/** * The default initial capacity - MUST be a power of two. */ 默认容量 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 /** * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. */ 最大容量 static final int MAXIMUM_CAPACITY = 1 << 30; /** * The load factor used when none specified in constructor. */ 默认负载因子 static final float DEFAULT_LOAD_FACTOR = 0.75f;
/** * 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). */ 修改次数（由于hashmap不是线程安全的，由此判断群体操作时的数据安全） transient int modCount; /** * The next size value at which to resize (capacity * load factor). * * @serial */ 桶元（容量*负载因子） // (The javadoc description is true upon serialization. // Additionally, if the table array has not been allocated, this // field holds the initial array capacity, or zero signifying // DEFAULT_INITIAL_CAPACITY.) int threshold; /** * The load factor for the hash table. * * @serial */ 负载因子 final float loadFactor;
/** * 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 */ 按照桶元找链表，通过链表寻找对应的Node，没有就创建一个Node 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; }
/** * Implements Map.get and related methods * * @param hash hash for key * @param key the key * @return the node, or null if none */ 按照桶元（ (n-1) & hash ）寻找链表，通过链表寻找Node final Node<K,V> getNode(int hash, Object key) {Node<K,V>[] tab; Node<K,V> first, e; int n; K k; if ((tab = table) != null && (n = tab.length) > 0 &&(first = tab[(n - 1) & hash]) != null) {if (first.hash == hash && // always check first node ((k = first.key) == key || (key != null && key.equals(k))))return first; if ((e = first.next) != null) {if (first instanceof TreeNode)return ((TreeNode<K,V>)first).getTreeNode(hash, key); do {if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))return e; } while ((e = e.next) != null); }}return null; }
/** * Basic hash bin node, used for most entries. (See below for * TreeNode subclass, and in LinkedHashMap for its Entry subclass.) */ 每一个Key对应的一个Node static class Node<K,V> implements Map.Entry<K,V> {final int hash; final K key; V value; 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; }public final K getKey() { return key; }public final V getValue() { return value; }public final String toString() { return key + "=" + value; }public final int hashCode() {return Objects.hashCode(key) ^ Objects.hashCode(value); }public final V setValue(V newValue) {V oldValue = value; value = newValue; return oldValue; }public final boolean equals(Object o) {if (o == this)return true; if (o instanceof Map.Entry) {Map.Entry<?,?> e = (Map.Entry<?,?>)o; if (Objects.equals(key, e.getKey()) &&Objects.equals(value, e.getValue()))return true; }return false; } }
/** * Computes key.hashCode() and spreads (XORs) higher bits of hash * to lower. Because the table uses power-of-two masking, sets of * hashes that vary only in bits above the current mask will * always collide. (Among known examples are sets of Float keys * holding consecutive whole numbers in small tables.) So we * apply a transform that spreads the impact of higher bits * downward. There is a tradeoff between speed, utility, and * quality of bit-spreading. Because many common sets of hashes * are already reasonably distributed (so don't benefit from * spreading), and because we use trees to handle large sets of * collisions in bins, we just XOR some shifted bits in the * cheapest possible way to reduce systematic lossage, as well as * to incorporate impact of the highest bits that would otherwise * never be used in index calculations because of table bounds. */ 计算一个hash static final int hash(Object key) {int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }
/** * 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 */ 创建或是扩容Node<K,V>[] 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; }