HashMap继承AbstractMap，实现了Map<K,V>, Cloneable, Serializable接口。HashMap的构造方法有3个，分别是
- public HashMap(int initialCapacity, float loadFactor)
- public HashMap(int initialCapacity)
- public HashMap()
这三个方法中第二个方法是调用第一个方法
public HashMap(int initialCapacity) {
//static final float DEFAULT_LOAD_FACTOR = 0.75f;
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
而第三个方法只设置hash表的负载因子
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
因此重点看一下第一个方法
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);
//设置负载系数，下一次的容量的大小
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
下来需要看一下常用的put方法
public V put(K key, V value) {
//将key的hash值和k,v传入
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//判断当前数组为空，则调用resize()，重新给数组赋值
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//通过数组最后一位下标和key的hash值按位与计算出来的tab为空，则直接赋值在这个数组的第一个位置
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)
//如果当前节点是树节点则调用putTreeVal进行put
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//当数组的第一个位置不为空，则需要赋值在next上面
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//当一个数组的下标有超过7个（即冲突有8个）的时候将链表转化成树的形式存储
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;
}
}
//当有相同key和hash值，则只改变相应的value值，不生成新的节点
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
//空方法，linkedhashMap使用
afterNodeAccess(e);
return oldValue;
}
}
//修改次数加一，如果容量大于下一次的容量则重新修改大小
++modCount;
if (++size > threshold)
resize();
//空方法，linkedhashMap使用
afterNodeInsertion(evict);
return null;
}
主要的几个辅助方法
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//旧的数组容量
int oldCap = (oldTab == null) ? 0 : oldTab.length;
//旧的阈值
int oldThr = threshold;
int newCap, newThr = 0;
//旧的数组容量大于0
if (oldCap > 0) {
//已经达到了最大值2的30次方。无法扩展，直接返回原有的数组
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//新的容量等于旧的容量的2倍小于最大容量值并且旧的容量值大于等于缺省的容量值（2的4次方）则新的容量值=旧的容量值*2
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
//如果旧的阈值大于0，则新的容量值等于旧的阈值否则新的容量值和新的阈值设置为缺省值
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);
}
//当 else if (oldThr > 0)时，赋值给新的阈值
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;
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) {//判断是否要改变索引位置
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
//需要改变索引位置
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
//不改变索引位置，直接赋值
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
//将要改变索引位置的链表赋值到扩展的地方
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
