一个简单的HashTable

package structures;

import java.util.TreeMap;

public class HashTable<K, V> {
    private TreeMap<K, V>[] hashtable;
    private int M;
    private int size;

    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(97);
    }

    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 ++;
        }
    }

    public V remove(K key) {
        V res = null;

        TreeMap<K, V> map = hashtable[hash(key)];

        if (map.containsKey(key)) {
            res = map.remove(key);
            size ++;
        }

        return res;
    }

    public void set(K key, V value) {
        TreeMap<K, V> map = hashtable[hash(key)];
        if (!map.containsKey(key)) {
            throw new IllegalArgumentException("key is not exists");
        }

        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);
    }
}