逼着面试官问了我ArrayList和LinkedList的区别，他对我彻底服了

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ArrayList 和 LinkedList 有什么区别，是面试官非常喜欢问的一个问题。可能大部分小伙伴和我一样，能回答出“ArrayList 是基于数组实现的，LinkedList 是基于双向链表实现的。”"}]},{"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":"关于这一点，我之前的文章里也提到过了。但说实话，这样苍白的回答并不能令面试官感到满意，他还想知道的更多。"}]},{"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":"那假如小伙伴们继续做出下面这样的回答："}]},{"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":"“ArrayList 在新增和删除元素时，因为涉及到数组复制，所以效率比 LinkedList 低，而在遍历的时候，ArrayList 的效率要高于 LinkedList。”"}]},{"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":"面试官会感到满意吗？我只能说，如果面试官比较仁慈的话，他可能会让我们回答下一个问题；否则的话，他会让我们回家等通知，这一等，可能意味着杳无音讯了。"}]},{"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":"为什么会这样呢？为什么为什么？回答的不对吗？"}]},{"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":"暴躁的小伙伴请喝口奶茶冷静一下。冷静下来后，请随我来，让我们一起肩并肩、手拉手地深入地研究一下 ArrayList 和 LinkedList 的数据结构、实现原理以及源码，可能神秘的面纱就揭开了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"01、ArrayList 是如何实现的？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/f5/f5e4865f3e9a2031f605d9f3237a1520.png","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"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":"ArrayList 实现了 List 接口，继承了 AbstractList 抽象类，底层是基于数组实现的，并且实现了动态扩容。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class ArrayList extends AbstractList\n implements List, RandomAccess, Cloneable, java.io.Serializable\n{\n private static final int DEFAULT_CAPACITY = 10;\n transient Object[] elementData;\n private int size;\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":"ArrayList 还实现了 RandomAccess 接口，这是一个标记接口："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface RandomAccess {\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":"内部是空的，标记“实现了这个接口的类支持快速（通常是固定时间）随机访问”。快速随机访问是什么意思呢？就是说不需要遍历，就可以通过下标（索引）直接访问到内存地址。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public E get(int index) {\n Objects.checkIndex(index, size);\n return elementData(index);\n}\nE elementData(int index) {\n return (E) elementData[index];\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":"ArrayList 还实现了 Cloneable 接口，这表明 ArrayList 是支持拷贝的。ArrayList 内部的确也重写了 Object 类的 "},{"type":"codeinline","content":[{"type":"text","text":"clone()"}]},{"type":"text","text":" 方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public Object clone() {\n try {\n ArrayList> v = (ArrayList>) super.clone();\n v.elementData = Arrays.copyOf(elementData, size);\n v.modCount = 0;\n return v;\n } catch (CloneNotSupportedException e) {\n // this shouldn't happen, since we are Cloneable\n throw new InternalError(e);\n }\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":"ArrayList 还实现了 Serializable 接口，同样是一个标记接口："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface Serializable {\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":"内部也是空的，标记“实现了这个接口的类支持序列化”。序列化是什么意思呢？Java 的序列化是指，将对象转换成以字节序列的形式来表示，这些字节序中包含了对象的字段和方法。序列化后的对象可以被写到数据库、写到文件，也可用于网络传输。"}]},{"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":"眼睛雪亮的小伙伴可能会注意到，ArrayList 中的关键字段 elementData 使用了 transient 关键字修饰，这个关键字的作用是，让它修饰的字段不被序列化。"}]},{"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":"这不前后矛盾吗？一个类既然实现了 Serilizable 接口，肯定是想要被序列化的，对吧？那为什么保存关键数据的 elementData 又不想被序列化呢?"}]},{"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":"这还得从 “ArrayList 是基于数组实现的”开始说起。大家都知道，数组是定长的，就是说，数组一旦声明了，长度（容量）就是固定的，不能像某些东西一样伸缩自如。这就很麻烦，数组一旦装满了，就不能添加新的元素进来了。"}]},{"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":"ArrayList 不想像数组这样活着，它想能屈能伸，所以它实现了动态扩容。一旦在添加元素的时候，发现容量用满了 "},{"type":"codeinline","content":[{"type":"text","text":"s == elementData.length"}]},{"type":"text","text":"，就按照原来数组的 1.5 倍（"},{"type":"codeinline","content":[{"type":"text","text":"oldCapacity >> 1"}]},{"type":"text","text":"）进行扩容。扩容之后，再将原有的数组复制到新分配的内存地址上 "},{"type":"codeinline","content":[{"type":"text","text":"Arrays.copyOf(elementData, newCapacity)"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private void add(E e, Object[] elementData, int s) {\n if (s == elementData.length)\n elementData = grow();\n elementData[s] = e;\n size = s + 1;\n}\n\nprivate Object[] grow() {\n return grow(size + 1);\n}\n\nprivate Object[] grow(int minCapacity) {\n int oldCapacity = elementData.length;\n if (oldCapacity > 0 || elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {\n int newCapacity = ArraysSupport.newLength(oldCapacity,\n minCapacity - oldCapacity, /* minimum growth */\n oldCapacity >> 1 /* preferred growth */);\n return elementData = Arrays.copyOf(elementData, newCapacity);\n } else {\n return elementData = new Object[Math.max(DEFAULT_CAPACITY, minCapacity)];\n }\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":"动态扩容意味着什么？大家伙想一下。嗯，还是我来告诉大家答案吧，有点迫不及待。"}]},{"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":"意味着数组的实际大小可能永远无法被填满的，总有多余出来空置的内存空间。"}]},{"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":"比如说，默认的数组大小是 10，当添加第 11 个元素的时候，数组的长度扩容了 1.5 倍，也就是 15，意味着还有 4 个内存空间是闲置的，对吧？"}]},{"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":"序列化的时候，如果把整个数组都序列化的话，是不是就多序列化了 4 个内存空间。当存储的元素数量非常非常多的时候，闲置的空间就非常非常大，序列化耗费的时间就会非常非常多。"}]},{"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":"于是，ArrayList 做了一个愉快而又聪明的决定，内部提供了两个私有方法 writeObject 和 readObject 来完成序列化和反序列化。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private void writeObject(java.io.ObjectOutputStream s)\n throws java.io.IOException {\n // Write out element count, and any hidden stuff\n int expectedModCount = modCount;\n s.defaultWriteObject();\n\n // Write out size as capacity for behavioral compatibility with clone()\n s.writeInt(size);\n\n // Write out all elements in the proper order.\n for (int i=0; i\n extends AbstractSequentialList\n implements List, Deque, Cloneable, java.io.Serializable\n{\n transient int size = 0;\n transient Node first;\n transient Node last;\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":" LinkedList 内部定义了一个 Node 节点，它包含 3 个部分：元素内容 item，前引用 prev 和后引用 next。代码如下所示："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private static class Node {\n E item;\n LinkedList.Node next;\n LinkedList.Node prev;\n\n Node(LinkedList.Node prev, E element, LinkedList.Node next) {\n this.item = element;\n this.next = next;\n this.prev = prev;\n }\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":"LinkedList 还实现了 Cloneable 接口，这表明 LinkedList 是支持拷贝的。"}]},{"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":"LinkedList 还实现了 Serializable 接口，这表明 LinkedList 是支持序列化的。眼睛雪亮的小伙伴可能又注意到了，LinkedList 中的关键字段 size、first、last 都使用了 transient 关键字修饰，这不又矛盾了吗？到底是想序列化还是不想序列化？"}]},{"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":"答案是 LinkedList 想按照自己的方式序列化，来看它自己实现的 "},{"type":"codeinline","content":[{"type":"text","text":"writeObject()"}]},{"type":"text","text":" 方法："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private void writeObject(java.io.ObjectOutputStream s)\n throws java.io.IOException {\n // Write out any hidden serialization magic\n s.defaultWriteObject();\n\n // Write out size\n s.writeInt(size);\n\n // Write out all elements in the proper order.\n for (LinkedList.Node x = first; x != null; x = x.next)\n s.writeObject(x.item);\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":"发现没？LinkedList 在序列化的时候只保留了元素的内容 item，并没有保留元素的前后引用。这样就节省了不少内存空间，对吧？"}]},{"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":"那有些小伙伴可能就疑惑了，只保留元素内容，不保留前后引用，那反序列化的时候怎么办？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private void readObject(java.io.ObjectInputStream s)\n throws java.io.IOException, ClassNotFoundException {\n // Read in any hidden serialization magic\n s.defaultReadObject();\n\n // Read in size\n int size = s.readInt();\n\n // Read in all elements in the proper order.\n for (int i = 0; i < size; i++)\n linkLast((E)s.readObject());\n}\n\nvoid linkLast(E e) {\n final LinkedList.Node l = last;\n final LinkedList.Node newNode = new LinkedList.Node<>(l, e, null);\n last = newNode;\n if (l == null)\n first = newNode;\n else\n l.next = newNode;\n size++;\n modCount++;\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":"注意 for 循环中的 "},{"type":"codeinline","content":[{"type":"text","text":"linkLast()"}]},{"type":"text","text":" 方法，它可以把链表重新链接起来，这样就恢复了链表序列化之前的顺序。很妙，对吧？"}]},{"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":"和 ArrayList 相比，LinkedList 没有实现 RandomAccess 接口，这是因为 LinkedList 存储数据的内存地址是不连续的，所以不支持随机访问。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"03、ArrayList 和 LinkedList 新增元素时究竟谁快？"}]},{"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":"前面我们已经从多个维度了解了 ArrayList 和 LinkedList 的实现原理和各自的特点。那接下来，我们就来聊聊 ArrayList 和 LinkedList 在新增元素时究竟谁快？"}]},{"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","marks":[{"type":"strong"}],"text":"1）ArrayList"}]},{"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":"ArrayList 新增元素有两种情况，一种是直接将元素添加到数组末尾，一种是将元素插入到指定位置。"}]},{"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":"添加到数组末尾的源码："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public boolean add(E e) {\n modCount++;\n add(e, elementData, size);\n return true;\n}\n\nprivate void add(E e, Object[] elementData, int s) {\n if (s == elementData.length)\n elementData = grow();\n elementData[s] = e;\n size = s + 1;\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":"很简单，先判断是否需要扩容，然后直接通过索引将元素添加到末尾。"}]},{"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":"插入到指定位置的源码："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public void add(int index, E element) {\n rangeCheckForAdd(index);\n modCount++;\n final int s;\n Object[] elementData;\n if ((s = size) == (elementData = this.elementData).length)\n elementData = grow();\n System.arraycopy(elementData, index,\n elementData, index + 1,\n s - index);\n elementData[index] = element;\n size = s + 1;\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":"先检查插入的位置是否在合理的范围之内，然后判断是否需要扩容，再把该位置以后的元素复制到新添加元素的位置之后，最后通过索引将元素添加到指定的位置。这种情况是非常伤的，性能会比较差。"}]},{"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","marks":[{"type":"strong"}],"text":"2）LinkedList"}]},{"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":"LinkedList 新增元素也有两种情况，一种是直接将元素添加到队尾，一种是将元素插入到指定位置。"}]},{"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":"添加到队尾的源码："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public boolean add(E e) {\n linkLast(e);\n return true;\n}\nvoid linkLast(E e) {\n final LinkedList.Node l = last;\n final LinkedList.Node newNode = new LinkedList.Node<>(l, e, null);\n last = newNode;\n if (l == null)\n first = newNode;\n else\n l.next = newNode;\n size++;\n modCount++;\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":"先将队尾的节点 last 存放到临时变量 l 中（不是说不建议使用 I 作为变量名吗？Java 的作者们明知故犯啊），然后生成新的 Node 节点，并赋给 last，如果 l 为 null，说明是第一次添加，所以 first 为新的节点；否则将新的节点赋给之前 last 的 next。"}]},{"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":"插入到指定位置的源码："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public void add(int index, E element) {\n checkPositionIndex(index);\n\n if (index == size)\n linkLast(element);\n else\n linkBefore(element, node(index));\n}\nLinkedList.Node node(int index) {\n // assert isElementIndex(index);\n\n if (index < (size >> 1)) {\n LinkedList.Node x = first;\n for (int i = 0; i < index; i++)\n x = x.next;\n return x;\n } else {\n LinkedList.Node x = last;\n for (int i = size - 1; i > index; i--)\n x = x.prev;\n return x;\n }\n}\nvoid linkBefore(E e, LinkedList.Node succ) {\n // assert succ != null;\n final LinkedList.Node pred = succ.prev;\n final LinkedList.Node newNode = new LinkedList.Node<>(pred, e, succ);\n succ.prev = newNode;\n if (pred == null)\n first = newNode;\n else\n pred.next = newNode;\n size++;\n modCount++;\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":"先检查插入的位置是否在合理的范围之内，然后判断插入的位置是否是队尾，如果是，添加到队尾；否则执行 "},{"type":"codeinline","content":[{"type":"text","text":"linkBefore()"}]},{"type":"text","text":" 方法。"}]},{"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":"在执行 "},{"type":"codeinline","content":[{"type":"text","text":"linkBefore()"}]},{"type":"text","text":" 方法之前，会调用 "},{"type":"codeinline","content":[{"type":"text","text":"node()"}]},{"type":"text","text":" 方法查找指定位置上的元素，这一步是需要遍历 LinkedList 的。如果插入的位置靠前前半段，就从队头开始往后找；否则从队尾往前找。也就是说，如果插入的位置越靠近 LinkedList 的中间位置，遍历所花费的时间就越多。"}]},{"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":"找到指定位置上的元素（succ）之后，就开始执行 "},{"type":"codeinline","content":[{"type":"text","text":"linkBefore()"}]},{"type":"text","text":" 方法了，先将 succ 的前一个节点（prev）存放到临时变量 pred 中，然后生成新的 Node 节点（newNode），并将 succ 的前一个节点变更为 newNode，如果 pred 为 null，说明插入的是队头，所以 first 为新节点；否则将 pred 的后一个节点变更为 newNode。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/27/27463bd2c268382f3bd008b945bc5936.png","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"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":"经过源码分析以后，小伙伴们是不是在想：“好像 ArrayList 在新增元素的时候效率并不一定比 LinkedList 低啊！”"}]},{"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":"当两者的起始长度是一样的情况下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果是从集合的头部新增元素，ArrayList 花费的时间应该比 LinkedList 多，因为需要对头部以后的元素进行复制。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class ArrayListTest {\n public static void addFromHeaderTest(int num) {\n ArrayList list = new ArrayList(num);\n int i = 0;\n\n long timeStart = System.currentTimeMillis();\n\n while (i < num) {\n list.add(0, i + \"沉默王二\");\n i++;\n }\n long timeEnd = System.currentTimeMillis();\n\n System.out.println(\"ArrayList从集合头部位置新增元素花费的时间\" + (timeEnd - timeStart));\n }\n}\n\n/**\n * @author 微信搜「沉默王二」，回复关键字 PDF\n */\npublic class LinkedListTest {\n public static void addFromHeaderTest(int num) {\n LinkedList list = new LinkedList();\n int i = 0;\n long timeStart = System.currentTimeMillis();\n while (i < num) {\n list.addFirst(i + \"沉默王二\");\n i++;\n }\n long timeEnd = System.currentTimeMillis();\n\n System.out.println(\"LinkedList从集合头部位置新增元素花费的时间\" + (timeEnd - timeStart));\n }\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":"num 为 10000，代码实测后的时间如下所示："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"ArrayList从集合头部位置新增元素花费的时间595\nLinkedList从集合头部位置新增元素花费的时间15"}]},{"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":"ArrayList 花费的时间比 LinkedList 要多很多。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果是从集合的中间位置新增元素，ArrayList 花费的时间搞不好要比 LinkedList 少，因为 LinkedList 需要遍历。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class ArrayListTest {\n public static void addFromMidTest(int num) {\n ArrayList list = new ArrayList(num);\n int i = 0;\n\n long timeStart = System.currentTimeMillis();\n while (i < num) {\n int temp = list.size();\n list.add(temp / 2 + \"沉默王二\");\n i++;\n }\n long timeEnd = System.currentTimeMillis();\n\n System.out.println(\"ArrayList从集合中间位置新增元素花费的时间\" + (timeEnd - timeStart));\n }\n}\n\npublic class LinkedListTest {\n public static void addFromMidTest(int num) {\n LinkedList list = new LinkedList();\n int i = 0;\n long timeStart = System.currentTimeMillis();\n while (i < num) {\n int temp = list.size();\n list.add(temp / 2, i + \"沉默王二\");\n i++;\n }\n long timeEnd = System.currentTimeMillis();\n\n System.out.println(\"LinkedList从集合中间位置新增元素花费的时间\" + (timeEnd - timeStart));\n }\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":"num 为 10000，代码实测后的时间如下所示："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"ArrayList从集合中间位置新增元素花费的时间1\nLinkedList从集合中间位置新增元素花费的时间101"}]},{"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":"ArrayList 花费的时间比 LinkedList 要少很多很多。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果是从集合的尾部新增元素，ArrayList 花费的时间应该比 LinkedList 少，因为数组是一段连续的内存空间，也不需要复制数组；而链表需要创建新的对象，前后引用也要重新排列。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class ArrayListTest {\n public static void addFromTailTest(int num) {\n ArrayList list = new ArrayList(num);\n int i = 0;\n\n long timeStart = System.currentTimeMillis();\n\n while (i < num) {\n list.add(i + \"沉默王二\");\n i++;\n }\n\n long timeEnd = System.currentTimeMillis();\n\n System.out.println(\"ArrayList从集合尾部位置新增元素花费的时间\" + (timeEnd - timeStart));\n }\n}\n\npublic class LinkedListTest {\n public static void addFromTailTest(int num) {\n LinkedList list = new LinkedList();\n int i = 0;\n long timeStart = System.currentTimeMillis();\n while (i < num) {\n list.add(i + \"沉默王二\");\n i++;\n }\n long timeEnd = System.currentTimeMillis();\n\n System.out.println(\"LinkedList从集合尾部位置新增元素花费的时间\" + (timeEnd - timeStart));\n }\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":"num 为 10000，代码实测后的时间如下所示："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"ArrayList从集合尾部位置新增元素花费的时间69\nLinkedList从集合尾部位置新增元素花费的时间193"}]},{"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":"ArrayList 花费的时间比 LinkedList 要少一些。"}]},{"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":"这样的结论和预期的是不是不太相符？ArrayList 在添加元素的时候如果不涉及到扩容，性能在两种情况下（中间位置新增元素、尾部新增元素）比 LinkedList 好很多，只有头部新增元素的时候比 LinkedList 差，因为数组复制的原因。"}]},{"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":"当然了，如果涉及到数组扩容的话，ArrayList 的性能就没那么可观了，因为扩容的时候也要复制数组。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"04、ArrayList 和 LinkedList 删除元素时究竟谁快？"}]},{"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","marks":[{"type":"strong"}],"text":"1）ArrayList"}]},{"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":"ArrayList 删除元素的时候，有两种方式，一种是直接删除元素（"},{"type":"codeinline","content":[{"type":"text","text":"remove(Object)"}]},{"type":"text","text":"），需要直先遍历数组，找到元素对应的索引；一种是按照索引删除元素（"},{"type":"codeinline","content":[{"type":"text","text":"remove(int)"}]},{"type":"text","text":"）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public boolean remove(Object o) {\n final Object[] es = elementData;\n final int size = this.size;\n int i = 0;\n found: {\n if (o == null) {\n for (; i < size; i++)\n if (es[i] == null)\n break found;\n } else {\n for (; i < size; i++)\n if (o.equals(es[i]))\n break found;\n }\n return false;\n }\n fastRemove(es, i);\n return true;\n}\npublic E remove(int index) {\n Objects.checkIndex(index, size);\n final Object[] es = elementData;\n\n @SuppressWarnings(\"unchecked\") E oldValue = (E) es[index];\n fastRemove(es, index);\n\n return oldValue;\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":"但从本质上讲，都是一样的，因为它们最后调用的都是 "},{"type":"codeinline","content":[{"type":"text","text":"fastRemove(Object, int)"}]},{"type":"text","text":" 方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private void fastRemove(Object[] es, int i) {\n modCount++;\n final int newSize;\n if ((newSize = size - 1) > i)\n System.arraycopy(es, i + 1, es, i, newSize - i);\n es[size = newSize] = 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":"从源码可以看得出，只要删除的不是最后一个元素，都需要数组重组。删除的元素位置越靠前，代价就越大。"}]},{"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","marks":[{"type":"strong"}],"text":"2）LinkedList"}]},{"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":"LinkedList 删除元素的时候，有四种常用的方式："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"remove(int)"}]},{"type":"text","text":"，删除指定位置上的元素"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public E remove(int index) {\n checkElementIndex(index);\n return unlink(node(index));\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":"先检查索引，再调用 "},{"type":"codeinline","content":[{"type":"text","text":"node(int)"}]},{"type":"text","text":" 方法（ 前后半段遍历，和新增元素操作一样）找到节点 Node，然后调用 "},{"type":"codeinline","content":[{"type":"text","text":"unlink(Node)"}]},{"type":"text","text":" 解除节点的前后引用，同时更新前节点的后引用和后节点的前引用："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" E unlink(Node x) {\n // assert x != null;\n final E element = x.item;\n final Node next = x.next;\n final Node prev = x.prev;\n\n if (prev == null) {\n first = next;\n } else {\n prev.next = next;\n x.prev = null;\n }\n\n if (next == null) {\n last = prev;\n } else {\n next.prev = prev;\n x.next = null;\n }\n\n x.item = null;\n size--;\n modCount++;\n return element;\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"remove(Object)"}]},{"type":"text","text":"，直接删除元素"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public boolean remove(Object o) {\n if (o == null) {\n for (LinkedList.Node x = first; x != null; x = x.next) {\n if (x.item == null) {\n unlink(x);\n return true;\n }\n }\n } else {\n for (LinkedList.Node x = first; x != null; x = x.next) {\n if (o.equals(x.item)) {\n unlink(x);\n return true;\n }\n }\n }\n return false;\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":"也是先前后半段遍历，找到要删除的元素后调用 "},{"type":"codeinline","content":[{"type":"text","text":"unlink(Node)"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"removeFirst()"}]},{"type":"text","text":"，删除第一个节点"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public E removeFirst() {\n final LinkedList.Node f = first;\n if (f == null)\n throw new NoSuchElementException();\n return unlinkFirst(f);\n}\nprivate E unlinkFirst(LinkedList.Node f) {\n // assert f == first && f != null;\n final E element = f.item;\n final LinkedList.Node next = f.next;\n f.item = null;\n f.next = null; // help GC\n first = next;\n if (next == null)\n last = null;\n else\n next.prev = null;\n size--;\n modCount++;\n return element;\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":"删除第一个节点就不需要遍历了，只需要把第二个节点更新为第一个节点即可。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"removeLast()"}]},{"type":"text","text":"，删除最后一个节点"}]}]}]},{"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":"删除最后一个节点和删除第一个节点类似，只需要把倒数第二个节点更新为最后一个节点即可。"}]},{"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":"可以看得出，LinkedList 在删除比较靠前和比较靠后的元素时，非常高效，但如果删除的是中间位置的元素，效率就比较低了。"}]},{"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":"这里就不再做代码测试了，感兴趣的小伙伴可以自己试试，结果和新增元素保持一致："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"从集合头部删除元素时，ArrayList 花费的时间比 LinkedList 多很多；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"从集合中间位置删除元素时，ArrayList 花费的时间比 LinkedList 少很多；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"从集合尾部删除元素时，ArrayList 花费的时间比 LinkedList 少一点。"}]}]}]},{"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":"我本地的统计结果如下所示，小伙伴们可以作为参考："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":""},"content":[{"type":"text","text":"ArrayList从集合头部位置删除元素花费的时间380\nLinkedList从集合头部位置删除元素花费的时间4\nArrayList从集合中间位置删除元素花费的时间381\nLinkedList从集合中间位置删除元素花费的时间5922\nArrayList从集合尾部位置删除元素花费的时间8\nLinkedList从集合尾部位置删除元素花费的时间12"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"05、ArrayList 和 LinkedList 遍历元素时究竟谁快？"}]},{"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","marks":[{"type":"strong"}],"text":"1）ArrayList"}]},{"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":"遍历 ArrayList 找到某个元素的话，通常有两种形式："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"get(int)"}]},{"type":"text","text":"，根据索引找元素"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public E get(int index) {\n Objects.checkIndex(index, size);\n return elementData(index);\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":"由于 ArrayList 是由数组实现的，所以根据索引找元素非常的快，一步到位。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"indexOf(Object)"}]},{"type":"text","text":"，根据元素找索引"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public int indexOf(Object o) {\n return indexOfRange(o, 0, size);\n}\n\nint indexOfRange(Object o, int start, int end) {\n Object[] es = elementData;\n if (o == null) {\n for (int i = start; i < end; i++) {\n if (es[i] == null) {\n return i;\n }\n }\n } else {\n for (int i = start; i < end; i++) {\n if (o.equals(es[i])) {\n return i;\n }\n }\n }\n return -1;\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":"根据元素找索引的话，就需要遍历整个数组了，从头到尾依次找。"}]},{"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","marks":[{"type":"strong"}],"text":"2）LinkedList"}]},{"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":"遍历 LinkedList 找到某个元素的话，通常也有两种形式："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"get(int)"}]},{"type":"text","text":"，找指定位置上的元素"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public E get(int index) {\n checkElementIndex(index);\n return node(index).item;\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":"既然需要调用 "},{"type":"codeinline","content":[{"type":"text","text":"node(int)"}]},{"type":"text","text":" 方法，就意味着需要前后半段遍历了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"indexOf(Object)"}]},{"type":"text","text":"，找元素所在的位置"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public int indexOf(Object o) {\n int index = 0;\n if (o == null) {\n for (LinkedList.Node x = first; x != null; x = x.next) {\n if (x.item == null)\n return index;\n index++;\n }\n } else {\n for (LinkedList.Node x = first; x != null; x = x.next) {\n if (o.equals(x.item))\n return index;\n index++;\n }\n }\n return -1;\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":"需要遍历整个链表，和 ArrayList 的 "},{"type":"codeinline","content":[{"type":"text","text":"indexOf()"}]},{"type":"text","text":" 类似。"}]},{"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":"那在我们对集合遍历的时候，通常有两种做法，一种是使用 for 循环，一种是使用迭代器（Iterator）。"}]},{"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":"如果使用的是 for 循环，可想而知 LinkedList 在 get 的时候性能会非常差，因为每一次外层的 for 循环，都要执行一次 "},{"type":"codeinline","content":[{"type":"text","text":"node(int)"}]},{"type":"text","text":" 方法进行前后半段的遍历。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"LinkedList.Node node(int index) {\n // assert isElementIndex(index);\n\n if (index < (size >> 1)) {\n LinkedList.Node x = first;\n for (int i = 0; i < index; i++)\n x = x.next;\n return x;\n } else {\n LinkedList.Node x = last;\n for (int i = size - 1; i > index; i--)\n x = x.prev;\n return x;\n }\n}"}]},{"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":"那如果使用的是迭代器呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"LinkedList list = new LinkedList();\nfor (Iterator it = list.iterator(); it.hasNext();) {\n it.next();\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":"迭代器只会调用一次 "},{"type":"codeinline","content":[{"type":"text","text":"node(int)"}]},{"type":"text","text":" 方法，在执行 "},{"type":"codeinline","content":[{"type":"text","text":"list.iterator()"}]},{"type":"text","text":" 的时候：先调用 AbstractSequentialList 类的 "},{"type":"codeinline","content":[{"type":"text","text":"iterator()"}]},{"type":"text","text":" 方法，再调用 AbstractList 类的 "},{"type":"codeinline","content":[{"type":"text","text":"listIterator()"}]},{"type":"text","text":" 方法，再调用 LinkedList 类的 "},{"type":"codeinline","content":[{"type":"text","text":"listIterator(int)"}]},{"type":"text","text":" 方法，如下图所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/a2/a20ecc2f1645269776f241b5f7f36436.png","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"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":"最后返回的是 LinkedList 类的内部私有类 ListItr 对象："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public ListIterator listIterator(int index) {\n checkPositionIndex(index);\n return new LinkedList.ListItr(index);\n}\n\nprivate class ListItr implements ListIterator {\n private LinkedList.Node lastReturned;\n private LinkedList.Node next;\n private int nextIndex;\n private int expectedModCount = modCount;\n\n ListItr(int index) {\n // assert isPositionIndex(index);\n next = (index == size) ? null : node(index);\n nextIndex = index;\n }\n\n public boolean hasNext() {\n return nextIndex < size;\n }\n\n public E next() {\n checkForComodification();\n if (!hasNext())\n throw new NoSuchElementException();\n\n lastReturned = next;\n next = next.next;\n nextIndex++;\n return lastReturned.item;\n }\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":"执行 ListItr 的构造方法时调用了一次 "},{"type":"codeinline","content":[{"type":"text","text":"node(int)"}]},{"type":"text","text":" 方法，返回第一个节点。在此之后，迭代器就执行 "},{"type":"codeinline","content":[{"type":"text","text":"hasNext()"}]},{"type":"text","text":" 判断有没有下一个，执行 "},{"type":"codeinline","content":[{"type":"text","text":"next()"}]},{"type":"text","text":" 方法下一个节点。"}]},{"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":"由此，可以得出这样的结论："},{"type":"text","marks":[{"type":"strong"}],"text":"遍历 LinkedList 的时候，千万不要使用 for 循环，要使用迭代器。"}]},{"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":"也就是说，for 循环遍历的时候，ArrayList 花费的时间远小于 LinkedList；迭代器遍历的时候，两者性能差不多。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"06、总结"}]},{"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":"花了两天时间，终于肝完了！相信看完这篇文章后，再有面试官问你 ArrayList 和 LinkedList 有什么区别的话，你一定会胸有成竹地和他扯上半小时。"}]},{"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","marks":[{"type":"strong"}],"text":"另外，我把自己看过的学习视频按照顺序分了类，共 500G，目录如下，还有 2020 年最新面试题，现在免费送给大家"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/66/66a09ec280921c8df0f3608edce7eac5.png","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"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":"链接:"},{"type":"link","attrs":{"href":"https://pan.baidu.com/s/1j2uB7-TF3t5BAzVXBgV7dA","title":""},"content":[{"type":"text","text":"https://pan.baidu.com/s/1j2uB7-TF3t5BAzVXBgV7dA"}]},{"type":"text","text":" 密码:cg1q"}]},{"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":"我是沉默王二，一枚沉默但有趣的程序员，感谢各位同学的："},{"type":"text","marks":[{"type":"strong"}],"text":"点赞、收藏"},{"type":"text","text":"和评论，我们下篇见！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}