K key){
        return hashtable[hash(key)].get(key);
    }

    private void resize(int newM){
        TreeMap<K, V>[] newHashTable = new TreeMap[newM];
        for(int i = 0 ; i < newM ; i ++)
            newHashTable[i] = new TreeMap<>();

        int oldM = M;
        this.M = newM;
        for(int i = 0 ; i < oldM ; i ++){
            TreeMap<K, V> map = hashtable[i];
            for(K key: map.keySet())
                newHashTable[hash(key)].put(key, map.get(key));
        }

        this.hashtable = newHashTable;
    }
}