[TOC] > ### `HashMap`類圖  <br/> > ### 構造函數 ``` // DEFAULT_LOAD_FACTOR = 0.75 // initialCapacity 16 public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted } public HashMap(int initialCapacity, float loadFactor) { } ``` <br/> > ### 哈希算法 * 在原生`hashCode`的基礎上做了一次與高16位相異或的處理，這樣做的目的是將哈希值的高位納入到取余運算中來，防止由于低位相同造成的頻繁沖突的情況。 * `hashCode()`，默認情況下，`Object`中的`hashCode()` 返回對象的32位jvm內存地址。也就是說如果對象不重寫該方法，則返回相應對象的32為JVM內存地址。 ``` static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); } ``` <br/> > ### `put` * onlyIfAbsent參數如果為true，那么對于已經存在的key，將不替換其值 。 * `transient`修飾的變量在類實例被序列化時不會被序列化。 * table即HashMap進行數據存儲的核心變量:`transient Node<K,V>[] table;` ``` final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; //1.初始化 if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; // Initializes (16) or doubles table size //2.bin為null，初始化第一個節點，通過(n - 1) & hash定位在數組中的位置 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)))) //指定key的值已存在，那么記錄下原先的值 e = p; else if (p instanceof TreeNode) //紅黑樹 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { //bin不為空，且與鏈表頭不相同(==或equals) //3. for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); //達到臨界值（8）轉為紅黑樹 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; //空實現，為LinkedHashMap預留 afterNodeAccess(e); return oldValue; } } ++modCount; //4. if (++size > threshold) resize(); //空實現，為LinkedHashMap預留 afterNodeInsertion(evict); return null; } ``` <br/> > ### `resize()` * ``` final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; int oldThr = threshold; int newCap, newThr = 0; //原table不為null， if (oldCap > 0) { //MAXIMUM_CAPACITY取1 << 30，即table數組的大小，如果已到達此值，那么無需擴容 if (oldCap >= MAXIMUM_CAPACITY) { //threshold，CAPACITY乘以負載因子即擴容的臨界值 threshold = Integer.MAX_VALUE; return oldTab; } //沒有達到最大值，兩倍擴容 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 { //初始化，默認大小為16 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; 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) { //數組的此位置含有元素 oldTab[j] = null; if (e.next == null) //如果桶位中只有一個元素=>直接設置 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; } ``` <br/> <br/> *** 參考： [HashMap](https://github.com/seaswalker/JDK/blob/master/note/HashMap/hashmap.md)