public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {// 序列号private static final long serialVersionUID = 362498820763181265L;    // 默认的初始容量是16static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;   // 最大容量static final int MAXIMUM_CAPACITY = 1 << 30; // 默认的填充因子static final float DEFAULT_LOAD_FACTOR = 0.75f;// 当桶(bucket)上的结点数大于这个值时会转成红黑树static final int TREEIFY_THRESHOLD = 8; // 当桶(bucket)上的结点数小于这个值时树转链表static final int UNTREEIFY_THRESHOLD = 6;// 树的最小的容量，至少是 4 x TREEIFY_THRESHOLD = 32 然后为了避免(resizing 和 treeification thresholds) 设置成64static final int MIN_TREEIFY_CAPACITY = 64;// 存储元素的数组，总是2的幂次倍transient Node<k,v>[] table; // 存放具体元素的集transient Set<map.entry<k,v>> entrySet;// 存放元素的个数，注意这个不等于数组的长度。transient int size;// 每次扩容和更改map结构的计数器transient int modCount;   // 临界值 当实际大小(容量*填充因子)超过临界值时，会进行扩容int threshold;// 填充因子final float loadFactor;
}

public HashMap(int initialCapacity, float loadFactor) {// 初始容量不能小于0，否则报错if (initialCapacity < 0)throw new IllegalArgumentException("Illegal initial capacity: " +initialCapacity);// 初始容量不能大于最大值，否则为最大值if (initialCapacity > MAXIMUM_CAPACITY)initialCapacity = MAXIMUM_CAPACITY;// 填充因子不能小于或等于0，不能为非数字if (loadFactor <= 0 || Float.isNaN(loadFactor))throw new IllegalArgumentException("Illegal load factor: " +loadFactor);// 初始化填充因子                                        this.loadFactor = loadFactor;// 初始化threshold大小this.threshold = tableSizeFor(initialCapacity);
}

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;}

public HashMap(int initialCapacity) {// 调用HashMap(int, float)型构造函数this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

public HashMap() {// 初始化填充因子this.loadFactor = DEFAULT_LOAD_FACTOR;
}

public HashMap(Map<? extends K, ? extends V> m) {// 初始化填充因子this.loadFactor = DEFAULT_LOAD_FACTOR;// 将m中的所有元素添加至HashMap中putMapEntries(m, false);
}

final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {int s = m.size();if (s > 0) {// 判断table是否已经初始化if (table == null) { // pre-size// 未初始化，s为m的实际元素个数float ft = ((float)s / loadFactor) + 1.0F;int t = ((ft < (float)MAXIMUM_CAPACITY) ?(int)ft : MAXIMUM_CAPACITY);// 计算得到的t大于阈值，则初始化阈值if (t > threshold)threshold = tableSizeFor(t);}// 已初始化，并且m元素个数大于阈值，进行扩容处理else if (s > threshold)resize();// 将m中的所有元素添加至HashMap中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);}}
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {Node<K,V>[] tab; Node<K,V> p; int n, i;// table未初始化或者长度为0，进行扩容if ((tab = table) == null || (n = tab.length) == 0)n = (tab = resize()).length;// (n - 1) & hash 确定元素存放在哪个桶中，桶为空，新生成结点放入桶中(此时，这个结点是放在数组中)if ((p = tab[i = (n - 1) & hash]) == null)tab[i] = newNode(hash, key, value, null);// 桶中已经存在元素else {Node<K,V> e; K k;// 比较桶中第一个元素(数组中的结点)的hash值相等，key相等if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))// 将第一个元素赋值给e，用e来记录e = p;// hash值不相等，即key不相等；为红黑树结点else if (p instanceof TreeNode)// 放入树中e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);// 为链表结点else {// 在链表最末插入结点for (int binCount = 0; ; ++binCount) {// 到达链表的尾部if ((e = p.next) == null) {// 在尾部插入新结点p.next = newNode(hash, key, value, null);// 结点数量达到阈值，转化为红黑树if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1sttreeifyBin(tab, hash);// 跳出循环break;}// 判断链表中结点的key值与插入的元素的key值是否相等if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))// 相等，跳出循环break;// 用于遍历桶中的链表，与前面的e = p.next组合，可以遍历链表p = e;}}// 表示在桶中找到key值、hash值与插入元素相等的结点if (e != null) { // 记录e的valueV oldValue = e.value;// onlyIfAbsent为false或者旧值为nullif (!onlyIfAbsent || oldValue == null)//用新值替换旧值e.value = value;// 访问后回调afterNodeAccess(e);// 返回旧值return oldValue;}}// 结构性修改++modCount;// 实际大小大于阈值则扩容if (++size > threshold)resize();// 插入后回调afterNodeInsertion(evict);return null;
}

final Node<K,V> getNode(int hash, Object key) {Node<K,V>[] tab; Node<K,V> first, e; int n; K k;// table已经初始化，长度大于0，根据hash寻找table中的项也不为空if ((tab = table) != null && (n = tab.length) > 0 &&(first = tab[(n - 1) & hash]) != null) {// 桶中第一项(数组元素)相等if (first.hash == hash && // always check first node((k = first.key) == key || (key != null && key.equals(k))))return first;// 桶中不止一个结点if ((e = first.next) != null) {// 为红黑树结点if (first instanceof TreeNode)// 在红黑树中查找return ((TreeNode<K,V>)first).getTreeNode(hash, key);// 否则，在链表中查找do {if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))return e;} while ((e = e.next) != null);}}return null;
}

final Node<K,V>[] resize() {// 当前table保存Node<K,V>[] oldTab = table;// 保存table大小int oldCap = (oldTab == null) ? 0 : oldTab.length;// 保存当前阈值 int oldThr = threshold;int newCap, newThr = 0;// 之前table大小大于0if (oldCap > 0) {// 之前table大于最大容量if (oldCap >= MAXIMUM_CAPACITY) {// 阈值为最大整形threshold = Integer.MAX_VALUE;return oldTab;}// 容量翻倍，使用左移，效率更高else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&oldCap >= DEFAULT_INITIAL_CAPACITY)// 阈值翻倍newThr = oldThr << 1; // double threshold}// 之前阈值大于0else if (oldThr > 0)newCap = oldThr;// oldCap = 0并且oldThr = 0，使用缺省值（如使用HashMap()构造函数，之后再插入一个元素会调用resize函数，会进入这一步）else {           newCap = DEFAULT_INITIAL_CAPACITY;newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);}// 新阈值为0if (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tableNode<K,V>[] newTab = (Node<K,V>[])new Node[newCap];table = newTab;// 之前的table已经初始化过if (oldTab != null) {// 复制元素，重新进行hashfor (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 orderNode<K,V> loHead = null, loTail = null;Node<K,V> hiHead = null, hiTail = null;Node<K,V> next;// 将同一桶中的元素根据(e.hash & oldCap)是否为0进行分割，分成两个不同的链表，完成rehashdo {next = e.next;if ((e.hash & oldCap) == 0) {if (loTail == null)loHead = e;elseloTail.next = e;loTail = e;}else {if (hiTail == null)hiHead = e;elsehiTail.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;
}