HashMap2
HashMap 源码重识
⭐ HashMap 重要的变量
// 默认的初始容量
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
// 最大的容量，且必须是2的倍数
static final int MAXIMUM_CAPACITY = 1 << 30;
// 默认的负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//链表转成红黑树的默认值
static final int TREEIFY_THRESHOLD = 8;
//反向转换回链表的最大值
static final int UNTREEIFY_THRESHOLD = 6;
//可将其分类为树木的最小桌子容量。（否则，如果bin中的节点过多，则将调整表的大小。）
//至少应为4 * TREEIFY_THRESHOLD,以避免冲突在调整大小和树化阈值之间。
static final int MIN_TREEIFY_CAPACITY = 64;
//该表在首次使用时初始化，并根据需要调整大小。 分配时，长度始终是2的幂。 （在某些操作中，我们还允许长度为零，以允许使用当前不需要的引导机制。）
transient Node<K,V>[] table;
// map 内元素的size
transient int size;⭐ 构造方法
// 默认负载因子为 0.75 初始化容量是 16
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
// 指定初始化容量 以及负载因子
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);
}
// 对于给定的目标容量，返回两倍大小的幂。
static final int tableSizeFor(int cap) {
int n = -1 >>> Integer.numberOfLeadingZeros(cap - 1);
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}注：
如果我们传入参数为
new HashMap(10);this.threshold = tableSizeFor(initialCapacity);为16当你
new HashMap(17);this.threshold = tableSizeFor(initialCapacity);为32
⭐ Put 方法
Map<String,String> map = new HashMap<>(17);
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
// 初始话数组
Node<K,V>[] tab; Node<K,V> p; int n, i;
// 当进行第一put时，会通过resize 进行tab的初始化
// 这时我们的经过 resize() tab.length = 32
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//这里进行 散列，将数据放入 tab 中
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
// 当 key 重复了
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//当节点为tree的时进行
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//循环像链表中添加
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 链表的长度 大于 8 转成 tree
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;
}
}
// 当key重复，直接替换value 并返回原来的value
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
// 这里将 size 进行 ++ ，并判断与threshold的关系，进行tab的阔容
// 例如 ： 17 > 32 > 32 * 0.75 > 24
// 这里大概的意思就是，我们在构造方法传入 17 ，但是经过计算 threshold =32 ，经过resize 后24
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}⭐ resize()方法
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
//这是的 threshold 经过构造方法为 32
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
//超过最大值就等于最大值
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//没超过最大值，就扩充为原来的2倍
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;
// 计算新的resize上限
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;
//进行数组的大小赋值，
@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;
}
// 原索引+oldCap放到bucket里
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}⭐ treeifyBin()方法
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
// tab 为 null 或者 length < 64 重新计算长度
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
// 进行Tree的创建 循环勾结节点
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)
//形成具体的树
hd.treeify(tab);
}
}⭐ Get方法
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
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) {
//这是特殊的判断，为了减少耗时，每次都乐观的假设一个节点就是目标数据
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
// 树节点查询
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
// 否则就循环的查询
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}⭐ getTreeNode
final TreeNode<K,V> getTreeNode(int h, Object k) {
return ((parent != null) ? root() : this).find(h, k, null);
}
//使用给定的哈希和密钥查找从根p开始的节点。
final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
TreeNode<K,V> p = this;
do {
int ph, dir; K pk;
TreeNode<K,V> pl = p.left, pr = p.right, q;
// 重置为 左节点
if ((ph = p.hash) > h)
p = pl;
// 重置为右节点
else if (ph < h)
p = pr;
else if ((pk = p.key) == k || (k != null && k.equals(pk)))
return p;
else if (pl == null)
p = pr;
else if (pr == null)
p = pl;
//说如果实现了 compare
else if ((kc != null ||
(kc = comparableClassFor(k)) != null) &&
(dir = compareComparables(kc, k, pk)) != 0)
p = (dir < 0) ? pl : pr;
// 递归
else if ((q = pr.find(h, k, kc)) != null)
return q;
else
p = pl;
} while (p != null);
return null;
}Last updated