[JDK源码] HashMap

1. 参数介绍

 

/**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    transient Entry[] table;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     * @serial
     */
    int threshold;

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient volatile int modCount;

 

 

Entry[] table:

用于存放Entry(key-value)的数组。Entry类实际上是个链结构，正因如此，对于table[i] (0 <= i <= table.length - 1)，里面可以存放任意数量的Entry。（看过hash算法的应该不陌生）

 

capacity：

该值并不是变量，等于table.length。DEFAULT_INITIAL_CAPACITY是改参数的默认值。该参数必须是2的n(n是自然数)次方，我猜是选用了移位算法作为Hash的缘故。 MAXIMUM_CAPACITY是该参数的最大值。 

 

size：

该参数记录HashMap中Entry的数量。当该值超过threshold的时候，HashMap会进行扩容并且rehash操作。

 

loadFactor:

一个算法参数，决定了HashMap何时会进行扩容和rehash。DEFAULT_LOAD_FACTOR是改参数的默认值。 该值越大，table所占空间就越小，Entry多的时候性能就越差。反之，table所占空间就越大，性能也就越好。

 

threshold:

该参数等于capacity*loadFactor。当size超过该值时，HashMap会进行扩容并且rehash操作。

 

modCount:

 

2. HashMap.Entry

 

 

static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        final int hash;

        /**
         * Creates new entry.
         */
        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();
        }

        /**
         * This method is invoked whenever the value in an entry is
         * overwritten by an invocation of put(k,v) for a key k that's already
         * in the HashMap.
         */
        void recordAccess(HashMap<K,V> m) {
        }

        /**
         * This method is invoked whenever the entry is
         * removed from the table.
         */
        void recordRemoval(HashMap<K,V> m) {
        }
    }

 

该类是用于保存HashMap中key-value的数据结构，采用单向链。

 

3. 重要的方法

 

 

/**
     * Applies a supplemental hash function to a given hashCode, which
     * defends against poor quality hash functions.  This is critical
     * because HashMap uses power-of-two length hash tables, that
     * otherwise encounter collisions for hashCodes that do not differ
     * in lower bits. Note: Null keys always map to hash 0, thus index 0.
     */
    static int hash(int h) {
        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

 

对某个h值进行hash计算。该方法一般用于对key对象的hashCode()调用。

 

/**
     * Returns index for hash code h.
     */
    static int indexFor(int h, int length) {
        return h & (length-1);
    }

 

根据上面方法生成的hash值计算table中所处的位置。因为用的是&(并操作)，保证了结果小于等于table的length。

 

 

/**
     * Adds a new entry with the specified key, value and hash code to
     * the specified bucket.  It is the responsibility of this
     * method to resize the table if appropriate.
     *
     * Subclass overrides this to alter the behavior of put method.
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {

        //将新Entry插入到table中指定位置，该位置原有链会作为该Entry的next(参考Entry的构造方法)
	Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
        
        //如果size超过threshold,扩容
        if (size++ >= threshold)
            resize(2 * table.length);
    }

 

/**
     * Rehashes the contents of this map into a new array with a
     * larger capacity.  This method is called automatically when the
     * number of keys in this map reaches its threshold.
     *
     * If current capacity is MAXIMUM_CAPACITY, this method does not
     * resize the map, but sets threshold to Integer.MAX_VALUE.
     * This has the effect of preventing future calls.
     *
     * @param newCapacity the new capacity, MUST be a power of two;
     *        must be greater than current capacity unless current
     *        capacity is MAXIMUM_CAPACITY (in which case value
     *        is irrelevant).
     */
    void resize(int newCapacity) {

        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;

        //如果已经是最大容量，则将threshold设置为最大并返回
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        
        //将oldTable里面的所有元素hash到newTable
        transfer(newTable);
        table = newTable;
        threshold = (int)(newCapacity * loadFactor);
    }

 

/**
     * Transfers all entries from current table to newTable.
     */
    void transfer(Entry[] newTable) {
        Entry[] src = table;
        int newCapacity = newTable.length;

        //循环处理oldTable里面每个Entry链
        for (int j = 0; j < src.length; j++) {
            Entry<K,V> e = src[j];
            if (e != null) {
                src[j] = null;
                
                //循环处理链里面的每个Entry，重新计算Entry所在newTable的位置，并把该Entry插入到newTable[i]的最前端
                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);
            }
        }
    }

 

 

4. 构造HashMap

 

 

/**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and load factor.
     *
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    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);

        // Find a power of 2 >= initialCapacity
        int capacity = 1;
        //保证capacity是2的n次方。
        while (capacity < initialCapacity)
            capacity <<= 1;

        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
        table = new Entry[capacity];
        //该方法是空方法，供继承类实现额外操作。
        init();
    }

 

 

5. put

 

 

/**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        
        //空key处理
        if (key == null)
            return putForNullKey(value);

        //生成key所对应的hash
        int hash = hash(key.hashCode());

        //根据hash获得table中所在的位置
        int i = indexFor(hash, table.length);

        //链式搜索是否已经存在相同key的Entry，如果存在，则替换掉oldValue并返回oldValue
        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++;

        //插入新的Entry到i位置
        addEntry(hash, key, value, i);

        return null;
    }