1.7 和 1.8 源码区别很大
面试要点：
1.7->数组、链表
1.8—>数组、链表、红黑树
主攻1.8
- 众所周知，HashMap是是哈希表，数组+链表。下面就是数组的默认大小。位移效率高。注释中强调这是2的整数幂（算法导论中有降到除数散列法：除数如果是一个不太接近2的整数幂的素数是一个好的选择，HashTable中确实不是2的整数幂，是11）HashMap这么做后面有大用处
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
- 数组扩展长度用的。如果被问为什么是0.75 怼之
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
- 1.8新增 转红黑树的阈值
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
static final int TREEIFY_THRESHOLD = 8;
- 构造 发现并没有创建 数组。相关操作在put方法中
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
- 哈希表中的数组，其长度为上面DEFAULT_INITIAL_CAPACITY，源码注释中也提到初始化在第一次使用的时候，所以映射上面的构造时啥也没有做。
/**
* The table, initialized on first use, and resized as
* necessary. When allocated, length is always a power of two.
* (We also tolerate length zero in some operations to allow
* bootstrapping mechanics that are currently not needed.)
*/
transient Node<K,V>[] table;
- 重头戏 put
/**
* 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);
}
- 这有一个特别重要的方法hash。key取hash。这也可以看出HashMap是允许key为null的。这后面的右移16位是为了取其高位，舍弃低位，因为低位容易出现hash碰撞。细节看源码注释，也能看出来为什么数组的长度要是2的整数幂
/**
* Computes key.hashCode() and spreads (XORs) higher bits of hash
* to lower. Because the table uses power-of-two masking, sets of
* hashes that vary only in bits above the current mask will
* always collide. (Among known examples are sets of Float keys
* holding consecutive whole numbers in small tables.) So we
* apply a transform that spreads the impact of higher bits
* downward. There is a tradeoff between speed, utility, and
* quality of bit-spreading. Because many common sets of hashes
* are already reasonably distributed (so don't benefit from
* spreading), and because we use trees to handle large sets of
* collisions in bins, we just XOR some shifted bits in the
* cheapest possible way to reduce systematic lossage, as well as
* to incorporate impact of the highest bits that would otherwise
* never be used in index calculations because of table bounds.
*/
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
- putVal
/**
* 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;
// 第一次插入的时候 table 即数组 是没有初始化的
if ((tab = table) == null || (n = tab.length) == 0)
// 在resize方法中 进行了初始化
n = (tab = resize()).length;
// (n-1)&hash 这个与操作 代替了mod操作 i->要放入的bin(桶)
if ((p = tab[i = (n - 1) & hash]) == null)
// 如果这个桶是空的 那么新建一个节点(这个节点是链表 还不是红黑树)
tab[i] = newNode(hash, key, value, null);
else {
// 如果桶不是空的 已经存在了节点
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
// p = tab[i = (n - 1) & hash] p代表的是链表上的节点 碰上key 相同的情况
e = p;
else if (p instanceof TreeNode)
// 如果不是链表是红黑树的话
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// 开始向链表上添加节点
for (int binCount = 0; ; ++binCount) {
// (e = p.next) == null 意思是找到末尾
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// TREEIFY_THRESHOLD默认是8 满足条件要开始转换数据结构
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 碰上key 相同的情况
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// key是唯一的 剔掉value
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
// 只有替换value的时候 返回值是旧值
return oldValue;
}
}
++modCount;
// 如果长度 达到了 threshold(跟加载因子有关)
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
- resize() 初始化的时候用到。数组长度扩展的时候也用到
/**
* 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;
// 初始化的时候是0
int oldCap = (oldTab == null) ? 0 : oldTab.length;
// 第一次的时候也是0
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
// 初始化
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"})
// 初始化数组
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
// 扩容的时候
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;
}
