Hashmap 源码分析之基本操作

我们从流程中看一下，这个hashmap究竟是这么实现的。
我们先了解一下put方法实现

public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}

接下来的才是真正实现:

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
//数组tab table是内部数组
Node<K,V>[] tab;
Node<K,V> p;
int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
// resize（）是扩容函数
n = (tab = resize()).length;
//取到hash地址，如果为null就可以填入
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
//不为null
Node<K,V> e; K k;
//如果map中已经存在，就将值覆盖
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果这个结点是红黑树的结点，那么使用putTreeVal来实现。
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
//如果是链表结构
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//假如这个链表结点个数大于等于TREEIFY_THRESHOLD - 1的时候的时候，直接转换成树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// key已经存在直接覆盖value
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;
//onlyIfAbsent if true, don't change existing value
if (!onlyIfAbsent || oldValue == null)
e.value = value;

            afterNodeAccess(e);
            return oldValue;
        }
    }

//这个是一个安全机制 modCount在LinkedList讲过
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}

至于afterNodeInsertion这些，在hashmap中其实是没有实现的，

// Callbacks to allow LinkedHashMap post-actions
void afterNodeAccess(Node<K,V> p) { }
void afterNodeInsertion(boolean evict) { }
void afterNodeRemoval(Node<K,V> p) { }

正如上面写的，应该是和LinkedhashMap有关系。

我们接下来看，get方法，注意hashmap是可以有null值的

public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}

同样真正实现是在

final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab;
Node<K,V> first, e;
int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
//如果第一个存在，并可以key值相同返回first
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//没有找到的话
if ((e = first.next) != null) {
//先判断是否是红黑树
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
//如果是链表开始判断
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}

接下来就是hashmap扩容了

final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
//如果超过了最大容量，不会扩容的
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//没有的话讲容量变成两倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
//初始化
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//重新计算下次扩容的临界值
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;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
//为null利于gc回收处理
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
//
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;

                            loTail = e;
                        }
                        else {
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);

//将旧的table对于到扩容的table上
//扩了一倍，那么其实是就是高位新增一位判断是1 还是0
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}

那么其实我们可以看到hashmap扩容其实非常的耗时间，如果可以预测这个map可以存的值数量，其实在初始化的时候定义是更加合算的。

读源码，是我们了解大神领域的一大捷径
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