直接进入正题，先看该类的属性
//默认初始化的容量 16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
//默认最大的容量。必须是2的冥
static final int MAXIMUM_CAPACITY = 1 << 30;
//构造函数中未指定时使用的负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//节点转换成树的阈值
static final int TREEIFY_THRESHOLD = 8;
//节点减少成链表的阈值
static final int UNTREEIFY_THRESHOLD = 6;
//
transient Node<K,V>[] table;
//键值对的数量
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;
//下一次扩容的值（capacity * load factor）
int threshold;
//加载因子
final float loadFactor;
通过上面的属性，能发现，hashMap是一个Node数组，那么接下来看Node类的属性
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
//下一个Node,形成了一个链表的关系
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;
}
.....
}
在来看构造函数
//无参构造函数
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//指定初始化容量的参数。默认的加载因子0.75
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
//指定初始化容量与加载因子
public HashMap(int initialCapacity, float loadFactor) {
//如果指定的初始化容量小于了0，抛异常
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
//如果指定的初始化容量大于了最大值，则使用最大的容量值
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
//返回给定目标容量的两个大小的幂
this.threshold = tableSizeFor(initialCapacity);
}
接下来查看，往HashMap中添加数据的时候，执行的代码
//往map中存放数据，先计算key的 hash值
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
//计算key的hash值
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
//重点
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//如果table 为空，则先进行扩容
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//通过key的hash与table的数组大小进行与运算，确定该key的位置，
//如果该位置为空，则创建一个node,并赋值
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//如果存放的key的hash值与该位置node的key的hash相同并且key值也相同
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果该位置的node是一个TreeNode
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//进行一个for的死循环
for (int binCount = 0; ; ++binCount) {
//通过一个node节点找下一个node,如果下一个node为空
if ((e = p.next) == null) {
//生成一个新的node，放在该node的下一个
p.next = newNode(hash, key, value, null);
//如果该node链表中的数量大于等于了8
if (binCount >= TREEIFY_THRESHOLD - 1)
treeifyBin(tab, hash);//转换成树
//跳出循环
break;
}
//如果该node与存放的key的hash相同，并且key值也相同,跳出循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
//通过for循环，挨个查找下个node
p = e;
}
}
//如果e不等于空，表示map中存在已有相同的key
if (e != null) {
//获取原来的value值
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value; //新的vlue值替换原有的value值
//LinkedHashMap post操作的回调（hashMap中是空操作）
afterNodeAccess(e);
return oldValue;//返回原来的value值
}
}
//修改次数加一
++modCount;
//如果当前的键值对数量大于了需要扩容的值（capacity * load factor），则进行扩容
if (++size > threshold)
resize();
//空操作不用理会
afterNodeInsertion(evict);
return null;
}
下面则查看resize 方法的代码
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;
}
//如果table的容量小于最大容量，并且大于默认的初始容量，则扩容阈值变成两倍
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;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//如果扩容阈值为0，则设置值
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
//将扩容后的table赋值给原来的table
table = newTab;
if (oldTab != null) {
//将原来的table中的值，存放到新的table中
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
//如果为空，表示该node链表上只有一个node
if (e.next == null)
//将该node进行hash重新定位到新的table中
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;
}
