1.在jdk1.8以前，hashmap的实现原理是数组+链表，在1.8以后实现就变成了数组+链表+红黑树。这样实现的好处是防止某个链表中的元素数量过多，导致hashmap的整体性能下降，所以在1.8以后改为当链表中的元数量大于8时，就把链表改成红黑树，以提高效率。在红黑树中元素的数量小于6时，就会变成链表。
2.关于hashmap的put()的具体实现如下：
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; //如果数组没有就初始化数组
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null); //如果数组中 该位置没有元素 就直接插入
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p; //数组中存在key
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); // 如果数组中存在该元素 并且元素类型为树节点 则进行红黑树的插入
else {
for (int binCount = 0; ; ++binCount) { //循环遍历该链表
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) //若不存在key 则插入该链表的最后，如果链表长度超过阈值 则树化
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) //若该链表中存在该key 跳出循环
break;
p = e;
}
}
if (e != null) { //如果该hashmap中存在该key 则返回原先的值
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold) //判断容量是否已经超过阈值，若超过 则进行扩容
resize();
afterNodeInsertion(evict);
return null;
}
3.关于hashmap 的get（）的具体实现如下：
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) { //判断数组不为空 为空则返回null
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k)))) //如果第一个节点是 则返回第一个节点
return first;
if ((e = first.next) != null) { //遍历
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key); //如果是树节点 则调用红黑树的取值
do { //循环遍历链表
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
3.关于hashmap的resize()方法具体实现如下：
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) {
if (oldCap >= MAXIMUM_CAPACITY) { //如果原先的容器容量已经大于等于规定的最大值（2的30次方）了，则扩容后的新的容量是2的31次方-1
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // 两倍扩容
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults 初始化的容量
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]; //创建一个新的容器 需要把原先所有的数据重新散列到新的容器中
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null; //原先数组中该元素复制为空 便于资源回收
if (e.next == null) //如果没有链表 直接插入到新的容器中
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode) //如果是树节点 则调用红黑树的分离插入
((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); //把一个链表分离成两个并插入到新的链表中
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}