瞭解HashMap數據結構，超詳細！

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"寫在前面"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"小夥伴兒們，大家好！今天來學習HashMap相關內容，作爲面試必問的知識點，來深入瞭解一波！"}]}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"思維導圖："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/d1/d1a4c006620f1e619dfa7e6a7cfe6ce5.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"學習框架圖"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"1，HashMap集合簡介"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"HashMap基於哈希表的Map接口實現，是以"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"key-value"},{"type":"text","text":"存儲形式存在，即主要用來存放鍵值對。HashMap的實現不是同步的，這意味着它不是線程安全的。它的key、value都可以爲null。此外，HashMap中的映射不是有序的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"JDK1.8之前的HashMap由數組+鏈表組成的，數組是HashMap的主體，鏈表則是主要爲了節解決"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"哈希碰撞"},{"type":"text","text":"(兩個對象調用的hashCode方法計算的哈希碼值一致導致計算的數組索引值相同)而存在的（“拉鍊法”解決衝突）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"JDK1.8之後在解決哈希衝突時有了較大的變化，當"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"鏈表長度大於閾值"},{"type":"text","text":"（或者紅黑樹的邊界值，默認爲8）並且當前"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"數組的長度大於64"},{"type":"text","text":"時，此時此索引位置上的所有數據改爲使用紅黑樹存儲。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"數組裏面都是key-value的實例，在JDK1.8之前叫做Entry，在JDK1.8之後叫做Node。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/1c/1c7f5d4f4839b72908faba89c2ab56d9.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"key-value實例"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由於它的key、value都爲null，所以在插入的時候會根據key的hash去計算一個index索引的值。計算索引的方法如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"/**\n * 根據key求index的過程\n * 1,先用key求出hash值\n */\nstatic final int hash(Object key) {\n int h;\n return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);\n}\n//2,再用公式index = (n - 1) & hash（n是數組長度）\nint hash=hash(key);\nindex=(n-1)&hash;\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這裏的Hash算法本質上就是三步：取key的hashCode值、高位運算、取模運算。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣的話比如說put(\"A\",王炸)，插入了key爲\"A\"的元素，這時候通過上述公式計算出插入的位置index，若index爲3則結果如下（即hash(\"A\")=3）："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/7c/7c21871ec7ee8c9f758992d568b6cd3f.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那麼，HashMap中的鏈表又是幹什麼用的呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"大家都知道數組的長度是有限的，"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"在有限的長度裏面使用哈希函數計算index的值時，很有可能插入的k值不同，但所產生的hash是相同的"},{"type":"text","text":"（也叫做哈希碰撞），這也就是哈希函數存在一定的概率性。就像上面的K值爲A的元素，如果再次插入一個K值爲a的元素，很有可能所產生的index值也爲3，也就是即hash(\"a\")=3；那這就形成了鏈表，這種解決哈希碰撞的方法也叫做拉鍊法。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/06/06c43bf7aa4b497d858503e7c76c4ba7.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"當這個鏈表長度大於閾值8並且數組長度大於64則進行將鏈表變爲紅黑樹。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"補充："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"將鏈表轉換成紅黑樹前會判斷，"},{"type":"text","marks":[{"type":"strong"}],"text":"如果閾值大於8，但是數組長度小64，此時並不會將鏈表變爲紅黑樹。而是選擇進行數組擴容。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣做的目的是因爲數組比較小，儘量避開紅黑樹結構，這種情況下變爲紅黑樹結構，反而會降低效率，因爲紅黑樹需要進行左旋，右旋，變色這些操作來保持平衡。同事數組長度小於64時，搜索時間相對快一些。所以綜上所述爲了提高性能和減少搜索時間，底層在閾值大於8並且數組長度大於64時，鏈表才轉換爲紅黑樹。具體可以參考treeifyBin方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當然雖然增了紅黑樹作爲底層數據結構，結構變得複雜了，但是閾值大於8並且數組長度大於64時，鏈表轉換爲紅黑樹時，效率也變得更高效。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"特點："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"存取無序的"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"鍵和值位置都可以是null，但是鍵位置只能是一個null"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"鍵位置是唯一的，底層的數據結構控制鍵的"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"jdk1.8前數據結構是：鏈表 + 數組  jdk1.8之後是 ：鏈表 + 數組  + 紅黑樹"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"閾值(邊界值) > 8 並且數組長度大於64，纔將鏈表轉換爲紅黑樹，變爲紅黑樹的目的是爲了高效的查詢。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2，HsahMap底層數據結構"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2.1，HashMap存儲數據的過程"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/33/3372df90aec7531aa0993b6c5fb5ce3e.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"每一個Node結點都包含"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"鍵值對的key，value"},{"type":"text","text":"還有計算出來的"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"hash值"},{"type":"text","text":"，還保存着下一個"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":" "},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"Node 的引用 next"},{"type":"text","text":"（如果沒有下一個 Node，next = null），來看看Node的源碼："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"static class Node implements Map.Entry {\n final int hash;\n final K key;\n V value;\n Node next;\n ...\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"HashMap存儲數據需要用到put()方法，關於這些方法的詳解，我們下節再說，這裏簡要說一下；"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public static void main(String[] args) {\n HashMap hmap=new HashMap<>();\n hmap.put(\"斑\",55);\n hmap.put(\"鏡\",63);\n hmap.put(\"帶土\",25);\n hmap.put(\"鼬\",9);\n hmap.put(\"佐助\",43);\n hmap.put(\"斑\",88);\n System.out.println(hmap);\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當創建HashMap集合對象的時候，在jdk1.8之前，構造方法中會創建很多"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"長度是16的Entry[] table"},{"type":"text","text":"用來存儲鍵值對數據的。在jdk1.8之後不是在HashMap的構造方法底層創建數組了，是在"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"第一次調用put方法"},{"type":"text","text":"時創建的數組，"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"Node[] table"},{"type":"text","text":"用來存儲鍵值對數據的。"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"比方說我們向哈希表中存儲\"斑\"-55的數據，根據K值(\"斑\")調用String類中重寫之後的hashCode()方法計算出值（數量級很大），然後結合數組長度採用取餘（(n-1)&hash）操作或者其他操作方法來計算出向Node數組中存儲數據的空間的索引值。如果計算出來的索引空間沒有數據，則直接將\"斑\"-55數據存儲到數組中。跟上面的\"A-王炸\"數據差不多。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們回到上方的數組圖，如果此時再插入\"A-蘑菇\"元素，那麼首先根據Key值(\"A\")調用hashCode()方法結合數組長度計算出索引肯定也是3，此時比較後存儲的\"A-蘑菇\"和已經存在的數據\"A-王炸\"的hash值是否相等，如果hash相等，此時發生hash碰撞。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那麼底層會調用\"A\"所屬"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#F5222D","name":"red"}},{"type":"strong"}],"text":"類String中的equals方法"},{"type":"text","text":"比較兩個key內容是否相等，若相等，則後添加的數據直接覆蓋已經存在的Value，也就是\"蘑菇\"直接覆蓋\"王炸\"；若不相等，繼續向下和其他數據的key進行比較，如果都不相等，則規劃出一個節點存儲數據。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/7d/7dce818b614372db22deffaffcba10c2.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"兩個結點key值比較，是否覆蓋"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2.2，哈希碰撞相關的問題"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"哈希表底層採用何種算法計算hash值？還有哪些算法可以計算出hash值？"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"底層是採用key的hashCode方法的值結合數組長度進行"},{"type":"text","marks":[{"type":"strong"}],"text":"無符號右移（>>>）"},{"type":"text","text":"、按位異或（^）、按位與（&）計算出索引的"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"還可以採用：平方取中法，取餘數，僞隨機數法。這三種效率都比較低。而無符號右移16位異或運算效率是最高的。"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當兩個對象的hashCode相等時會怎麼樣？"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"會產生哈希碰撞，若"},{"type":"text","marks":[{"type":"strong"}],"text":"key值內容相同則替換舊的value"},{"type":"text","text":".否則連接到鏈表後面，"},{"type":"text","marks":[{"type":"strong"}],"text":"鏈表長度超過閾值8"},{"type":"text","text":"就轉換爲紅黑樹存儲。"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"何時發生哈希碰撞和什麼是哈希碰撞,如何解決哈希碰撞？"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"只要"},{"type":"text","marks":[{"type":"strong"}],"text":"兩個元素的key計算的哈希值相同"},{"type":"text","text":"就會發生哈希碰撞。jdk8前使用鏈表解決哈希碰撞。jdk8之後使用鏈表+紅黑樹解決哈希碰撞。"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果兩個鍵的hashcode相同，如何存儲鍵值對？"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"hashcode相同，通過equals比較內容是否相同。"},{"type":"text","marks":[{"type":"strong"}],"text":"相同：則新的value覆蓋之前的value 不相同：則將新的鍵值對添加到哈希表中"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2.3，紅黑樹結構"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當位於一個鏈表中的元素較多，即hash值相等但是內容不相等的元素較多時，通過key值依次查找的效率較低。而jdk1.8中，哈希表存儲採用數組+鏈表+紅黑樹實現，當鏈表長度(閥值)超過 8 時且當前數組的長度 > 64時，將鏈表轉換爲紅黑樹，這樣大大減少了查找時間。jdk8在哈希表中"},{"type":"text","marks":[{"type":"strong"}],"text":"引入紅黑樹的原因只是爲了查找效率更高。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/2d/2d6bf6020091655bf7dade250e72cbc8.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"紅黑樹結構"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"JDK 1.8 以前 HashMap 的實現是 數組+鏈表，即使哈希函數取得再好，也很難達到元素百分百均勻分佈。當 HashMap 中有大量的元素都存放到同一個桶中時，這個桶下有一條長長的鏈表，這個時候 HashMap 就相當於一個單鏈表，假如單鏈表有 n 個元素，遍歷的時間複雜度就是 O(n)，完全失去了它的優勢。針對這種情況，JDK 1.8 中引入了 紅黑樹（"},{"type":"text","marks":[{"type":"strong"}],"text":"查找時間複雜度爲 O(logn)"},{"type":"text","text":"）來優化這個問題。當鏈表長度很小的時候，即使遍歷，速度也非常快，但是當鏈表長度不斷變長，肯定會對查詢性能有一定的影響，所以才需要轉成樹。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2.4，存儲流程圖"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"HashMap存放數據是用的put方法，put 方法內部調用的是 putVal() 方法，所以對 put 方法的分析也是對 putVal 方法的分析，整個過程比較複雜，流程圖如下："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e4/e40955237c51d77cbddd5d04b53a66f6.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"來看看put()源碼："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public V put(K key, V value) {\n //對key的hashCode()做hash，調用的是putVal方法\n return putVal(hash(key), key, value, false, true);\n }\n\n final V putVal(int hash, K key, V value, boolean onlyIfAbsent,\n boolean evict) {\n Node[] tab; Node p; int n, i;\n /*\n 1，tab爲空則開始創建，\n 2，(tab = table) == null 表示將空的table賦值給tab,然後判斷tab是否等於null，第一次肯定是null\n 3，(n = tab.length) == 0 表示沒有爲table分配內存\n 4，tab爲空，執行代碼 n = (tab = resize()).length; 進行擴容。並將初始化好的數組長度賦值給n.\n 5，執行完n = (tab = resize()).length，數組tab每個空間都是null\n */\n \n if ((tab = table) == null || (n = tab.length) == 0)\n //調用resize()方法進行擴容\n n = (tab = resize()).length;\n /*\n 1，i = (n - 1) & hash 表示計算數組的索引賦值給i，即確定元素存放在哪個桶中\n 2，p = tab[i = (n - 1) & hash]表示獲取計算出的位置的數據賦值給節點p\n 3，(p = tab[i = (n - 1) & hash]) == null 判斷節點位置是否等於null，\n 如果爲null，則執行代碼：tab[i] = newNode(hash, key, value, null);根據鍵值對創建新的節點放入該位置的桶中\n 小結：如果當前桶沒有哈希碰撞衝突，則直接把鍵值對插入空間位置\n */ \n if ((p = tab[i = (n - 1) & hash]) == null)\n //節點位置爲null，則直接進行插入操作\n tab[i] = newNode(hash, key, value, null);\n //節點位置不爲null，表示這個位置已經有值了，於是需要進行比較hash值是否相等\n else {\n Node e; K k;\n /*\n 比較桶中第一個元素(數組中的結點)的hash值和key是否相等\n 1，p.hash == hash 中的p.hash表示原來存在數據的hash值 hash表示後添加數據的hash值 比較兩個hash值是否相等\n 2，(k = p.key) == key ：p.key獲取原來數據的key賦值給k key表示後添加數據的key 比較兩個key的地址值是否相等\n 3，key != null && key.equals(k)：能夠執行到這裏說明兩個key的地址值不相等，那麼先判斷後添加的key是否等於null，如果不等於null再調用equals方法判斷兩個key的內容是否相等\n */\n if (p.hash == hash &&\n ((k = p.key) == key || (key != null && key.equals(k))))\n /*\n 說明：兩個元素哈希值相等（哈希碰撞），並且key的值也相等\n 將舊的元素整體對象賦值給e，用e來記錄\n */ \n e = p;\n // hash值不相等或者key不相等；判斷p是否爲紅黑樹結點\n else if (p instanceof TreeNode)\n // 是紅黑樹，調用樹的插入方法\n e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);\n // 說明是鏈表節點，這時進行插入操作\n else {\n /*\n 1，如果是鏈表的話需要遍歷到最後節點然後插入\n 2，採用循環遍歷的方式，判斷鏈表中是否有重複的key\n */\n for (int binCount = 0; ; ++binCount) {\n /*\n 1)e = p.next 獲取p的下一個元素賦值給e\n 2)(e = p.next) == null 判斷p.next是否等於null，等於null，說明p沒有下一個元 素，那麼此時到達了鏈表的尾部，還沒有找到重複的key,則說明HashMap沒有包含該鍵\n 將該鍵值對插入鏈表中\n */\n if ((e = p.next) == null) {\n p.next = newNode(hash, key, value, null);\n //插入後發現鏈表長度大於8，轉換成紅黑樹結構\n if (binCount >= TREEIFY_THRESHOLD - 1) \n //轉換爲紅黑樹\n treeifyBin(tab, hash);\n break;\n }\n //key值以及存在直接覆蓋value\n if (e.hash == hash &&\n ((k = e.key) == key || (key != null && key.equals(k))))\n break;\n p = e;\n }\n }\n //若結點爲null，則不進行插入操作\n if (e != null) { \n V oldValue = e.value;\n if (!onlyIfAbsent || oldValue == null)\n e.value = value;\n afterNodeAccess(e);\n return oldValue;\n }\n }\n //修改記錄次數\n ++modCount;\n // 判斷實際大小是否大於threshold閾值，如果超過則擴容\n if (++size > threshold)\n resize();\n // 插入後回調\n afterNodeInsertion(evict);\n return null;\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"小結："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"根據哈希表中元素個數確定是"},{"type":"text","marks":[{"type":"strong"}],"text":"擴容還是樹形化"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"如果是樹形化遍歷桶中的元素，創建相同個數的樹形節點，複製內容，建立起聯繫"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"然後讓桶中的第一個元素指向新創建的樹根節點，替換桶的鏈表內容爲樹形化內容"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"3，HashMap的擴容機制"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們知道，數組的容量是有限的，多次插入數據的話，到達一定數量就會進行擴容；先來看兩個問題"}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"什麼時候需要擴容？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當HashMap中的"},{"type":"text","marks":[{"type":"strong"}],"text":"元素個數超過數組長度loadFactor(負載因子)"},{"type":"text","text":"時，就會進行數組擴容，loadFactor的默認值是0.75,這是一個折中的取值。也就是說，默認情況下，數組大小爲16，那麼當HashMap中的元素個數超過16×0.75=12(這個值就是閾值)的時候，就把數組的大小擴展爲2×16=32，即擴大一倍，然後"},{"type":"text","marks":[{"type":"strong"}],"text":"重新計算每個元素在數組中的位置"},{"type":"text","text":"，而這是一個非常耗性能的操作，所以如果我們已經預知HashMap中元素的個數，那麼預知元素的個數能夠有效的提高HashMap的性能。"}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"怎麼進行擴容的？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"HashMap在進行擴容時使用 resize() 方法，計算 table 數組的新容量和 Node 在新數組中的新位置，將舊數組中的值複製到新數組中，從而實現自動擴容。因爲每次擴容都是翻倍，與原來計算的 (n-1)&hash的結果相比，只是多了一個bit位，所以節點要麼就在原來的位置，要麼就被分配到\""},{"type":"text","marks":[{"type":"strong"}],"text":"原位置+舊容量"},{"type":"text","text":"\"這個位置。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因此，我們在擴充HashMap的時候，不需要重新計算hash，只需要看看原來hash值新增的那個bit是1還是0就可以了，是0的話索引沒變，是1的話索引變成“原索引+oldCap("},{"type":"text","marks":[{"type":"strong"}],"text":"原位置+舊容量"},{"type":"text","text":")”。這裏不再詳細贅述，可以看看下圖爲16擴充爲32的resize示意圖："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/fc/fc1b19df3ec331e181c92fa047b08dff.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"hashmap擴容"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"4，HashMap數組長度爲什麼是2的次冪"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們先看看它的成員變量："}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"序列化版本號"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private static final long serialVersionUID = 362498820763181265L;"}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"集合的初始化容量initCapacity"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"//默認的初始容量是16 -- 1<<4相當於1*2的4次方---1*16\nstatic final int DEFAULT_INITIAL_CAPACITY = 1 << 4; "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"初始化容量默認是16，容量過大，遍歷時會減慢速度，效率低；容量過小，那麼擴容的次數變多，非常耗費性能。"}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"負載因子"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"/**\n * The load factor used when none specified in constructor.\n */\n static final float DEFAULT_LOAD_FACTOR = 0.75f;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"初始默認值爲0.75，若過大，會導致哈希衝突的可能性更大；若過小，擴容的次數也會提高。"}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"爲什麼必須是2的n次冪？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當向HashMap中添加一個元素的時候，需要根據key的hash值，去確定其在數組中的具體位置。HashMap爲了提高存取效率，要儘量較少碰撞，就是要儘量把數據分配均勻，每個鏈表長度大致相同，這個實現就在把數據存到哪個鏈表中的算法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個算法實際就是取模，hash%length，計算機中直接求餘效率不如位移運算。所以源碼中做了優化,使用 "},{"type":"codeinline","content":[{"type":"text","text":"hash&(length-1)"}]},{"type":"text","text":"，而實際上"},{"type":"codeinline","content":[{"type":"text","text":"hash%length"}]},{"type":"text","text":"等於"},{"type":"codeinline","content":[{"type":"text","text":"hash&(length-1)"}]},{"type":"text","text":"的前提是length是2的n次冪。"}]},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"如果輸入值不是2的冪會怎麼樣？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果數組長度不是2的n次冪，計算出的索引特別容易相同，及其容易發生hash碰撞，導致其餘數組空間很大程度上並沒有存儲數據，鏈表或者紅黑樹過長，效率降低。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"小結："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"1，當根據key的hash確定其在數組的位置時，如果n爲2的冪次方"},{"type":"text","marks":[{"type":"strong"}],"text":"，可以保證數據的均勻插入"},{"type":"text","text":"，如果n不是2的冪次方，可能數組的一些位置永遠不會插入數據，"},{"type":"text","marks":[{"type":"strong"}],"text":"浪費數組的空間，加大hash衝突。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2，一般可能會想通過 % 求餘來確定位置，這樣也可以，只不過性能不如 & 運算。而且當n是2的冪次方時：hash & (length - 1) == hash % length"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"3，因此，HashMap 容量爲2次冪的原因，就是"},{"type":"text","marks":[{"type":"strong"}],"text":"爲了數據的的均勻分佈，減少hash衝突，"},{"type":"text","text":"畢竟hash衝突越大，代表數組中一個鏈的長度越大，這樣的話會降低hashmap的性能"}]},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"微信搜索公衆號《"},{"type":"text","marks":[{"type":"size","attrs":{"size":12}},{"type":"color","attrs":{"color":"#F5222D","name":"red"}}],"text":"程序員的時光"},{"type":"text","text":"》"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"black"}}],"text":"好了，今天就先分享到這裏了，下期繼續給大家帶來HashMap面試內容！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"black"}}],"text":"更多幹貨、優質文章，歡迎關注我的原創技術公衆號~"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}