源码解读

JDK 1.8

• 静态常量

    /**
     * The default initial capacity - MUST be a power of two.
     * 初始容量为16，要求必须为2的幂
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 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.
     * 最大容量为2的30次方
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;
 
    /**
     * The load factor used when none specified in constructor.
     * 扩展因子为0.75，当容量超过当前容量0.75时自动扩容
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
 
    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     * hash冲突使用单链表存储，当单链表结点数超过8之后，转为红黑树存储
     */
    static final int TREEIFY_THRESHOLD = 8;
 
    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     * hash冲突红黑树中的结点数少于6时传华为单链表存储
     */
    static final int UNTREEIFY_THRESHOLD = 6;
 
    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     * hash冲突单链表转化为红黑树的条件除了结点数大于8之外，还要求数组容量大于64
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

• 存储方式

    //以数组的形式存储Hash值，当存在hash冲突时使用单链表存储
        
    transient Node<K,V>[] table;
    /**
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;
     
        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }
     
        public final K getKey()        { return key; }
        public final V getValue()      { return value; }
        public final String toString() { return key + "=" + value; }
     
        public final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }
     
        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }
     
        public final boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof Map.Entry) {
                Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            }
            return false;
        }
    }

• 插入元素 1.7版本

    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;
            //覆盖原有值
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        //插入新结点
        addEntry(hash, key, value, i);
        return null;
    }

    void addEntry(int hash, K key, V value, int bucketIndex) {
        //判断是否需要扩容
        if ((size >= threshold) && (null != table[bucketIndex])) {
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
            bucketIndex = indexFor(hash, table.length);
        }
        //插入新的结点
        createEntry(hash, key, value, bucketIndex);
    }

    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++;
    }


        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            //将原链表连接到新元素的后边，即从头部插入
            next = n;
            key = k;
            hash = h;
        }

jdk1.8版本

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
 
 /**
     * Implements Map.put and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent if true, don't change existing value
     * @param evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            //hash冲突时，采用尾插法
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //判断此时是红黑树还是链表
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

JDK1.8中首先判断此时是否为树结构，如果是，插入新元素为树中的一个结点，否则遍历链表道末尾，将新元素未插入到链表末尾，然后判断链表长度是否超过转化数的临界值。

JDK 1.7