本文分析基于 JDK1.8
HashMap是使用数组 + 链表 + 红黑树 的结构来存储的。当hash冲突时使用链表来存储数据，当链表上的数量大于等于8时，转换成红黑树存储元素。
hashCode 方法
HashMap中处处用到了hashCode，先说一下hashCode方法。
HashCode#hash(Object key)
/**
* 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;
// 用hashCode的高位与低位进行异或运算，让高位参与运算，防止hashCode过小时冲突问题
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
知识点补充：
^运算是相应位数的什不一样时取1，否则取0- 正数在计算机底层是以原码形式存储，而负数是以补码形式存储
Integer#hashCode()值是本身；Long#hashCode()与HashMap的相似(int)(value ^ (value >>> 32))；String#hashCode()是h = 31 * h + val[i];把原本的 hash 乘以31后再加上每个字符串起来。
HashMap 的构造器
无参构造器
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 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
}
指定了默认的加载因子，Node 的初始化在第一次放元素的时候。
有参构造器
有参构造器最后调用的都是一个。
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
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);
}
注意的是 tableSizeFor(initialCapacity) 的方法，取比自己最大且最近的2的幂次方的数。
/**
* Returns a power of two size for the given target capacity.
*/
static final int tableSizeFor(int cap) {
// 先减1，防止该数刚好是2的幂次方
int n = cap - 1;
// 利用最高位的1把低全变成1，这样变变成全部的1了。比如：9
// 1001 | 100 = 1101
n |= n >>> 1;
// 1101 | 10 = 1111，上次或后最差的情况会把前两位变成1，这一步操作后就能把前4位变成1
n |= n >>> 2;
// 1111 | 1 = 1111
n |= n >>> 4;
// 1111 | 0 = 1111
n |= n >>> 8;
// 1111 | 0 = 1111
n |= n >>> 16;
// 因为是以前面或后的结果再移动，所以移动16次刚好把int的数给处理完。
// 再加1返回，刚好是2的幂次方
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
举例说明：
n = 0100 0000 0000 0000
0100 0000 0000 0000
010 0000 0000 0000 n |= n >>> 1
0110 0000 0000 0000
01 1000 0000 0000 n |= n >>> 2
0111 1000 0000 0000
0111 1000 0000 n |= n >>> 4
put方法
putVal() 方法
/**
* 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
*/
// onlyIfAbsent key 存在时是否覆盖 value，true为不覆盖
// evict HashMap中什么也没有做，在子类LinkHashMap中用来删除头节点。
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put时，会初始化数组
if ((tab = table) == null || (n = tab.length) == 0)
// 初始化 & 扩容 后面分析
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
// 当该位置为null时直接放入该位置
tab[i] = newNode(hash, key, value, null);
else {
// 当该数组的位置不为null时，然后判断是链表还是红黑树
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
// 如果插入元素的key与position上key相等，就把赋值给临时变量e
e = p;
else if (p instanceof TreeNode)
// 如果是红黑树节点，就放入树中
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// key不等于链表的头节点，也不是红黑树时，遍历链表，放到链表的最后面
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
// 放置链表最后面
p.next = newNode(hash, key, value, null);
// >= 7时，即链表上有8个元素的时候进行转换成红黑树
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))))
// 等于链表上存在的某一个节点时，退出，走下面的是否覆盖的操作
break;
p = e;
}
}
if (e != null) { // existing mapping for key
// 修改值并返回旧值
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
// 扩容
resize();
afterNodeInsertion(evict);
return null;
}
resize() 方法
/**
* 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;
// 旧数组的阈值
int oldThr = threshold;
int newCap, newThr = 0;
// 扩容而非初始化
if (oldCap > 0) {
// 容量达到最大，无法扩容
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
// 扩容
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;
// 初始化数组
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
// 计算扩容的阈值
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;
// 该位置上只有一个元素，直接放到新数组
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
// 该位置是红黑树结构
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
// 链表
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;
// 在新数组的下标还是当前的下标
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
// 在新数组的下标是 oldCap + index
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;
}
链表迁移
假如旧链表在旧数组的下标是3，capacity 是16，扩容后，capacity 是32。链表迁移是把链表上的数据分成两组，一组数据在新数组的下标是3，另一 组的下标应该是19（oldCapacity + index）。分组的依据是使用 e.hash & oldCap 因为在旧的数组上计算下脚标使用的是 e.hash & oldCap - 1 ，所以扩容后，只比较 oldCap 的最高位就能确定在扩容后数组的位置。
get方法
get的代码比较简单。
get(Object)
/**
* 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;
}
getNode(int, Object)
/**
* 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) {
// 如果是数组元素（链表/红黑树的第一个值），直接返回
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)
// 查找红黑树
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
// 遍历查看链表
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}