之前hashmap处理冲突单纯使用链表法进行链接，1.8中，当一散列值中对应的链表长度超过8个后，会将链表转化为红黑树进行存储。
1.8中源码
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; //如果tab为空或长度为0，则分配内存resize() if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; //(n - 1) & hash找到put位置，如果为空,则直接put if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); else { Node<k,v> e; K k; //第一节节点hash值同，且key值与插入key相同 if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k)))) e = p; else if (p instanceof TreeNode)//属于红黑树处理冲突 e = ((TreeNode<k,v>)p).putTreeVal(this, tab, hash, key, value); else { //链表处理冲突 for (int binCount = 0; ; ++binCount) { //p第一次指向表头,以后依次后移 if ((e = p.next) == null) { //e为空，表示已到表尾也没有找到key值相同节点，则新建节点 p.next = newNode(hash, key, value, null); //新增节点后如果节点个数到达阈值，则将链表转换为红黑树 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st treeifyBin(tab, hash); break; } //容许null==null if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k)))) break; p = e;//更新p指向下一个节点 } } //更新hash值和key值均相同的节点Value值 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; }
1.7源码
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*/
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
//这里的循环是关键
//当新增的key所对应的索引i，对应table[i]中已经有值时，进入循环体
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
//判断是否存在本次插入的key，如果存在用本次的value替换之前oldValue，相当于update操作
//并返回之前的oldValue
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
//如果本次新增key之前不存在于HashMap中，modCount加1，说明结构改变了
modCount++;
addEntry(hash, key, value, i);
return null;
}
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tab为空或长度为0，则分配内存resize()
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//(n - 1) & hash找到put位置，如果为空,则直接put
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<k,v> e; K k;
//第一节节点hash值同，且key值与插入key相同
if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)//属于红黑树处理冲突
e = ((TreeNode<k,v>)p).putTreeVal(this, tab, hash, key, value);
else {
//链表处理冲突
for (int binCount = 0; ; ++binCount) {
//p第一次指向表头,以后依次后移
if ((e = p.next) == null) {
//e为空，表示已到表尾也没有找到key值相同节点，则新建节点
p.next = newNode(hash, key, value, null);
//新增节点后如果节点个数到达阈值，则将链表转换为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//容许null==null
if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;//更新p指向下一个节点
}
}
//更新hash值和key值均相同的节点Value值
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;
}