HashMap代碼解析

 

hashmap (jdk 1.7)使用 「數組-鏈表」 方式進行存儲，圖形化表示以下：

即，前面是一個數組，後面跟一個鏈表，那麼數據結構這個對應到HashMap的代碼裏面是什麼樣子的呢？

在HashMap中定義了一個類型爲Entry<K,V>的數組table，上圖就是顯示了這個table。

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

類型Entry<K,V>的定義以下：

static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        int hash;
        // 省略構造/get/set等函數
}

由Entry<K,V>的定義可知，上圖每一個節點中其實存了4個變量：

    key表示鍵，即存入map的鍵值

    value表示值，即存入map的值

    next表示下一個Entry節點

    hash表示key的哈希值。

那麼上圖準確表示應該是：

對於HashMap，最經常使用的莫過於直接使用默認構造函數建立一個Map對象了

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

這裏HashMap調用了

    /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

其中，DEFAULT_INITIAL_CAPACITY是

    /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

DEFAULT_LOAD_FACTOR是

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

 this()調用的是

    /**
     * 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);

        this.loadFactor = loadFactor;
        threshold = initialCapacity;
        init();
    }

瞭解了基本結構以後，看一下HashMap的put()和get()方法是如何實現的。

 

首先，看put()方法，再瞭解put()方法以前，先了解幾個put()方法會調用的幾個輔助方法：

1. inflateTable()，給表充氣 or 讓表膨脹？原來table對象是空的，因此須要將table對象初始化

    /**
     * Inflates the table.
     */
    private void inflateTable(int toSize) {
        // Find a power of 2 >= toSize
        // capacity 表示HashpMap的容量，必須是2的倍數
        int capacity = roundUpToPowerOf2(toSize);
        // threshold 表示須要resize的閾值
        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
        // 初始化大小爲capacity的table對象
        table = new Entry[capacity];
        // 初始化 hashSeed
        initHashSeedAsNeeded(capacity);
    }

　　 

2. putForNullKey()

    /**
     * Offloaded version of put for null keys
     */
    private V putForNullKey(V value) {
        // 遍歷table[0]，若是已經有key爲null的元素，直接返回對應的value
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        // 若是沒有key爲null的元素，
        // 將HashMap的修改次數+1
        modCount++;
        // 將key爲null的元素添加到HashMap中
        addEntry(0, null, value, 0);
        return null;
    }

　疑問：key爲null的元素的hash值必定爲0嗎？

3. hash()，求對象的hash值

    /**
     * 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 = hashSeed;
        if (0 != h && k instanceof String) {
            return sun.misc.Hashing.stringHash32((String) k);
        }

        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);
    }

　　　　

4. indexFor()，根據對象的hash值以及HashMap table的長度，尋找該對象的索引位置

    /**
     * Returns index for hash code h.
     */
    static int indexFor(int h, int length) {
        // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
        return h & (length-1);
    }

這裏沒有使用hash值h對長度length取餘，而是使用的位運算？其實二者結果是同樣的，h % length == h & (length -1)

 

5. addEntry()

/**
     * 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) {
        // size是HashMap中元素的個數
        // threshhold = capacity * load factor，表示須要擴容resize的閾值
        // 若是size > threshold，而且table在當前索引處有元素，不爲null，則須要擴容HashMap，並重新計算索引值
        if ((size >= threshold) && (null != table[bucketIndex])) {
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
            bucketIndex = indexFor(hash, table.length);
        }
        // 將元素加入到HashMap
        createEntry(hash, key, value, bucketIndex);
    }

　　 

6. createEntry()

    /**
     * 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) {
        // 這裏是獲取某個鏈表的第一個節點e，
        // 由於每次插入都是往鏈表的頭部插入的，所以e就做爲了新節點的next值
        Entry<K,V> e = table[bucketIndex];
        // e做爲新節點的next值
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        // HashMap的size加1
        size++;
    }

　　　　

7. resize()

    /**
     * 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;
        // 若是HashMap中table的長度（這裏是指table數組的長度，不是鏈表的長度）
        // 已經達到了MAXIMUN_CAPACITY = 1 << 30，直接將閾值threshold設置爲Integer的最大值。
        // 不在擴容HashMap
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }
　　　　 // newCapacity是原table數組長度的2倍
        Entry[] newTable = new Entry[newCapacity];
        // 將原table中的值遷移到擴容後的newTable中
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
        // 更新table和閾值threshold
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

　　　　

8. transfer()， 將原HashMap中的元素遷移到擴容後的HashMap中

    /**
     * Transfers all entries from current table to newTable.
     */
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        // 遍歷table數組
        for (Entry<K,V> e : table) {
            // 這裏e是table數組某個鏈表的第一個元素，後面e會依次指向鏈表中全部的元素
            // 若是table數組的元素不爲null
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    // 若是e.key是null，hash值是0
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                // 獲取元素e在新table中的索引值
                int i = indexFor(e.hash, newCapacity);
                // 將e的next指向新table的第一個元素（這裏仍是要記住，插入鏈表是從頭部插入的）
                // newTable[i]是鏈表的第一個元素
                e.next = newTable[i];
                // 將e賦值給鏈表的第一個元素newTable[i]，這樣e就取代了鏈表原來的第一個元素，做爲鏈表新的第一個元素，引領鏈表！
                newTable[i] = e;
                e = next;
            }
        }
    }

　　

 

瞭解了上述n個方法以後，是時候看一下HashMap的put()方法的真面目了！

    /**
     * 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) {
        // 若是table是空的，須要初始化table
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        // 若是key是null，調用putForNullKey方法插入元素
        if (key == null)
            return putForNullKey(value);
        // 求key的hash值
        int hash = hash(key);
        // 根據key的hash值和table的長度求元素在table中的索引
        int i = indexFor(hash, table.length);
        // 遍歷table[i]引領的鏈表
        // 若是已經存在了相同的key，則更新value並返回old value，不然插入新元素
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            // key已經存在，更新value，返回old value
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        // HashMap的修改次數加1，modCount是modified times
        modCount++;
        // 插入新元素
        addEntry(hash, key, value, i);
        // 若是key沒有重複，返回值是null
        return null;
    }

　　

　　 

其次，看一下get()方法，再瞭解get()方法以前，一樣先了解幾個get()方法會調用的幾個輔助方法：

1. getForNullKey()

    /**
     * Offloaded version of get() to look up null keys.  Null keys map
     * to index 0.  This null case is split out into separate methods
     * for the sake of performance in the two most commonly used
     * operations (get and put), but incorporated with conditionals in
     * others.
     */
    private V getForNullKey() {
        if (size == 0) {
            return null;
        }
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }

　　

2. getEntry()

    /**
     * Returns the entry associated with the specified key in the
     * HashMap.  Returns null if the HashMap contains no mapping
     * for the key.
     */
    final Entry<K,V> getEntry(Object key) {
        if (size == 0) {
            return null;
        }

        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;
    }

瞭解了上述2個方法以後，get()方法就比較簡單了

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        if (key == null)
            return getForNullKey();
        Entry<K,V> entry = getEntry(key);

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

　　

最後，再學習一下remove()方法的實現

    /**
     * Removes the mapping for the specified key from this map if present.
     *
     * @param  key key whose mapping is to be removed from the map
     * @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 remove(Object key) {
        Entry<K,V> e = removeEntryForKey(key);
        return (e == null ? null : e.value);
    }

    /**
     * Removes and returns the entry associated with the specified key
     * in the HashMap.  Returns null if the HashMap contains no mapping
     * for this key.
     */
    final Entry<K,V> removeEntryForKey(Object key) {
        if (size == 0) {
            return null;
        }
        // 找到hash值
        int hash = (key == null) ? 0 : hash(key);
        // 求索引位置
        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;
            // 尋找到key的位置，是當前的e
            if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) {
                // HashMap的修改次數加1
                modCount++;
                // HashMap的size減1
                size--;
                // 若是是鏈表的第一個元素，next是null，直接將table[i]設置爲null
                if (prev == e)
                    table[i] = next;
                else
                    // prev的next是e，next是e.next，即[prev]-> [e] -> [next]
                    // prev.next = next，即[prev] -> [next]，直接將元素e移除掉了
                    prev.next = next;
                e.recordRemoval(this);
                return e;
            }
            prev = e;
            e = next;
        }

        return e;
    }