重写Object.hashCode如何提高hashtable的效率

首先，我个人对Hash算法不熟悉，参考了这位大神的博客:

http://www.cnblogs.com/dolphin0520/archive/2012/09/28/2700000.html

粗略的理解是，Hash算法在分块的时候，如果分的过多会导致查找元素效率低。并做了以下的实验:

/*
 * Copyright 2001-2004 The Apache Software Foundation.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
/*
 * $Id: Hashtable.java,v 1.2.4.1 2005/09/06 11:05:18 pvedula Exp $
 */

import java.awt.List;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.TreeMap;

/**
 * IMPORTANT NOTE:
 * This code was taken from Sun's Java1.1 JDK java.util.HashTable.java
 * All "synchronized" keywords and some methods we do not need have been 
 * all been removed.
 */

/**
 * Object that wraps entries in the hash-table
 * 
 * @author Morten Jorgensen
 */
class HashtableEntry {
    int hash;
    Object key;
    Object value;
    HashtableEntry next;

    protected Object clone() {
        HashtableEntry entry = new HashtableEntry();
        entry.hash = hash;
        entry.key = key;
        entry.value = value;
        entry.next = (next != null) ? (HashtableEntry) next.clone() : null;
        return entry;
    }
}

/**
 * The main hash-table implementation
 */
public class MyHashTable {

    private transient HashtableEntry table[]; // hash-table entries
    private transient int count; // number of entries
    private int threshold; // current size of hash-tabke
    private float loadFactor; // load factor

    private HashMap<Integer, Integer> indexcount = new HashMap<Integer, Integer>();

    /**
     * Constructs a new, empty hashtable with the specified initial capacity and
     * the specified load factor.
     */
    public MyHashTable(int initialCapacity, float loadFactor) {
        if (initialCapacity <= 0)
            initialCapacity = 11;
        if (loadFactor <= 0.0)
            loadFactor = 0.75f;
        this.loadFactor = loadFactor;
        table = new HashtableEntry[initialCapacity];
        threshold = (int) (initialCapacity * loadFactor);
    }

    /**
     * Constructs a new, empty hashtable with the specified initial capacity and
     * default load factor.
     */
    public MyHashTable(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty hashtable with a default capacity and load
     * factor.
     */
    public MyHashTable() {
        this(101, 0.75f);
    }

    /**
     * Returns the number of keys in this hashtable.
     */
    public int size() {
        return count;
    }

    /**
     * Tests if this hashtable maps no keys to values.
     */
    public boolean isEmpty() {
        return count == 0;
    }

    /**
     * Returns an enumeration of the keys in this hashtable.
     */
    public Enumeration keys() {
        return new HashtableEnumerator(table, true);
    }

    /**
     * Returns an enumeration of the values in this hashtable. Use the
     * Enumeration methods on the returned object to fetch the elements
     * sequentially.
     */
    public Enumeration elements() {
        return new HashtableEnumerator(table, false);
    }

    /**
     * Tests if some key maps into the specified value in this hashtable. This
     * operation is more expensive than the <code>containsKey</code> method.
     */
    public boolean contains(Object value) {

        if (value == null)
            throw new NullPointerException();

        int i;
        HashtableEntry e;
        HashtableEntry tab[] = table;

        for (i = tab.length; i-- > 0;) {
            for (e = tab[i]; e != null; e = e.next) {
                if (e.value.equals(value)) {
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Tests if the specified object is a key in this hashtable.
     */
    public boolean containsKey(Object key) {
        HashtableEntry e;
        HashtableEntry tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;

        for (e = tab[index]; e != null; e = e.next)
            if ((e.hash == hash) && e.key.equals(key))
                return true;

        return false;
    }

    /**
     * Returns the value to which the specified key is mapped in this hashtable.
     */
    public Object get(Object key) {
        HashtableEntry e;
        HashtableEntry tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;

        for (e = tab[index]; e != null; e = e.next)
            if ((e.hash == hash) && e.key.equals(key))
                return e.value;

        return null;
    }

    /**
     * Rehashes the contents of the hashtable into a hashtable with a larger
     * capacity. This method is called automatically when the number of keys in
     * the hashtable exceeds this hashtable's capacity and load factor.
     */
    protected void rehash() {
        HashtableEntry e, old;
        int i, index;
        int oldCapacity = table.length;
        HashtableEntry oldTable[] = table;

        int newCapacity = oldCapacity * 2 + 1;
        HashtableEntry newTable[] = new HashtableEntry[newCapacity];

        threshold = (int) (newCapacity * loadFactor);
        table = newTable;

        for (i = oldCapacity; i-- > 0;) {
            for (old = oldTable[i]; old != null;) {
                e = old;
                old = old.next;
                index = (e.hash & 0x7FFFFFFF) % newCapacity;
                e.next = newTable[index];
                newTable[index] = e;
            }
        }
    }

    /**
     * Maps the specified <code>key</code> to the specified <code>value</code>
     * in this hashtable. Neither the key nor the value can be <code>null</code>
     * .
     * <p>
     * The value can be retrieved by calling the <code>get</code> method with a
     * key that is equal to the original key.
     */
    public Object put(Object key, Object value) {
        // Make sure the value is not null
        if (value == null)
            throw new NullPointerException();

        // Makes sure the key is not already in the hashtable.
        HashtableEntry e;
        HashtableEntry tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        if (indexcount.get(Integer.valueOf(index)) != null) {
            indexcount.put(Integer.valueOf(index),
                    indexcount.get(Integer.valueOf(index)) + 1);
        } else {
            indexcount.put(Integer.valueOf(index), 1);
        }

        for (e = tab[index]; e != null; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                Object old = e.value;
                e.value = value;
                return old;
            }
        }

        // Rehash the table if the threshold is exceeded
        if (count >= threshold) {
            rehash();
            return put(key, value);
        }

        // Creates the new entry.
        e = new HashtableEntry();
        e.hash = hash;
        e.key = key;
        e.value = value;
        e.next = tab[index];
        tab[index] = e;
        count++;
        return null;
    }

    public void printIndexCount() {
        Collection<Integer> values = indexcount.values();
        ArrayList<Integer> arrays = new ArrayList<Integer>(values);
        Collections.sort(arrays,new Comparator<Integer>() {

            @Override
            public int compare(Integer o1, Integer o2) {
                return o2.compareTo(o1);
            }
        });
        HashMap<Integer, Integer> hMap = new HashMap<Integer, Integer>();
        for (Integer integer : arrays) {
            if(hMap.containsKey(integer)){
                hMap.put(integer, hMap.get(integer)+1);
            }else{
                hMap.put(integer, 1);
            }
        }
        System.out.println(hMap);
    }

    /**
     * Removes the key (and its corresponding value) from this hashtable. This
     * method does nothing if the key is not in the hashtable.
     */
    public Object remove(Object key) {
        HashtableEntry e, prev;
        HashtableEntry tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (e = tab[index], prev = null; e != null; prev = e, e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                if (prev != null)
                    prev.next = e.next;
                else
                    tab[index] = e.next;
                count--;
                return e.value;
            }
        }
        return null;
    }

    /**
     * Clears this hashtable so that it contains no keys.
     */
    public void clear() {
        HashtableEntry tab[] = table;
        for (int index = tab.length; --index >= 0;)
            tab[index] = null;
        count = 0;
    }

    /**
     * Returns a rather long string representation of this hashtable. Handy for
     * debugging - leave it here!!!
     */
    public String toString() {
        int i;
        int max = size() - 1;
        StringBuffer buf = new StringBuffer();
        Enumeration k = keys();
        Enumeration e = elements();
        buf.append("{");

        for (i = 0; i <= max; i++) {
            String s1 = k.nextElement().toString();
            String s2 = e.nextElement().toString();
            buf.append(s1 + "=" + s2);
            if (i < max)
                buf.append(", ");
        }
        buf.append("}");
        return buf.toString();
    }

    /**
     * A hashtable enumerator class. This class should remain opaque to the
     * client. It will use the Enumeration interface.
     */
    class HashtableEnumerator implements Enumeration {
        boolean keys;
        int index;
        HashtableEntry table[];
        HashtableEntry entry;

        HashtableEnumerator(HashtableEntry table[], boolean keys) {
            this.table = table;
            this.keys = keys;
            this.index = table.length;
        }

        public boolean hasMoreElements() {
            if (entry != null) {
                return true;
            }
            while (index-- > 0) {
                if ((entry = table[index]) != null) {
                    return true;
                }
            }
            return false;
        }

        public Object nextElement() {
            if (entry == null) {
                while ((index-- > 0) && ((entry = table[index]) == null))
                    ;
            }
            if (entry != null) {
                HashtableEntry e = entry;
                entry = e.next;
                return keys ? e.key : e.value;
            }
            return null;
        }
    }

}

public class TestHash {

    public static void main(String[] args) {
        int length = 10000 * 20;
        MyHashTable map = new MyHashTable();
        for (int i = 0; i < length; i++) {
            map.put(new Person(i + "", i + ""), i + "");
        }
        map.printIndexCount();
    }
}

/**
 * 具有相同身份证信息的人就是同一个人
 * 
 * @author Lucifer
 * 
 */
public class Person {

    private String name;
    private String identityCard;
    private long age;

    public Person(String name, String identityCard) {
        this.name = name;
        this.setIdentityCard(identityCard);
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getIdentityCard() {
        return identityCard;
    }

    public void setIdentityCard(String identityCard) {
        this.identityCard = identityCard;
    }

    @Override
    public int hashCode() {
        return this.name.hashCode();
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Person other = (Person) obj;
        if (age != other.age)
            return false;
        if (identityCard == null) {
            if (other.identityCard != null)
                return false;
        } else if (!identityCard.equals(other.identityCard))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }

    public long getAge() {
        return age;
    }

    public void setAge(long age) {
        this.age = age;
    }
}

结果是，如果Person类重写了hashCode方法，hashtable的分块会更加细致。这样查找的效率就提高了。

我的理解有限，如果哪位大神看到了，请指点一二，非常感谢。