总的要点
- new的时候，初始化了加载因子是0.75， 数组是空的
- put第一元素的时候，初始化了数组容量是16，阀值是12，一旦存入第十三个元素，数组长度就变成32
- 根据key的hash值和数组的长度 取余决定了元素在数组中的存放位置，如果两个key的hash值计算以后，数组中的位置是一样的，就会产生链表
- get的时候，（1）根据key的hashcode值确定数组中的位置 （2）如果存在链表，会执行key的equals方法，来确定节点
- 通过get方法的过程可以理解到，为什么重写equals方法一定要重写hashcode方法。 比如用户类，重写了equals方法，只要id属性的值相同，就认为是同一个对象，equals方法就返回true,但是忘记重写hashcode方法， 这个时候，如果有两个user对象A和B，id属性相同，使用equals方法返回true,认为是同一个对象。 当我们把用户对象作为hashmap的key的时候，A作为key存入一个字符串的值，然后用B来get就会get不到，因为hashcode不同，在数组的位置已经不同了。
JAVA 7 HashMap
使用无参构造方法，创建一个HashMap HashMap h1=new HashMap();
只是是初始化了加载因子，默认值是0.75.
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
h1.put(“key1”,“tt”);
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
putVal 方法源码
/**
* 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;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
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)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
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;
}
}
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;
}
如果是put第一个元素，那么原来的数组table肯定是空的，所以会执行如下
n = (tab = resize()).length;
resize方法的源码
/**
* 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;
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
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"})
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) {
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;
}
第一次的时候，肯定会运行如下
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
上面这两句话，实现了
- 把数组的容量初始化成了16（1<<4）.
- 把阀值设置成了12（16*加载因子0.75）. 也就是一旦数组的存入第13个元素，那么数组的容量就会变成32.
然后会执行
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
这个地方实现了
- 把全局变量threshold的值变成了12
- 把全局变量 table 变成了一个长度是16的Node类型的数组
回到putVal方法
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
实现了如下
- 把全局变量modCount 从0变成了1