java.utils.HashMap数据结构分析(转)

 

上图为Hashmap的数据结构图，具体实线是采用数组结合链表实现，链表是为了解决在hash过程中因hash值一样导致的碰撞问题。

所以在使用自定义对象做key的时候，一定要去实现hashcode方法，不然hashmap就成了纯粹的链表，查找性能非常的慢，添加节点元素也非常的慢。如

import java.util.HashMap;

import java.util.Map;

public class User {

private String username;

public boolean equals(Object obj) {

User user=(User)obj;

return username.equals(user.username);}

//手动将hashCode 返回一样的值

public int hashCode() {

return 1;

}

public static void main(String args[]){

Map<User,String>map=new HashMap<User,String>();

for(int i=0;i<10000;i++){

User one=new User();

one.setUsername(i+" user");

map.put(one, i+"");

}

}

}

debug发现，添加9个user对象后数组table的entry通过hash后的到数组index为1，即数组第二个位置，每次都是一样的值，导致hash碰撞，所有的元素都通过链表形式加入到entry当中，并没有均匀分布到16个位置当中（默认使用的map构造方法），所以如果在查找的时候就是纯粹的线性查找（链表）。性能相当相当的低。

 

具体HashMap分析如下：

---------------------------------------------------------------------------------------------

public class HashMap<K,V>

    extends AbstractMap<K,V>

    implements Map<K,V>, Cloneable, Serializable

{

    static final int DEFAULT_INITIAL_CAPACITY = 16;//初始容量

 

    static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量 2的30次方

    static final float DEFAULT_LOAD_FACTOR = 0.75f;//默认加载因子

    transient Entry[] table;//条目（entry），大小跟容量大小一致（capacity）

    transient int size; //map所包含键-值对的数量 每增加一个k-v，根据k来判断是否自增长

    int threshold; //容量与加载因子的乘积，当map的size（entry个数）大于等于这个值时，会重新构造map-table的大小（为原来size的2倍大小，而此时threshold=size＊loadFactor）

    final float loadFactor;//加载因子（人为指定，即在构造对象的时候指定合适的加载因子）

    transient int modCount;//当条目增加或者删除的时候modCount会自增长，这个主要用来在防止在非线程安全下迭代访问map的时候发生变化会抛出ConcurrentModificationException异常

 

    public HashMap(int initialCapacity, float loadFactor) {

        if (initialCapacity < 0)

            throw new IllegalArgumentException("Illegal initial capacity: " +

                                               initialCapacity);

        if (initialCapacity > MAXIMUM_CAPACITY)

            initialCapacity = MAXIMUM_CAPACITY;

        if (loadFactor <= 0 || Float.isNaN(loadFactor))

            throw new IllegalArgumentException("Illegal load factor: " +

                                               loadFactor);

        //Map容量大小必须为2的幂次方，这里通过算法找出合适的容量大小，如您给定initialCapacity为17，它 //的二进制数为10001

       //当capacity16的时候（10000）已经左移了4次，16<17,所以会将capacity再左移1位，即 //32（100000）,所以在创建对象使用

       //Map map=new HashMap(17,0.75)，此时真正capacity=32，而不是你开始给的17.

       //想要创建17个容量大小的时候，实际上为您创建了32个容量大小的map

        int capacity = 1;

        while (capacity < initialCapacity)

            capacity <<= 1;

 

        this.loadFactor = loadFactor;

        threshold = (int)(capacity * loadFactor);//32*0.75=24, 当条目达到24的时候会重新构造map结构

        table = new Entry[capacity];//创建条目，大小为32

        init();

    }

    public HashMap(int initialCapacity) {

        this(initialCapacity, DEFAULT_LOAD_FACTOR);

    }

    public HashMap() {

        this.loadFactor = DEFAULT_LOAD_FACTOR;

        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);//12（默认值）

        table = new Entry[DEFAULT_INITIAL_CAPACITY];//16个（默认值）

        init();

    }

    public HashMap(Map<? extends K, ? extends V> m) {

        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,

                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);

        putAllForCreate(m);

    }

 

    void init() {

    }

 

    static int hash(int h) ｛

        h ^= (h >>> 20) ^ (h >>> 12);

        return h ^ (h >>> 7) ^ (h >>> 4);

    }

 

   //h&(length-1)等价于h%length，取模运算

    static int indexFor(int h, int length) {

        return h & (length-1);

    }

 

    

    public int size() {

        return size;

    }

 

    public boolean isEmpty() {

        return size == 0;

    }

   //根据KEY找出V，如果key==null，会返回table[0](如果table[0]不为null，并且table[0]对应的key==null)

  //如果table[0]不为null，则检查table[0]的下一个节点（线性链表）是否满足上述情况，满足则返回value,否

  //则没有该key对应的value。

 //key不为null，则计算出key的hash，并根据hash得出在table中的位置，该位置不一定是真正Value对应的

 //位置，还要根据table位置的entry的key的hash以及key值进行比较，不相等则要该位置entry的下一个节点

 //是否满足，满足返回，否则返回null

    public V get(Object key) {

        if (key == null)

            return getForNullKey();

        int hash = hash(key.hashCode());

        for (Entry<K,V> e = table[indexFor(hash, table.length)];

             e != null;

             e = e.next) {

            Object k;

            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))

                return e.value;

        }

        return null;

    }

   ......

   ......

//根据Key的hash值得出在table中的位置，该位置可能会被占用，如果占用entry的hash以及key值完全跟put的key相等，则对该entry进行update，如果不相等，则发生了碰撞，测试要判断当期entry是否有（next）下一个节点（entry），有则继续上一步判断，没有则新增一个entry节点到当前节点。

//这里可以hash不可能保证每次都不一样，所以我们使用的key的对象如果是自定义的对象，一定要重写hashcode方法保证每个对象的唯一性，这洋就能减少碰撞，如果hashcode一样，这洋在查找对象的时候等于是线性查找，算法复杂度近似O（n），并不能达到hashmap设计本来近似的O（1)

    public V put(K key, V value) {

        if (key == null)

            return putForNullKey(value);

        int hash = hash(key.hashCode());

        int i = indexFor(hash, table.length);

        for (Entry<K,V> e = table[i]; e != null; e = e.next) {

            Object k;

            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {

                V oldValue = e.value;

                e.value = value;

                e.recordAccess(this);

                return oldValue;

            }

        }

 

        modCount++;

        addEntry(hash, key, value, i);

        return null;

    }

    ......

    ......

    //重构map的大小，及重新hash所有元素

 //newCapacity=table.length*2 (即原始table的大小乘以2），按照前面给定的值，这里是32*2=64

//重构后capacity=64，table的length=64，threshold=64*0.75=48，即当entry的size达到48的时候会再次重构

    void resize(int newCapacity) {

        Entry[] oldTable = table;

        int oldCapacity = oldTable.length;

        if (oldCapacity == MAXIMUM_CAPACITY) {

            threshold = Integer.MAX_VALUE;

            return;

        }

 

        Entry[] newTable = new Entry[newCapacity];

        transfer(newTable);

        table = newTable;

        threshold = (int)(newCapacity * loadFactor);

    }

//entry-table的复制，复制过程中重新计算hash，算出在新table中的位置

    void transfer(Entry[] newTable) {

        Entry[] src = table;

        int newCapacity = newTable.length;

        for (int j = 0; j < src.length; j++) {

            Entry<K,V> e = src[j];

            if (e != null) {

                src[j] = null;

                do {

                    Entry<K,V> next = e.next;

                    int i = indexFor(e.hash, newCapacity);

                    e.next = newTable[i];

                    newTable[i] = e;

                    e = next;

                } while (e != null);

            }

        }

    }

 

    public void putAll(Map<? extends K, ? extends V> m) {

        int numKeysToBeAdded = m.size();

        if (numKeysToBeAdded == 0)

            return;

 

        if (numKeysToBeAdded > threshold) {

            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);

            if (targetCapacity > MAXIMUM_CAPACITY)

                targetCapacity = MAXIMUM_CAPACITY;

            int newCapacity = table.length;

            while (newCapacity < targetCapacity)

                newCapacity <<= 1;

            if (newCapacity > table.length)

                resize(newCapacity);

        }

 

        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())

            put(e.getKey(), e.getValue());

    }

 

   //移除Key对应的entry，如果table中存在因为碰撞问题导致的横向拉链（链表），要对链表进行操作，保证链表的连续性

    public V remove(Object key) {

        Entry<K,V> e = removeEntryForKey(key);

        return (e == null ? null : e.value);

    }

 

    final Entry<K,V> removeEntryForKey(Object key) {

        int hash = (key == null) ? 0 : hash(key.hashCode());

        int i = indexFor(hash, table.length);

        Entry<K,V> prev = table[i];

        Entry<K,V> e = prev;

 

        while (e != null) {

            Entry<K,V> next = e.next;

            Object k;

            if (e.hash == hash &&

                ((k = e.key) == key || (key != null && key.equals(k)))) {

                modCount++;

                size--;

                if (prev == e)

                    table[i] = next;

                else

                    prev.next = next;

                e.recordRemoval(this);

                return e;

            }

            prev = e;

            e = next;

        }

 

        return e;

    }

  .....

......

    public void clear() {

        modCount++;

        Entry[] tab = table;

        for (int i = 0; i < tab.length; i++)

            tab[i] = null;

        size = 0;

    }

 

    .......

    .......

    public Object clone() {

        HashMap<K,V> result = null;

        try {

            result = (HashMap<K,V>)super.clone();

        } catch (CloneNotSupportedException e) {

            // assert false;

        }

        result.table = new Entry[table.length];

        result.entrySet = null;

        result.modCount = 0;

        result.size = 0;

        result.init();

        result.putAllForCreate(this);

 

        return result;

    }

 

   //entry数据结构，真正Key和Value保存的地方

    static class Entry<K,V> implements Map.Entry<K,V> {

        final K key;

        V value;

        Entry<K,V> next;

        final int hash;

 

       

        Entry(int h, K k, V v, Entry<K,V> n) {

            value = v;

            next = n;

            key = k;

            hash = h;

        }

 

        public final K getKey() {

            return key;

        }

 

        public final V getValue() {

            return value;

        }

 

        public final V setValue(V newValue) {

            V oldValue = value;

            value = newValue;

            return oldValue;

        }

 

        public final boolean equals(Object o) {

            if (!(o instanceof Map.Entry))

                return false;

            Map.Entry e = (Map.Entry)o;

            Object k1 = getKey();

            Object k2 = e.getKey();

            if (k1 == k2 || (k1 != null && k1.equals(k2))) {

                Object v1 = getValue();

                Object v2 = e.getValue();

                if (v1 == v2 || (v1 != null && v1.equals(v2)))

                    return true;

            }

            return false;

        }

 

        public final int hashCode() {

            return (key==null   ? 0 : key.hashCode()) ^

                   (value==null ? 0 : value.hashCode());

        }

 

        public final String toString() {

            return getKey() + "=" + getValue();

        }

 

        void recordAccess(HashMap<K,V> m) {

        }

 

       

        void recordRemoval(HashMap<K,V> m) {

        }

    }

    void addEntry(int hash, K key, V value, int bucketIndex) {

        Entry<K,V> e = table[bucketIndex];

        table[bucketIndex] = new Entry<>(hash, key, value, e);

        if (size++ >= threshold)

            resize(2 * table.length);

    }

 

    void createEntry(int hash, K key, V value, int bucketIndex) {

        Entry<K,V> e = table[bucketIndex];

        table[bucketIndex] = new Entry<>(hash, key, value, e);

        size++;

    }

 

 

  ......

 

    ....

}

 

 

http://blog.sina.com.cn/s/blog_7d17f3cc01014dva.html