不必東奔西走，Java 集合框架看這一篇就夠了

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/2f/2fd5b329f34282e71ec45dbbf1cb3ce3.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"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},"content":[{"type":"text","text":"Java 集合，也稱作容器，主要是由兩大接口 (Interface) 派生出來的：Collection 和 Map"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"顧名思義，容器就是用來存放數據的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那麼這兩大接口的不同之處在於："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Collection 存放單一元素；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Map 存放 key-value 鍵值對。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"就是單身狗放 Collection 裏面，couple 就放 Map 裏。（所以你屬於哪裏？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"學習這些集合框架，我認爲有 4 個目標："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"明確每個接口和類的對應關係；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"對每個接口和類，熟悉常用的 API；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"對不同的場景，能夠選擇合適的數據結構並分析優缺點；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"學習源碼的設計，面試要會答啊。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"關於 Map，之前那篇 HashMap 的文章已經講的非常透徹詳盡了，所以本文不再贅述。如果還沒看過那篇文章的小夥伴，快去公衆號內回覆「"},{"type":"text","marks":[{"type":"strong"}],"text":"HashMap"},{"type":"text","text":"」看文章吧～"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Collection"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"先來看最上層的 Collection."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/42/42cda8bc6653e542aafe2ceeb12953ff.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"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},"content":[{"type":"text","text":"Collection 裏還定義了很多方法，這些方法也都會繼承到各個子接口和實現類裏，而這些 API 的使用也是日常工作和麪試常見常考的，所以我們先來看下這些方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"操作集合，無非就是「增刪改查」四大類，也叫 CRUD:"}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Create, Read, Update, and Delete."}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那我也把這些 API 分爲這四大類："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"功能方法增add()/addAll()刪remove()/ removeAll()改Collection Interface 裏沒有查contains()/ containsAll()其他isEmpty()/size()/toArray()"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面具體來看："}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"增："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"boolean add(E e);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"add() 方法傳入的數據類型必須是 Object，所以當寫入基本數據類型的時候，會做自動裝箱 auto-boxing 和自動拆箱 unboxing。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"還有另外一個方法 addAll()，可以把另一個集合裏的元素加到此集合中。"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"boolean addAll(Collection extends E> c);"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"刪："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"boolean remove(Object o);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"remove()是刪除的指定元素。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那和 addAll() 對應的，自然就有removeAll()，就是把集合 B 中的所有元素都刪掉。"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"boolean removeAll(Collection> c);"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"改："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Collection Interface 裏並沒有直接改元素的操作，反正刪和增就可以完成改了嘛！"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"查："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"查下集合中有沒有某個特定的元素："}]}]}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"boolean contains(Object o);"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"查集合 A 是否包含了集合 B："}]}]}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"boolean containsAll(Collection> c);"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"還有一些對集合整體的操作："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"判斷集合是否爲空："}]}]}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"boolean isEmpty();"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"集合的大小："}]}]}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"int size();"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"把集合轉成數組："}]}]}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Object[] toArray();"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上就是 Collection 中常用的 API 了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在接口裏都定義好了，子類不要也得要。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當然子類也會做一些自己的實現，這樣就有了不同的數據結構。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那我們一個個來看。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"List"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/af/af9915995e82ab2f487c5e3be72acd88.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"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":"List 最大的特點就是：有序，可重複。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"看官網說的："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"An ordered collection (also known as a sequence)."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Unlike sets, lists typically allow duplicate elements."}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這一下把 Set 的特點也說出來了，和 List 完全相反，Set 是 無序，不重複的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"List 的實現方式有 LinkedList 和 ArrayList 兩種，那面試時最常問的就是這兩個數據結構如何選擇。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於這類選擇問題：一是考慮數據結構是否能"},{"type":"text","marks":[{"type":"strong"}],"text":"完成需要的功能"},{"type":"text","text":"；如果都能完成，二是考慮哪種"},{"type":"text","marks":[{"type":"strong"}],"text":"更高效"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"（萬事都是如此啊。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那具體來看這兩個 classes 的 API 和它們的時間複雜度："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"功能方法ArrayListLinkedList增add(E e)O(1)O(1)增add(int index, E e)O(n)O(n)刪remove(int index)O(n)O(n)刪remove(E e)O(n)O(n)改set(int index, E e)O(1)O(n)查get(int index)O(1)O(n)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"稍微解釋幾個："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"add(E e) 是在尾巴上加元素，雖然 ArrayList 可能會有擴容的情況出現，但是均攤複雜度（amortized time complexity）還是 O(1) 的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"add(int index, E e)是在特定的位置上加元素，LinkedList 需要先找到這個位置，再加上這個元素，雖然單純的「加」這個動作是 O(1) 的，但是要找到這個位置還是 O(n) 的。（這個有的人就認爲是 O(1)，和麪試官解釋清楚就行了，拒絕扛精。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"remove(int index)是 remove 這個 index 上的元素，所以"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ArrayList 找到這個元素的過程是 O(1)，但是 remove 之後，後續元素都要往前移動一位，所以均攤複雜度是 O(n)；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"LinkedList 也是要先找到這個 index，這個過程是 O(n) 的，所以整體也是 O(n)。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"remove(E e)是 remove 見到的第一個這個元素，那麼"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ArrayList 要先找到這個元素，這個過程是 O(n)，然後移除後還要往前移一位，這個更是 O(n)，總的還是 O(n)；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"LinkedList 也是要先找，這個過程是 O(n)，然後移走，這個過程是 O(1)，總的是 O(n)."}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那造成時間複雜度的區別的原因是什麼呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"答"},{"type":"text","text":"："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲 ArrayList 是用數組來實現的。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而數組和鏈表的最大區別就是"},{"type":"text","marks":[{"type":"strong"}],"text":"數組是可以隨機訪問的（random access）"},{"type":"text","text":"。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個特點造成了在數組裏可以通過下標用 O(1) 的時間拿到任何位置的數，而鏈表則做不到，只能從頭開始逐個遍歷。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"也就是說在「改查」這兩個功能上，因爲數組能夠隨機訪問，所以 ArrayList 的效率高。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那「增刪」呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果不考慮找到這個元素的時間，"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"數組因爲物理上的連續性，當要增刪元素時，在尾部還好，但是其他地方就會導致後續元素都要移動，所以效率較低；而鏈表則可以輕鬆的斷開和下一個元素的連接，直接插入新元素或者移除舊元素。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是呢，實際上你不能不考慮找到元素的時間啊。。。而且如果是在尾部操作，數據量大時 ArrayList 會更快的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"所以說："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"改查選擇 ArrayList；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"增刪在尾部的選擇 ArrayList；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"其他情況下，如果時間複雜度一樣，推薦選擇 ArrayList，因爲 overhead 更小，或者說內存使用更有效率。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"Vector"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那作爲 List 的最後一個知識點，我們來聊一下 Vector。這也是一個年齡暴露帖，用過的都是大佬。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那 Vector 和 ArrayList 一樣，也是繼承自 java.util.AbstractList，底層也是用數組來實現的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是現在已經被棄用了，因爲...它加了太多的 synchronized！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"任何好處都是有代價的，線程安全的成本就是效率低，在某些系統裏很容易成爲瓶頸，所以現在大家不再在數據結構的層面加 synchronized，而是把這個任務轉移給我們程序員=="}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"那麼面試常問題：Vector 和 ArrayList 的區別是什麼，只答出來這個還還不太全面。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"來看 stack overflow 上的高票回答："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/94/948a096e64ffc05730c6a4af39818221.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一是剛纔已經說過的線程安全問題；二是擴容時擴多少的區別。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個得看看源碼："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9d/9d41808847e0823e5c5ee00014714e33.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"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":"這是 ArrayList 的擴容實現，這個"},{"type":"text","marks":[{"type":"strong"}],"text":"算術右移"},{"type":"text","text":"操作是把這個數的二進制往右移動一位，最左邊"},{"type":"text","marks":[{"type":"strong"}],"text":"補符號位"},{"type":"text","text":"，但是因爲容量沒有負數，所以還是補 0."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那右移一位的效果就是除以 2，那麼定義的新容量就是原容量的 "},{"type":"text","marks":[{"type":"strong"}],"text":"1.5 倍"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"再來看 Vector 的："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/54/54a8ec9117e23a381cec7773530b60ec.jpeg","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"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":"因爲通常 capacityIncrement 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