HashMap
==> Constructs an empty HashMap with the specified initial capacity and load factor
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;
this.threshold = tableSizeFor(initialCapacity);
}
tableSizeFor
==> Returns a power of two size for the given target capacity.
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
HashMap数据插入原理
1.调用put(K key, V value)方法来进行插入，在内部调用putVal方法
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
- 内部调用putVal方法
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//判断数组是否为空，为空进行初始化
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//不为空，计算 k 的 hash 值，通过(n - 1) & hash计算应当存放在数组中的下标 index
if ((p = tab[i = (n - 1) & hash]) == null)
//查看 table[index] 是否存在数据，没有数据就构造一个Node节点存放在 table[index] 中
tab[i] = newNode(hash, key, value, null);
//存在数据，说明发生了hash冲突(存在二个节点key的hash值一样)
else {
Node<K,V> e; K k;
//继续判断key是否相等，相等，用新的value替换原数据(onlyIfAbsent为false)
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果不相等，判断当前节点类型是不是树型节点
else if (p instanceof TreeNode)
//如果是树型节点，创造树型节点调用putTreeVal方法，将当前值放入红黑树中
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
//如果不是树型节点，创建普通Node加入链表中
else {
for (int binCount = 0; ; ++binCount) {
//如果遍历到了链接末尾，则创建链表节点，将数据存储到链表结尾
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//判断链表长度是否大于 8， 大于的话链表转换为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//如果在链表中找到完全相同的key，则直接替换value
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//e!=null说明只是遍历到中间就break了，该种情况就是在链表中找到了完全相等的key,该if块中就是对value的替换操作
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;
}
}
++modCount;
//插入完成之后判断当前节点数是否大于阈值，如果大于开始扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
- resize()的作用进行初始化或者进行2倍数扩容(Initializes or doubles table size.)
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) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
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);
}
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) {
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);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
