面试一、jdk1.8之hashMap
1、hashmap
1.1、线程不安全
1.2、数据结构
jdk1.7版本为Table数组+ Entry链表、jdk1.8版本为Table数组+ Node链表/红黑树
key值hash细节：
先取key的hashcode，然后将高位16位和自身异或
为什么是16位：因为key到hashcode值int类型为32位，16位刚好一半即高位
为什么要异或：一个数转化为2进制后，其中为1的位置代表了这个数字的特性，
让高位和低位异或实现了高位特性在低位上体现，让数据分布更均匀减少冲突
也因此，hash默认大小为16，扩容时设计为扩容2倍
1 /** 2 * Computes key.hashCode() and spreads (XORs) higher bits of hash 3 * to lower. Because the table uses power-of-two masking, sets of 4 * hashes that vary only in bits above the current mask will 5 * always collide. (Among known examples are sets of Float keys 6 * holding consecutive whole numbers in small tables.) So we 7 * apply a transform that spreads the impact of higher bits 8 * downward. There is a tradeoff between speed, utility, and 9 * quality of bit-spreading. Because many common sets of hashes 10 * are already reasonably distributed (so don't benefit from 11 * spreading), and because we use trees to handle large sets of 12 * collisions in bins, we just XOR some shifted bits in the 13 * cheapest possible way to reduce systematic lossage, as well as 14 * to incorporate impact of the highest bits that would otherwise 15 * never be used in index calculations because of table bounds. 16 */ 17 static final int hash(Object key) { 18 int h; 19 return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); 20 }
1.3、重要变量
DEFAULT_INITIAL_CAPACITY（初始化默认长度）：Table数组的初始化长度： 1 << 42^4=16
MAXIMUM_CAPACITY（最大长度）：2^30
DEFAULT_LOAD_FACTOR（默认负载因子）：默认值为0.75，当map的size>table数组长度*负载因子时，table数组会扩容为原来的2倍
TREEIFY_THRESHOLD（链表转红黑树阀值）：默认为8，表示table一个node下的数量超过8个时，链表结构会转换为红黑树
UNTREEIFY_THRESHOLD（红黑树转链表阀值）：默认为6，当table数组自动扩容时，如果table一个node下的数量小于6个，红黑树结构会转换为链表
MIN_TREEIFY_CAPACITY（最小树化阀值）：默认为64，当table的length>64时才会进行红黑树转换，防止map前期扩容频繁和树化冲突
1.4、实现原理(jdk1.8)
采用table数组+链表，HashMap采⽤Node数组来存储key-value对，每⼀个键值对组成了⼀个Node实体，
Node类实际上是⼀个单向的链表结构，它具有Next指针，可以连接下⼀个Node实体。 只是在JDK1.8中，链表⻓度⼤于8的时候，链表会转成红⿊树！
1.5、基本方法
put：对key取hash()，和当前table数组大小-1相与（hash(key) & n - 1），作为坐标。
1、如果当前位置为空，则new Node，将Node插入第一个
2、如果不为空，判断key是否和当前Node链表第一个Node的key相等，如果是则替换value
3、如果不是，判断是否为红黑树，是红黑树直接插入
4、如果是链表，则遍历查询key是否在当前链表中，如果是则替换value，
5、如果不是，则new Node，并插入到最后。此过程如果一个链表的数量大于TREEIFY_THRESHOLD会触发树化，当table数组小于MIN_TREEIFY_CAPACITY时用扩容代替树化
6、当map.size>threshold时，会触发扩容。threshold见resize()的计算newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY，即table数组大小*扩容因子
/**
* Returns <tt>true</tt> if this map contains a mapping for the
* specified key.
*
* @param key The key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
* key.
*/
public boolean containsKey(Object key) {
return getNode(hash(key), key) != null;
}
/**
* 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.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
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;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
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;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
get：对key取hash()，和当前table数组大小-1相与（hash(key) & n - 1），作为坐标取出值Node，如果Node是树结构则调用TreeNode查询，否则遍历Node整个链表取出key相等的值。
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Implements Map.get and related methods.
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
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) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
/**
* Finds the node starting at root p with the given hash and key.
* The kc argument caches comparableClassFor(key) upon first use
* comparing keys.
*/
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;
if ((ph = p.hash) > h)
p = pl;
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;
else if ((kc != null ||
(kc = comparableClassFor(k)) != null) &&
(dir = compareComparables(kc, k, pk)) != 0)
p = (dir < 0) ? pl : pr;
else if ((q = pr.find(h, k, kc)) != null)
return q;
else
p = pl;
} while (p != null);
return null;
}
/**
* Calls find for root node.
*/
final TreeNode<K,V> getTreeNode(int h, Object k) {
return ((parent != null) ? root() : this).find(h, k, null);
}
resize：当map.size大于table数组长度*负载因子时会触发自动扩容，table数组长度会扩容为原先的2倍。
扩容后遍历原先table数组，如果Node是红黑树则调用split方法分裂原先树，如果是链表则计算新位置。
计算：hash(key)&oldCap == 0判断是否在原位置（参考图例)，如果等于1则说明需要在新扩展的列表中，新坐标为原先坐标+oldCap
loTail和loHead表示原链表
hiTail和hiHead表示扩容出来的链表
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
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;
}
treeifyBin：将Node链表替换为红黑树，只有在table数组长度大于MIN_TREEIFY_CAPACITY会进行替换，否则进行扩容（防止在前期table数组大小较小时，频繁扩容和树化冲突）
/**
* Replaces all linked nodes in bin at index for given hash unless
* table is too small, in which case resizes instead.
*/
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
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 {
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)
hd.treeify(tab);
}
}
remove:
posted on 2021-08-24 21:15 Iversonstear 阅读(47) 评论(0) 编辑 收藏 举报
