哈，標題開個玩笑，0202 年的段子哈。
一、首先看一下 HashMap 的構造函數
/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and load factor. * * @param initialCapacity the initial capacity * @param loadFactor the load factor * @throws IllegalArgumentException if the initial capacity is negative * or the load factor is nonpositive */ public HashMap(int initialCapacity, float loadFactor) { if (initialCapacity < 0) throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity); if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal load factor: " + loadFactor); this.loadFactor = loadFactor; this.threshold = tableSizeFor(initialCapacity); // 奇怪的是這裏初始化閾值沒有用到負載因子。 }
第一個參數是初始化容量大小，第二個參數是負載因子。
對這兩個參數有如下介紹：
* <p>An instance of <tt>HashMap</tt> has two parameters that affect its * performance: <i>initial capacity</i> and <i>load factor</i>. The * <i>capacity</i> is the number of buckets in the hash table, and the initial * capacity is simply the capacity at the time the hash table is created. The * <i>load factor</i> is a measure of how full the hash table is allowed to * get before its capacity is automatically increased. When the number of * entries in the hash table exceeds the product of the load factor and the * current capacity, the hash table is <i>rehashed</i> (that is, internal data * structures are rebuilt) so that the hash table has approximately twice the * number of buckets.
機翻的意思就是：
HashMap 的實例有兩個影響其性能的參數:初始容量和負載因子。
容量是哈希表中的桶數，初始容量就是創建哈希表時的容量。
負載因子是一種度量方法，用來衡量在自動增加哈希表的容量之前，哈希表允許達到的滿度。
當哈希表中的條目數超過負載因子和當前容量的乘積時，哈希表將被重新哈希(即重新構建內部數據結構)，
這樣哈希表的桶數大約是原來的兩倍。
/** * 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. */ static final int MAXIMUM_CAPACITY = 1 << 30;
最大的容量是 2 的 30 次方，因爲 int 類型最大值是 2 的 31 次方減一。容量還必須是 2 的冪。
* <p>This implementation provides constant-time performance for the basic * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function * disperses the elements properly among the buckets. Iteration over * collection views requires time proportional to the "capacity" of the * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number * of key-value mappings). Thus, it's very important not to set the initial * capacity too high (or the load factor too low) if iteration performance is * important.
另外不要將容量設置太高，或者將負載因子設置太低，這都會影響性能。
// The next size value at which to resize (capacity * load factor). int threshold;
閾值，等於容量和負載因子的乘積，如果 table.length 大於 閾值，就得進行 2 倍擴容。
接下來看看 tableSizeFor 方法，也就是計算閾值的：
/** * Returns a power of two size for the given target capacity. */ 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; }
翻譯的意思是返回給定目標容量的 2 的冪，也就是大於且最接近給定目標容量的最小 2 的冪。
這段代碼可能看着不太好理解，我們假設 n 的最高位的 1 在位置 i 上，>>> 表示無符號右移。
(>>> 和 >> 的區別就是前者高位不管正數負數都取0，後者正數取 0，負數取 1)。
右移一位再和原來的值進行或操作，那麼結果位置 i 和 i-1 的值一定也爲 1。
同理，最後結果一定是 0 ~ i 位都爲 1，再加 1 的話，就是最接近給定值的最小2的冪。
另外如果 cap 爲 0 的話，那麼就是所有位都是 1 了，n 就小於 0，閾值就爲 1。
現在我們在來看下另外的三個構造函數：
/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and the default load factor (0.75). * * @param initialCapacity the initial capacity. * @throws IllegalArgumentException if the initial capacity is negative. */ public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** * Constructs an empty <tt>HashMap</tt> with the default initial capacity * (16) and the default load factor (0.75). */ public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted } /** * Constructs a new <tt>HashMap</tt> with the same mappings as the * specified <tt>Map</tt>. The <tt>HashMap</tt> is created with * default load factor (0.75) and an initial capacity sufficient to * hold the mappings in the specified <tt>Map</tt>. * * @param m the map whose mappings are to be placed in this map * @throws NullPointerException if the specified map is null */ public HashMap(Map<? extends K, ? extends V> m) { this.loadFactor = DEFAULT_LOAD_FACTOR; putMapEntries(m, false); }
對於前兩個就不用說了，默認的負載因子爲 0.75，爲什麼要取這個值呢？
* <p>As a general rule, the default load factor (.75) offers a good * tradeoff between time and space costs. Higher values decrease the * space overhead but increase the lookup cost (reflected in most of * the operations of the <tt>HashMap</tt> class, including * <tt>get</tt> and <tt>put</tt>). The expected number of entries in * the map and its load factor should be taken into account when * setting its initial capacity, so as to minimize the number of * rehash operations. If the initial capacity is greater than the * maximum number of entries divided by the load factor, no rehash * operations will ever occur.
機翻如下：
作爲一般規則，默認的負載係數(.75)在時間和空間成本之間提供了一個很好的折衷。
較高的值減少了空間開銷，但增加了查找成本(反映在HashMap類的大部分操作中，包括get和put)。
在設置初始容量時，應該考慮映射中的預期條目數及其負載因子，以便最小化重散列操作的數量。
如果初始容量大於最大條目數除以負載因子(初始容量和負載因子的乘積大於最大條目數)，則不會發生重新散列操作。
接下來看看 putMapEntries 方法，最初構造時爲假，否則爲真。
/** * Implements Map.putAll and Map constructor. * * @param m the map * @param evict false when initially constructing this map, else * true (relayed to method afterNodeInsertion). */ final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) { int s = m.size(); if (s > 0) { if (table == null) { // pre-size // table 沒有被初始化過(也就是構建函數是調用的)，就初始化一下閾值 float ft = ((float)s / loadFactor) + 1.0F; int t = ((ft < (float)MAXIMUM_CAPACITY) ? (int)ft : MAXIMUM_CAPACITY); if (t > threshold) threshold = tableSizeFor(t); } else if (s > threshold) // table 已經被初始化過了，長度大於閾值需要進行擴容處理 resize(); 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); } } }
table 就是存儲 Map 鍵值對的數組，並根據需要調整大小，長度總是 2 的冪。
/** * The table, initialized on first use, and resized as * necessary. When allocated, length is always a power of two. * (We also tolerate length zero in some operations to allow * bootstrapping mechanics that are currently not needed.) */ transient Node<K,V>[] table;
Node 就是一個靜態內部類，也就是存儲 Map 的實體。
/** * Basic hash bin node, used for most entries. (See below for * TreeNode subclass, and in LinkedHashMap for its Entry subclass.) */ 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; } }
我們目前看的是構建時候調用，就只看構建時候走的邏輯。那麼我們看下 putVal 方法 和 hash 方法：
hash 方法(每個 key 的 hash 是不會變的，這個無符號右移 16 位的操作可以減少衝突)：
/**
* 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.
*/
static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); // 保留高16位，將高16位和低16位進行異或的結果作爲低16位。 } public final int hashCode() { return Objects.hashCode(key) ^ Objects.hashCode(value); }
putVal 方法：
/** * 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) // table 還未被初始化過或者數據被清空，進行初始化。 n = (tab = resize()).length; if ((p = tab[i = (n - 1) & hash]) == null) // 如果這個表裏沒有這個 key 的哈希，就把這個鍵值對存表裏,因爲是與操作(n是2的冪)，所以擴容對位置沒有影響1 tab[i] = newNode(hash, key, value, null); else { // 如果表裏已經有這個 key 的哈希了，再進行進一步的比對，判斷是否存在 Node<K,V> e; K k; if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) // 先比對第一個節點的值 e = p; else if (p instanceof TreeNode) // 如果第一個節點不同，判斷是否是紅黑樹結構，進行比對 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { // 說明是鏈表結構，進行比對 for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { // 如果找不到，那麼就將這個鍵值對存進去，並判斷是否到達了要轉換成紅黑樹的條件 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) // 如果爲 onlyIfAbsent 爲 true，不改變現在的值 e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; // 用於記錄修改映射的數量，該字段用於使 HashMap 集合視圖上的迭代器快速失效 if (++size > threshold) // 說明找不到該 key 的鍵值對，就插入進去 resize(); afterNodeInsertion(evict); return null; }
如果 table 爲空的話，我們來看下 resize 方法，蠻長的，初始化或者給表的長度加倍：
/** * 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) { // 原來的 table 有值 if (oldCap >= MAXIMUM_CAPACITY) { // 容量大於最大容量，閾值設爲最大。 threshold = Integer.MAX_VALUE; return oldTab; } else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && // 新容量擴展爲原來兩倍並且小於最大容量大於初始容量，就把新新的閾值設爲原來兩倍。 oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } else if (oldThr > 0) // initial capacity was placed in threshold // 原來的 table 已經初始化過，但是table 裏沒有數據，新容量等於原來的閾值。 newCap = oldThr; else { // zero initial threshold signifies using defaults // table 還沒有初始化過，進行初始化。 newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } if (newThr == 0) { // 原來的 table 已經初始化過，但是 table 裏沒有數據，計算一下新閾值。 float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } threshold = newThr; // 設置閾值 @SuppressWarnings({"rawtypes","unchecked"}) Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; table = newTab; if (oldTab != null) { // 如果原來 table 有值，就把值放進新的 table 裏. for (int j = 0; j < oldCap; ++j) { Node<K,V> e; if ((e = oldTab[j]) != null) { oldTab[j] = null; if (e.next == null) // 這個節點沒有發生過沖突 newTab[e.hash & (newCap - 1)] = e; else if (e instanceof TreeNode) // 發生過沖突，並且是紅黑樹結構 ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); else { // preserve order // 發生過沖突，是鏈表結構 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) { // 分成兩段鏈表，暫時不知道爲什麼。 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; }
二、我們再來看看常用的 get 和 set 方法(感覺沒啥好解釋的了)：
public V get(Object key) { Node<K,V> e; return (e = getNode(hash(key), key)) == null ? null : e.value; } public V put(K key, V value) { return putVal(hash(key), key, value, false, true); } 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; }