集合——HashMap

HashMap

• 数组+链表+红黑树
• key不能重复，但值可以，允许使用null值和null键
• 如果添加相同的key，则会覆盖原来的key-value
• 无序
• 线程不安全

实现

public class test {
    public static void main(String[] args) {
        //创建HashMap对象
        /*
         *   public HashMap() {
               //初始化加载因子 0.75f
               this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
             }
        */
        HashMap map = new HashMap();
        /*  map.put(k,v);
            public V put(K key, V value) {
              return putVal(hash(key), key, value, false, true);
            }
        */
        map.put("",1);
        map.put(1,"");
        map.put(null,null);
        map.put(null,1);
        System.out.println(map);
    }
}

• return putVal(hash(key), key, value, false, true);

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable 
{
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //如果node为一个空数组,调用resize()初始化
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
//如果hash计算出的数组下标值为空，创建一个新的节点给当前下标

if ((p = tab[i = (n - 1) & hash]) == null)

tab[i] = newNode(hash, key, value, null);

else {

Node<K,V> e; K k;

//如果下标存在同一地址或相同内容的key时进入

if (p.hash == hash &&

((k = p.key) == key || (key != null && key.equals(k))))

e = p;

//如果当前节点是一棵红黑树则进入

else if (p instanceof TreeNode)

e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);

else {

for (int binCount = 0; ; ++binCount) {

//如果已经来到链表的尾节点，则在尾节点追加一个新的节点

if ((e = p.next) == null) {

p.next = newNode(hash, key, value, null);

//如果链表长度>8，考虑将链表转为红黑树结构

if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st

treeifyBin(tab, hash);

break;

}

//如果下标存在同一地址或相同内容的key时，退出循环

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)

//覆盖同一已存在key的vlaue

e.value = value;

afterNodeAccess(e);

return oldValue;

}

}

++modCount;

//如果size+1>length*0.75，则要对数组扩容

if (++size > threshold)

resize();

afterNodeInsertion(evict);

return null;

}

}

• resize();

    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //当前数组的临界点值 假设为初始的12
        int oldThr = threshold;
        int newCap, newThr = 0;
        //如果当前数组长度大于0
        if (oldCap > 0) {
            //如果长度大于最大数
            if (oldCap >= MAXIMUM_CAPACITY) {
                //加载因子为int最大值
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //如果当前数组长度*2<最大值 and 数组长度>=默认数组长度16
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                //加载因子 * 2
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            //如果加载因子>0，将初始容量设为阀值
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            //初识容量设为16，阀值设为12
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        //...省略代码块
        return newTab;
    }

• treeifyBin(tab, hash);

    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        //如果数组为空 或者 数组长度<64，先对数组进行扩容
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        //否则树化
        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);
        }
    }