OpenJDK 源代碼閱讀之 HashMap

概要

• 類繼承關係

java.lang.Object
    java.util.AbstractMap<K,V>
        java.util.HashMap<K,V>

• 定義

public class TreeMap<K,V>
extends AbstractMap<K,V>
implements NavigableMap<K,V>, Cloneable, Serializable

• 要點

1) 與 Hashtable 區別在於：非同步，允許 null

2) 不保證次序，甚至不保證次序隨時間不變 

3) 基本操作 put, get 常量時間 

4) 遍歷操作 與 capacity+size 成正比 

5) HashMap 性能與 capacity 和 load factor 相關，load factor 是當前元素個數與capacity 的比值，通常設定爲 0.75，如果此值過大，空間利用率高，但是衝突的可能性增加，因而可能導致查找時間增加，如果過小，反之。當元素個數大於 capacity * load_factor 時，HashMap 會重新安排 Hash 表。因此高效地使用 HashMap 需要預估元素個數，設置最佳的 capacity 和 load factor ，使得重新安排 Hash 表的次數下降。

實現

• capacity

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;
    while (capacity < initialCapacity)
        capacity <<= 1;

    this.loadFactor = loadFactor;
    threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
    table = new Entry[capacity];
    useAltHashing = sun.misc.VM.isBooted() &&
            (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
    init();
}

注意，HashMap 並不會按照你指定的 initialCapacity 來確定 capacity 大小，而是會找到一個比它大的數，並且是 2的n次方。

爲什麼要是 2 的n次方呢？

• hash

/**
 * Retrieve object hash code and applies a supplemental hash function to the
 * result hash, 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.
 */
final int hash(Object k) {
    int h = 0;
    if (useAltHashing) {
        if (k instanceof String) {
            return sun.misc.Hashing.stringHash32((String) k);
        }
        h = hashSeed;
    }

    h ^= k.hashCode();

    // 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);
}

如果 k 是 String 類型，使用了特別的 hash 函數，否則首先得到 hashCode，然後又對 h 作了移位，異或操作，問題：

爲什麼這裏要作移位，異或操作呢？

at 22: 
h = abcdefgh
h1 = h >>> 20 = 00000abc
h2 = h >>> 12 = 000abcde
h3 = h1 ^ h2 = [0][0][0][a][b][a^c][b^d][c^e]
h4 = h ^ h3 = [a][b][c][a^d][b^e][a^c^f][b^d^g][c^e^h]
h5 = h4 >>> 4 = [0][a][b][c][a^d][b^e][a^c^f][b^d^g]
h6 = h4 >>> 7 = ([0][:3])[0][0][a][b][c][a^d][b^e][a^c^f]([a^c^f][0])
h7 = h4 ^ h6 = 太兇殘了。。。

• 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) {
    if (key == null)
        return putForNullKey(value);
    int hash = hash(key);
    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;
}

從 put 其實可以看出各個 hash 表是如何實現的，首先取得 hash 值，然後由 indexFor 找到鏈表頭的 index，然後開始遍歷鏈表，如果鏈表裏的一個元素 hash 值與當前 key 的 hash 值相同，或者元素 key 的引用與當前 key 相同，或者 equals 相同，就說明當前 key 已經在 hash 表裏了，那麼修改它的值，返回舊值。

如果不在表裏，會調用 addEntry，將這一 (key, value) 對添加進去。

/**
 * 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) {
    if ((size >= threshold) && (null != table[bucketIndex])) {
        resize(2 * table.length);
        hash = (null != key) ? hash(key) : 0;
        bucketIndex = indexFor(hash, table.length);
    }

    createEntry(hash, key, value, bucketIndex);
}

/**
 * Like addEntry except that this version is used when creating entries
 * as part of Map construction or "pseudo-construction" (cloning,
 * deserialization).  This version needn't worry about resizing the table.
 *
 * Subclass overrides this to alter the behavior of HashMap(Map),
 * clone, and readObject.
 */
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++;
}

可以看出，新增加元素時，可能會調整 hash 表的大小，原因之前已經討論過。直接的添加在 createEntry 中完成，但是這裏並沒有體現出如何處理衝突。

Entry(int h, K k, V v, Entry<K,V> n) {
    value = v;
    next = n;
    key = k;
    hash = h;
}

注意這裏，將 n 賦值給了 next，這其實就是將新添加的項指向了當前鏈表頭。這一操作在 Entry 的構造函數中完成。

put 操作的基本思路在到這裏已經很清楚了，有了這個思路，不難想象 get 是如何動作的。

public V get(Object key) {
    if (key == null)
        return getForNullKey();
    Entry<K,V> entry = getEntry(key);

    return null == entry ? null : entry.getValue();
}

final Entry<K,V> getEntry(Object key) {
    int hash = (key == null) ? 0 : hash(key);
    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 != null && key.equals(k))))
            return e;
    }
    return null;
}

和 put 差不多，只是找到了就會返回相應的 value ，找不到就返回 null。

如果對代碼有更多見解，可以在這個頁面添加註釋： rtfcode-HashMap