首先从一条简单的语句开始，创建了一个hashmap对象：
Map<String,String> hashmap = new HashMap<String,String>();
调用hashmap类无参数构造方法，使用默认转载因子，
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
默认装载因子只有在构造器不指定的情况下使用
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
hashmap还有三个重载的构造方法，分别是：
有装载因子入参的构造方法
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
有初始化容器容量以及装载因子的构造方法
public HashMap(int initialCapacity, float loadFactor) {
//如果初始容量小于0，则抛出非法初始化容量异常
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
//如果初始容量大于最多容量，则容量为最多容量
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
//如果装载因子不大于0或者装载因子不是数字，则抛出装载因子非法异常
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
//根据初始化容量计算hashmap的阈值容量(hashmap真正的容量，必须是2的次方)
this.threshold = tableSizeFor(initialCapacity);
}
计算hashmap的容量
// 根据给定的目标容量，计算一个2的次方
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
有参数map的构造方法：
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
将map复制一份到本地，这个复制如果对于数值型则为数值，对于引用型则为地址
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
//原来map的长度大于0处理
if (s > 0) {
if (table == null) { // pre-size
//根据实际数量计算需要多大的容量
float ft = ((float) s / loadFactor) + 1.0F;
int t = ((ft < (float) MAXIMUM_CAPACITY) ? (int) ft : MAXIMUM_CAPACITY);
//如果预估容量大于现有容量，则需要根据预估容量计算一个2的次方数
if (t > threshold)
threshold = tableSizeFor(t);
}
//如果实际容量大于阈值则需要扩容
else if (s > threshold)
resize();
//扩容之后，将m的key和value复制一份到本地
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
//第一次调用初始化或者扩容
final Node<K,V>[] resize() {
//首次调用时为null,则oldCap为0 | 扩容时，
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
//扩容时,如果目前容量已经超过最多容量，那么默认值为int最大，否则容量为现在2倍
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
}
//初始化时newCap = oldThr = 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);
}
//初始化时newThr=newCap * loadFactor
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"})
//初始化时，创建一个长度为threshold的Node数组,然后返回
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;
}