本篇内容是学习的记录，可能会有所不足。
一：JDK1.7中的HashMap
JDK1.7的hashMap是由数组 + 链表组成
/** 1 << 4，表示1，左移4位，变成10000，即16，以二进制形式运行，效率更高
* 默认的hashMap数组长度
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
* hashMap的最大容量
*/
static final int MAXIMUM_CAPACITY = 1 << 30; //1 073 741 824
/**
* The load factor used when none specified in constructor.
* 负载因子
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* An empty table instance to share when the table is not inflated.
*/
static final Entry<?,?>[] EMPTY_TABLE = {};
/**
* The table, resized as necessary. Length MUST Always be a power of two.
* hashTable，根据需要调整大小。长度一定是2的幂。
*/
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
/**
* The number of key-value mappings contained in this map.
* hashMap中元素的个数
*/
transient int size;
/**
* The next size value at which to resize (capacity * load factor).
* @serial
*/
// If table == EMPTY_TABLE then this is the initial capacity at which the
// table will be created when inflated.
int threshold;
/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
* 记录hashMap元素被修改的次数
*/
transient int modCount;
1：DEFAULT_INITIAL_CAPACITY，是hashMap默认的初始容量，它的大小一定是2的幂。
2：MAXIMUM_CAPACITY，hashMap支持的最大容量。
3：DEFAULT_LOAD_FACTOR，hashMap默认的负载因子，值为0.75，它决定hashMap数据的密度。
4：Entry<K,V>[] table，hashMap数组，可以根据自己的需要调整大小，长度一定是2的幂。
5：size，主要是记录hashMap中元素的数量。
6：threshold，调整hashMap后的值，即容量*负载因子。
7：loadFactor，可以调整的负载因子。
8：modCount，用来记录hashMap结构被修改的次数。
hashMap源码中有四个构造函数，初始化的时候可以知道容量和负载因子的大小。
/** 做了两件事：1、为threshold、loadFactor赋值 2、调用init()
* 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) //限制最大容量
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor)) //检查 loadFactor
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
//真正在做的，只是记录下loadFactor、initialCpacity的值
this.loadFactor = loadFactor; //记录下loadFactor
threshold = initialCapacity; //初始的 阈值threshold=initialCapacity=16
init();
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/** 默认的初始化容量、默认的加载因子
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() { //16 0.75
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
inflateTable(threshold);
putAllForCreate(m);
}接下来看下put方法：
public V put(K key, V value) {
if (Entry<K,V>[] table == EMPTY_TABLE) {
inflateTable(threshold); //初始化表 （初始化、扩容 合并为了一个方法）
}
if (key == null) //对key为null做特殊处理
return putForNullKey(value);
int hash = hash(key); //计算hash值
int i = indexFor(hash, table.length); //根据hash值计算出index下标
for (Entry<K,V> e = table[i]; e != null; e = e.next) { //遍历下标为i处的链表
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //如果key值相同，覆盖旧值，返回新值
V oldValue = e.value;
e.value = value; //新值 覆盖 旧值
e.recordAccess(this); //do nothing
return oldValue; //返回旧值
}
}
modCount++; //修改次数+1，类似于一个version number
addEntry(hash, key, value, i);
return null;
}可以看到到table是空的时候，调用了一个方法：
private void inflateTable(int toSize) {
// Find a power of 2 >= toSize
int capacity = roundUpToPowerOf2(toSize);
//
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity]; //初始化表
initHashSeedAsNeeded(capacity);
}这个方法用来初始化table和table的扩容，roundUpToPowerOf2可以保证hashMap的容量一定是2的幂。
hashMap put元素时，会先根据hash运算计算出hash值，然后根据hash值和table的长度进行取模，计算出元素在table中的下标，如果key相同就覆盖原来的旧值，如果不相同就加入链表中。
/**
* Returns index for hash code h.
* 计算元素在table中的下标位置
*/
static int indexFor(int h, int length) {
// assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
return h & (length-1);
} /**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket. It is the responsibility of this
* method to resize the table if appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) { //如果size大于threshold && table在下标为index的地方已经有entry了
resize(2 * table.length); //扩容，将数组长度变为原来两倍
hash = (null != key) ? hash(key) : 0; //重新计算 hash 值
bucketIndex = indexFor(hash, table.length); //重新计算下标
}
createEntry(hash, key, value, bucketIndex); //创建entry
}
/**
* Like addEntry except that this version is used when creating entries
* as part of Map construction or "pseudo-construction" (cloning,
* deserialization). This version needn't worry about resizing the table.
*
* Subclass overrides this to alter the behavior of HashMap(Map),
* clone, and readObject.
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex]; //获取table中存的entry
table[bucketIndex] = new Entry<>(hash, key, value, e); //将新的entry放到数组中，next指向旧的table[i]
size++; //修改map中元素个数
}当put的元素个数大于12时，即大于hashMap的容量*负载因子计算后的值，那么就会进行扩容，上述源代码可以看到扩容的条件， 除了大于12，还要看当前put进table所处的位置，是否为null，若是null，就不进行扩容，否则就扩容成原来容量的2倍，扩容后需要重新计算hash和计算下标，由于table的长度发生了变化，需要重新计算。
接下来看下get方法：
public V get(Object key) {
if (key == null)
return getForNullKey();
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
/**
* Returns the entry associated with the specified key in the
* HashMap. Returns null if the HashMap contains no mapping
* for the key.
*/
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}get方法也是需要先计算hash然后计算下标，再去寻找元素。
二：JDK1.8中的HashMap
JDK1.8中的hashMap和1.7最大的区别就是引入了红黑树
/**
* 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;
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet;
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
transient int modCount;
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
int threshold;
/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 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;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
static final int MIN_TREEIFY_CAPACITY = 64;
/**
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}下面看下put方法：
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods. 添加元素
*
* @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) //若table为null
n = (tab = resize()).length; //resize
if ((p = tab[i = (n - 1) & hash]) == null) //计算下标i，取出i处的元素为p，如果p为null
tab[i] = newNode(hash, key, value, null); //创建新的node，放到数组中
else { //若 p!=null
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //若key相同
e = p; //直接覆盖
else if (p instanceof TreeNode) //如果为 树节点
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //放到树中
else { //如果key不相同，也不是treeNode
for (int binCount = 0; ; ++binCount) { //遍历i处的链表
if ((e = p.next) == null) { //找到尾部
p.next = newNode(hash, key, value, null); //在末尾添加一个node
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //如果链表长度 >= 8
treeifyBin(tab, hash); //将链表转成共黑树
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) //若果key相同，直接退出循环
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;
}可以看到，上述源代码中，put的时候加入了红黑树，当put元素时，若链表的长度大于8，即源代码中的TREEIFY_THRESHOLD的值，这个时候链表就会转化为红黑树结构；当进行扩容的时候，红黑树转移后，若元素个数小于6，那么就会重新转化为链表。
三：JDK1.7中的ConcurrentHashMap
JDK1.7中的ConcurrentHashMap和JDK1.7中的HashMap的区别就是数组所存的元素，我们知道ConcurrentHashMap 是线程安全的。
public V put(K key, V value) {
Segment<K,V> s;
if (value == null)
throw new NullPointerException();
int hash = hash(key); // 计算Hash值
int j = (hash >>> segmentShift) & segmentMask; //计算下标j
if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck
(segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment
s = ensureSegment(j); //若j处有segment就返回，若没有就创建并返回
return s.put(key, hash, value, false); //将值put到segment中去
}
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
HashEntry<K,V> node = tryLock() ? null :
scanAndLockForPut(key, hash, value); //如果tryLock成功，就返回null，否则。。。
V oldValue;
try {
HashEntry<K,V>[] tab = table;
int index = (tab.length - 1) & hash; //根据table数组的长度 和 hash值计算index小标
HashEntry<K,V> first = entryAt(tab, index); //找到table数组在 index处链表的头部
for (HashEntry<K,V> e = first;;) { //从first开始遍历链表
if (e != null) { //若e!=null
K k;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) { //如果key相同
oldValue = e.value; //获取旧值
if (!onlyIfAbsent) { //若absent=false
e.value = value; //覆盖旧值
++modCount; //
}
break; //若已经找到，就退出链表遍历
}
e = e.next; //若key不相同，继续遍历
}
else { //直到e为null
if (node != null) //将元素放到链表头部
node.setNext(first);
else
node = new HashEntry<K,V>(hash, key, value, first); //创建新的Entry
int c = count + 1; //count 用来记录元素个数
if (c > threshold && tab.length < MAXIMUM_CAPACITY) //如果hashmap元素个数超过threshold，并且table长度小于最大容量
rehash(node); //rehash跟resize的功能差不多,将table的长度变为原来的两倍，重新打包entries，并将给定的node添加到新的table
else //如果还有容量
setEntryAt(tab, index, node); //就在index处添加链表节点
++modCount; //修改操作数
count = c; //将count+1
oldValue = null; //
break;
}
}
} finally {
unlock(); //执行完操作后，释放锁
}
return oldValue; //返回oldValue
}
private Segment<K,V> ensureSegment(int k) {
final Segment<K,V>[] ss = this.segments;
long u = (k << SSHIFT) + SBASE; // raw offset 获取下标k处的offset，
Segment<K,V> seg;
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { //如果下标k处没有元素
Segment<K,V> proto = ss[0]; // use segment 0 as prototype
int cap = proto.table.length; //根据proto 获得 cap参数
float lf = proto.loadFactor; //。。。
int threshold = (int)(cap * lf); //计算threshold
HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
== null) { // recheck //如果下标k处仍然没有元素
Segment<K,V> s = new Segment<K,V>(lf, threshold, tab); //创建segment
while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
== null) { //若下标k处仍然没有元素，自旋
if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s)) //若通过CAS更新成功，则退出
break;
}
}
}
return seg;
}/** segments中每个元素都是一个专用的hashtable
* The segments, each of which is a specialized hash table.
*/
final Segment<K,V>[] segments;可以看到1.7中的ConcurrentHashMap数组中所存的是segments，每个segments下都是一个hashTable。当put元素时，会加锁，然后计算hash和下标，计算下标会计算两次，一次是在数组中的segments的位置，一次是在hashTable的位置。
四：JDK1.8中的ConcurrentHashMap
JDK1.8中的ConcurrentHashMap和JDK1.8中的HashMap结构一样，只是在处理上有区别
public V put(K key, V value) {
return putVal(key, value, false);
}
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode()); //计算hash值
int binCount = 0;
for (Node<K,V>[] tab = table;;) { //自旋
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0) //table==null || table.length==0
tab = initTable(); //就initTable
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //若下标 i 处的元素为null
if (casTabAt(tab, i, null, //直接用CAS操作，i处的元素
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin 想emptybin中假如元素的时候，不需要加锁
}
else if ((fh = f.hash) == MOVED) //若下标 i 处的元素不为null，且f.hash==MOVED MOVED为常量值-1
tab = helpTransfer(tab, f); //
else { //如果是一般的节点
V oldVal = null;
synchronized (f) { //当头部元素不为null，且不需要转换成树时，需要进行同步操作
if (tabAt(tab, i) == f) {
if (fh >= 0) { //若 链表头部hash值 >=0
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) { //如果key相同
oldVal = e.val;
if (!onlyIfAbsent) //且不为absent
e.val = value; //旧值覆盖新值
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null), { //如果链表遍历完成，还没退出，说明没有相同的key存在，在尾部添加节点
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) { //如果f是Tree的节点
Node<K,V> p;
binCount = 2;
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}当put元素时，会使用CAS操作，去判断数组中所要put到的位置元素是否为空，为空就修改为当前的put的元素，若CAS操作失败，那么会自旋，这个时候发现数组里已经有元素了，那么就会锁住链表或者红黑树头部，把元素放入链表或者红黑树下面 。
五：hash冲突
当put的时候需要计算hash和下标，这个时候计算出来的值可能存在一样的，那么存到数组中的相同位置，就会发生hash冲突，
计算出的hash值一样一定会发生hash冲突，但是hash值一样的概率很小，计算出的下标值是一样的概率很大，所以hash冲突主要是由下标位置一样引起的，hashMap的解决方式是使用链地址法，即使用链表的方式解决，key一样的时候才会覆盖，否则就把元素放到链表的下一个位置。
