学习Java基础知识，打通面试关八—Map系列学习

     */
    public synchronized int size() {
        return count;
    }

    /**
     * Tests if this hashtable maps no keys to values.
     *
     * @return  <code>true</code> if this hashtable maps no keys to values;
     *          <code>false</code> otherwise.
     */
    public synchronized boolean isEmpty() {
        return count == 0;
    }

    /**
     * Returns an enumeration of the keys in this hashtable.
     *
     * @return  an enumeration of the keys in this hashtable.
     * @see     Enumeration
     * @see     #elements()
     * @see     #keySet()
     * @see     Map
     */
    public synchronized Enumeration<K> keys() {
        return this.<K>getEnumeration(KEYS);
    }
    //都是线程安全的代码 

```
 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;  //初始化 设置大小 ，尽量自己设置好
        this.threshold = tableSizeFor(initialCapacity);
    }
```

```
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;   //java8中使用了node数组结构  
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;    //在这里我们可以看到如果tab 为null ，首先会初始化,其中resize 在容量不足的情况下 ，对Map集合进行扩容
        if ((p = tab[i = (n - 1) & hash]) == null)    /
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            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)   //在这里 进行比较 是否大于初始化 设定的容量大小 ，大于就进行扩容操作
            resize();
        afterNodeInsertion(evict);
        return null;
    }
```

```
  final Node<K,V>[] resize() {

 if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {    //在这里 进行判断 是否大于 该最大值  ，MAXIMUM_CAPACITY 大小为 2的30次方
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold 
// 在这里我们看到门限值 是在成倍的增加，只要超过这个门限的大小就进行扩容，
//扩容后还需要把原先的数据放到新的数组上，这个是扩容的开销.
//如果内存紧张的情况下，尽量避免使用集合，采用数组的方式来实现。 并且门限值时等于 负载因子乘以 容量的

        }
}
```

```
 final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                TreeNode<K,V> p = replacementTreeNode(e, null);
                if (tl == null)
                    hd = p;
                else {
                    p.prev = tl;
                    tl.next = p;
                }
                tl = p;
            } while ((e = e.next) != null);
            if ((tab[index] = hd) != null)
                hd.treeify(tab);
        }
    }
```