Java源码阅读-HashMap

添加元素

/**     * 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 (table == EMPTY_TABLE) { // 还未添加过元素，先填充存储键值对的表            inflateTable(threshold);        }        if (key == null)            return putForNullKey(value);        int hash = hash(key); // 计算hash值        int i = indexFor(hash, table.length);        for (Entry<K,V> e = table[i]; e != null; e = e.next) { // 遍历所有桶，如果要插入的key的hash值和桶的hash值相同则插入            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;    }

/**     * 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) { // 计算hash值        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);    }

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

根据key获取value

/**     * 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); // 调用私有方法getEntry(key)        return null == entry ? null : entry.getValue();    }

/**     * 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); // 计算查找的key的hash值        for (Entry<K,V> e = table[indexFor(hash, table.length)]; // 遍历桶，如果key的hash值和桶的hash值相同则取出值             e != null;             e = e.next) {            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k))))                return e;        }        return null;    }