ArrayList

基于数组实现，无容量的限制。

在执行插入元素时可能要扩容，在删除元素时并不会减小数组的容量，在查找元素时要遍历数组，对于非null的元素采取equals的方式寻找。

是非线程安全的。

注意点：

（1）ArrayList随机存取元素时间复杂度O(1)，插入删除操作需大量移动元素，效率较低

（2）为了节约内存，当新建容器为空时，会共享

Object[] EMPTY_ELEMENTDATA = {}和

Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}空数组

（3）容器底层采用数组存储，每次扩容为1.5倍

（4）ArrayList的实现中大量地调用了Arrays.copyof()和System.arraycopy()方法，其实Arrays.copyof()内部也是调用System.arraycopy()。System.arraycopy()为Native方法

（5）两个ToArray方法

Object[] toArray()方法。该方法有可能会抛出java.lang.ClassCastException异常

<T> T[] toArray(T[] a)方法。该方法可以直接将ArrayList转换得到的Array进行整体向下转型

（6）ArrayList可以存储null值

（7）ArrayList每次修改（增加、删除）容器时，都是修改自身的modCount；在生成迭代器时，迭代器会保存该modCount值，迭代器每次获取元素时，会比较自身的modCount与ArrayList的modCount是否相等，来判断容器是否已经被修改，如果被修改了则抛出异常（fast-fail机制）。

源码解析

package java.util;import sun.misc.SharedSecrets;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;public class ArrayList<E> extends AbstractList<E>implements List<E>, RandomAccess, Cloneable, java.io.Serializable {private static final long serialVersionUID = 8683452581122892189L;/*** 默认容量*/private static final int DEFAULT_CAPACITY = 10;/*** 对象数组*/private static final Object[] EMPTY_ELEMENTDATA = {};/*** 默认的空数组，无参构造函数创建的数组*/private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};/*** 存放数据的数组的缓存变量*/transient Object[] elementData;/*** 元素数量* */private int size;/*** 带有容量的构造方法* * @param 数组的初始容量* @throws IllegalArgumentException 参数为负*/public ArrayList(int initialCapacity) {// 参数>0if (initialCapacity > 0) {// new一个object数组赋给elementDatathis.elementData = new Object[initialCapacity];} else if (initialCapacity == 0) {// 参数=0// 将空数组赋给elementDatathis.elementData = EMPTY_ELEMENTDATA;} else {//参数<0,抛出IllegalArgumentException异常throw new IllegalArgumentException("Illegal Capacity: " +initialCapacity);}}/*** 不带参数构造方法*/public ArrayList() {// 将空数组赋给elementDatathis.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}/*** 带参数Collection的构造方法* * @param c*            其元素将被放入此列表中的集合* @throws NullPointerException*             集合为空*/public ArrayList(Collection<? extends E> c) {elementData = c.toArray();if ((size = elementData.length) != 0) {// c.toArray可能（错误地）不返回对象[]（JAVA BUG编号6260652）if (elementData.getClass() != Object[].class)// Array.copyOf()主要用来将原数组拷贝到一个新的数组，适用于数组扩容。elementData = Arrays.copyOf(elementData, size, Object[].class);} else {// 空数组this.elementData = EMPTY_ELEMENTDATA;}}/*** 因为容量基本会大于实际元素的数量。内存紧张时，可以调用该方法调整容量为元素实际数量。* 如果确定不会有元素添加进来时也可以调用该方法来节约空间*/public void trimToSize() {modCount++;// 如果size小于lengthif (size < elementData.length) {// 将elementData设置大小为sizeelementData = (size == 0)? EMPTY_ELEMENTDATA: Arrays.copyOf(elementData, size);}}/*** 使用指定参数设置数组容量* * @param minCapacity*            所需的最小容量*/public void ensureCapacity(int minCapacity) {// 如果数组为空，容量预取0，否则去默认值(10)int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)// any size if not default element table? 0// larger than default for default empty table. It's already// supposed to be at default size.: DEFAULT_CAPACITY;// 若参数大于预设的容量，再使用该参数进一步设置数组容量if (minCapacity > minExpand) {ensureExplicitCapacity(minCapacity);}}private static int calculateCapacity(Object[] elementData, int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {return Math.max(DEFAULT_CAPACITY, minCapacity);}return minCapacity;}/*** 得到最小扩容量* * @param minCapacity*/private void ensureCapacityInternal(int minCapacity) {ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));}/*** 判断是否需要扩容* * @param minCapacity*/private void ensureExplicitCapacity(int minCapacity) {modCount++;// 最小需要空间比elementData的内存空间大if (minCapacity - elementData.length > 0)grow(minCapacity);}/*** 数组的最大容量，可能会导致内存溢出*/private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;/*** 扩容* * @param minCapacity*            所需的最小容量*/private void grow(int minCapacity) {// ArrayList中elementData的内存空间长度int oldCapacity = elementData.length;// 扩容原来的1.5倍int newCapacity = oldCapacity + (oldCapacity >> 1);// 判断新数组的容量够不够// 不够就将数组长度设置为需要的长度if (newCapacity - minCapacity < 0)newCapacity = minCapacity;// 预设值>默认的最大值,检查溢出if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);// 放到新数组中elementData = Arrays.copyOf(elementData, newCapacity);}/*** 检查是否溢出，若没有溢出，返回最大整数值或默认最大值* * @param minCapacity* @return*/private static int hugeCapacity(int minCapacity) {if (minCapacity < 0)   // 溢出throw new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}/*** 返回ArrayList的大小* * @return ArrayList中的元素数量*/public int size() {return size;}/*** 返回是否为空* * @return true 如果ArrayList中无元素*/public boolean isEmpty() {return size == 0;}/*** 是否包含一个数 返回bool* * @param o*            被检查元素* @return true 如果ArrayList中包含o元素*/public boolean contains(Object o) {return indexOf(o) >= 0;}/*** 返回一个值首次出现的位置，不存在就返回-1。时间复杂度O(N)* * 返回第一个null* @param o* @return*/public int indexOf(Object o) {if (o == null) {for (int i = 0; i < size; i++)if (elementData[i] == null)return i;} else {for (int i = 0; i < size; i++)if (o.equals(elementData[i]))return i;}return -1;}/*** 返回一个值最后一次出现的位置，不存在就返回-1。时间复杂度O(N)* 返回最后一个null* @param o* @return*/public int lastIndexOf(Object o) {if (o == null) {for (int i = size - 1; i >= 0; i--)if (elementData[i] == null)return i;} else {for (int i = size - 1; i >= 0; i--)if (o.equals(elementData[i]))return i;}return -1;}/*** 返回副本，元素本身没有被复制，复制过程中数组发生改变会抛出异常* * @return v ArrayList副本*/public Object clone() {try {// 调用Object类的clone方法得到ArrayList副本ArrayList<?> v = (ArrayList<?>) super.clone();// 调用copyOf，将ArrayList的elementData数组赋给副本的elementData数组v.elementData = Arrays.copyOf(elementData, size);v.modCount = 0;return v;} catch (CloneNotSupportedException e) {// this shouldn't happen, since we are Cloneablethrow new InternalError(e);}}/*** 转换为Object数组* * @return 一个数组包含所有列表中的元素*/public Object[] toArray() {return Arrays.copyOf(elementData, size);}/*** 将ArrayList里面的元素赋值到一个数组中去* a的长度小于ArrayList的长度，直接调用Arrays类的copyOf* a的长度大于ArrayList的长度，调用System.arraycopy，然后把size位置赋值为空。* * @param a*            如果它的长度大的话，列表元素存储在这个数组中; 否则分配一个新数组。* @return 一个包含ArrayList元素的数组* @throws ArrayStoreException*             将与数组类型不兼容的值赋值给数组元素时抛出的异常* @throws NullPointerException*             数组为空*/@SuppressWarnings("unchecked")public <T> T[] toArray(T[] a) {if (a.length < size)// 创建一个新的a的运行时类型数组，内容不变return (T[]) Arrays.copyOf(elementData, size, a.getClass());System.arraycopy(elementData, 0, a, 0, size);if (a.length > size)a[size] = null;return a;}/*** 返回指定位置的值* @param index* @return*/@SuppressWarnings("unchecked")E elementData(int index) {return (E) elementData[index];}/*** 返回指定位置的值，先检查是否超出数组长度* * @param index*            元素的索引* @return ArrayList中指定位置的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public E get(int index) {// 检查是否越界rangeCheck(index);// 返回ArrayList的elementData数组index位置的元素return elementData(index);}/*** 设置指定位置为一个新值，返回之前的值* * @param index*            要替换的元素的索引* @param element*            要存储在指定位置的元素* @return 之前在指定位置的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public E set(int index, E element) {// 检查越界rangeCheck(index);// 获取当前位置的值E oldValue = elementData(index);// 将element赋值到index位置elementData[index] = element;return oldValue;}/*** 添加一个值，首先会确保容量* * @param e*            要添加到此列表中的元素* @return <tt>true</tt> (as specified by {@link Collection#add})*/public boolean add(E e) {// 扩容ensureCapacityInternal(size + 1);// 将e赋值给elementData的size+1的位置elementData[size++] = e;return true;}/*** index位置添加元素element，会检查添加的位置和容量* * @param index*            指定元素将被插入的索引* @param element*            要插入的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public void add(int index, E element) {// 判断越界rangeCheckForAdd(index);// 扩容ensureCapacityInternal(size + 1);  // Increments modCount!!// 将elementData从index位置开始，复制到elementData的index+1开始的连续空间System.arraycopy(elementData, index, elementData, index + 1,size - index);// 在index位置赋值elementelementData[index] = element;// ArrayList的大小++size++;}/*** 移除index位置的元素,会检查去除的位置* * @param index*            要删除的元素的索引* @return 删除的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public E remove(int index) {// 判断越界rangeCheck(index);modCount++;// 读取旧值E oldValue = elementData(index);// 获取index位置开始到最后一个位置的个数int numMoved = size - index - 1;if (numMoved > 0)// index+1位置开始拷贝到从index开始的空间System.arraycopy(elementData, index + 1, elementData, index,numMoved);elementData[--size] = null; // 便于垃圾回收器回收return oldValue;}/*** 移除对象为O的元素，跟indexOf方法思想基本一致* @param o*            要从该列表中删除的元素（如果存在）* @return true 如果这个列表包含指定的元素*/public boolean remove(Object o) {if (o == null) {for (int index = 0; index < size; index++)if (elementData[index] == null) {fastRemove(index);return true;}} else {for (int index = 0; index < size; index++)if (o.equals(elementData[index])) {fastRemove(index);return true;}}return false;}/*** 快速删除指定位置的值，不需要检查和返回值* * @param index*/private void fastRemove(int index) {modCount++;int numMoved = size - index - 1;if (numMoved > 0)System.arraycopy(elementData, index + 1, elementData, index,numMoved);elementData[--size] = null; // 便于垃圾回收器回收}/*** 清空数组，把每一个值设为null,方便垃圾回收(不同于reset，数组默认大小有改变的话不会重置)*/public void clear() {modCount++;// 便于垃圾回收器回收for (int i = 0; i < size; i++)elementData[i] = null;size = 0;}/*** 添加一个集合的元素到末端* * @param c*            包含要添加到此列表中的元素的集合* @return true 如果该列表因添加而改变* @throws NullPointerException*             如果指定的集合是空的*/public boolean addAll(Collection<? extends E> c) {// c转换为数组aObject[] a = c.toArray();// a占的内存空间长度赋值给numNewint numNew = a.length;// 扩容至size + numNewensureCapacityInternal(size + numNew);// 将a的第0位开始拷贝至elementData的size位开始，拷贝长度为numNewSystem.arraycopy(a, 0, elementData, size, numNew);// 将size增加numNewsize += numNew;return numNew != 0;}/*** 从第index位开始，将c全部拷贝到ArrayList* * @param index*            在哪个索引开始插入* @param c*            包含要添加到此列表中的元素的集合* @return true 如果该列表因添加而改变* @throws IndexOutOfBoundsException*             {@inheritDoc}* @throws NullPointerException*             如果指定的集合是空的*/public boolean addAll(int index, Collection<? extends E> c) {rangeCheckForAdd(index);// 将c转换为数组aObject[] a = c.toArray();int numNew = a.length;// 扩容至size + numNewensureCapacityInternal(size + numNew);// 获取需要添加的个数int numMoved = size - index;if (numMoved > 0)System.arraycopy(elementData, index, elementData, index + numNew,numMoved);System.arraycopy(a, 0, elementData, index, numNew);size += numNew;return numNew != 0;}/*** 删除指定范围元素。* * @throws IndexOutOfBoundsException*             if {@code fromIndex} or {@code toIndex} is out of range ({@code fromIndex < 0 ||*             fromIndex >= size() || toIndex > size() || toIndex <*             fromIndex})*/protected void removeRange(int fromIndex, int toIndex) {modCount++;int numMoved = size - toIndex;// 后段保留的长度System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);// 便于垃圾回收int newSize = size - (toIndex - fromIndex);for (int i = newSize; i < size; i++) {elementData[i] = null;}size = newSize;}/*** 检查index是否超出数组长度*/private void rangeCheck(int index) {// 如果下标超过ArrayList的数组长度if (index >= size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}/*** 检查是否溢出*/private void rangeCheckForAdd(int index) {if (index > size || index < 0)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}/*** 抛出的异常的详情*/private String outOfBoundsMsg(int index) {return "Index: " + index + ", Size: " + size;}/*** ArrayList移除集合c中的所有元素* * @param c*            包含要从此列表中移除的元素的集合* @return {@code true} 如果该列表因移除而改变* @throws ClassCastException*             if the class of an element of this list is incompatible with*             the specified collection (<a*             href="Collection.html#optional-restrictions">optional</a>)* @throws NullPointerException*             if this list contains a null element and the specified*             collection does not permit null elements (<a*             href="Collection.html#optional-restrictions">optional</a>),*             or if the specified collection is null* @see Collection#contains(Object)*/public boolean removeAll(Collection<?> c) {// 如果c为空，则抛出空指针异常Objects.requireNonNull(c);// 调用batchRemove移除c中的元素return batchRemove(c, false);}/*** 仅保留指定集合c中的元素* * @param c*            collection containing elements to be retained in this list* @return {@code true} if this list changed as a result of the call* @throws ClassCastException*             if the class of an element of this list is incompatible with*             the specified collection (<a*             href="Collection.html#optional-restrictions">optional</a>)* @throws NullPointerException*             if this list contains a null element and the specified*             collection does not permit null elements (<a*             href="Collection.html#optional-restrictions">optional</a>),*             or if the specified collection is null* @see Collection#contains(Object)*/public boolean retainAll(Collection<?> c) {Objects.requireNonNull(c);// 调用batchRemove保留c中的元素return batchRemove(c, true);}/*** 根据complement值，将ArrayList中包含c中元素的元素删除或者保留* * @param c* @param complement*            true时从数组保留指定集合中元素的值，为false时从数组删除指定集合中元素的值。* @return 数组中重复的元素都会被删除(而不是仅删除一次或几次)，有任何删除操作都会返回true*/private boolean batchRemove(Collection<?> c, boolean complement) {final Object[] elementData = this.elementData;// 定义一个w，一个r，两个同时右移int r = 0, w = 0;boolean modified = false;try {// r先右移for (; r < size; r++)// 如果c中不包含elementData[r]这个元素if (c.contains(elementData[r]) == complement)// 则直接将r位置的元素赋值给w位置的元素，w自增elementData[w++] = elementData[r];} finally {// 防止抛出异常导致上面r的右移过程没完成if (r != size) {// 将r未右移完成的位置的元素赋值给w右边位置的元素System.arraycopy(elementData, r,elementData, w,size - r);// 修改w值增加size-rw += size - r;}// 如果有被覆盖掉的元素，则将w后面的元素都赋值为nullif (w != size) {// clear to let GC do its workfor (int i = w; i < size; i++)elementData[i] = null;modCount += size - w;// 改变的次数// 新的大小为保留的元素的个数size = w;modified = true;}}return modified;}/*** 保存数组实例的状态到一个流（即序列化）。写入过程数组被更改会抛出异常* * @serialData The length of the array backing the <tt>ArrayList</tt>*             instance is emitted (int), followed by all of its elements*             (each an <tt>Object</tt>) in the proper order.*/private void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException {// Write out element count, and any hidden stuffint expectedModCount = modCount;// 执行默认的反序列化/序列化过程。将当前类的非静态和非瞬态字段写入此流s.defaultWriteObject();// 写入大小s.writeInt(size);// 写入所有元素for (int i = 0; i < size; i++) {s.writeObject(elementData[i]);}if (modCount != expectedModCount) {throw new ConcurrentModificationException();}}/*** 从流中重构ArrayList实例（即反序列化）。*/private void readObject(java.io.ObjectInputStream s)throws java.io.IOException, ClassNotFoundException {elementData = EMPTY_ELEMENTDATA;// 执行默认的序列化/反序列化过程s.defaultReadObject();// 读入数组长度s.readInt(); // ignoredif (size > 0) {int capacity = calculateCapacity(elementData, size);SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);ensureCapacityInternal(size);Object[] a = elementData;// 读入所有元素for (int i = 0; i < size; i++) {a[i] = s.readObject();}}}/*** 返回一个从index开始的ListIterator对象* * @throws IndexOutOfBoundsException*             {@inheritDoc}*/public ListIterator<E> listIterator(int index) {if (index < 0 || index > size)throw new IndexOutOfBoundsException("Index: " + index);return new ListItr(index);}/*** 返回一个ListIterator对象，ListItr为ArrayList的一个内部类，实现了ListIterator<E> 接口* * @see #listIterator(int)*/public ListIterator<E> listIterator() {return new ListItr(0);}/*** 返回一个Iterator对象，Itr为ArrayList的一个内部类，实现了Iterator<E>接口* * @return an iterator over the elements in this list in proper sequence*/public Iterator<E> iterator() {return new Itr();}/*** 通用的迭代器实现*/private class Itr implements Iterator<E> {int cursor;       // 下一个元素的索引，默认为0int lastRet = -1; // 上次访问的元素的位置int expectedModCount = modCount;// 迭代过程修改数组抛出异常// 是否还有下一个public boolean hasNext() {return cursor != size;}// 下一个元素@SuppressWarnings("unchecked")public E next() {checkForComodification();// 检查数组是否被修改int i = cursor;if (i >= size)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length)throw new ConcurrentModificationException();cursor = i + 1;// 向后移动游标return (E) elementData[lastRet = i];// 设置访问的位置并返回这个值}// 删除元素public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();// 检查数组是否被修改try {ArrayList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}@Override@SuppressWarnings("unchecked")public void forEachRemaining(Consumer<? super E> consumer) {Objects.requireNonNull(consumer);final int size = ArrayList.this.size;int i = cursor;if (i >= size) {return;}final Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length) {throw new ConcurrentModificationException();}while (i != size && modCount == expectedModCount) {consumer.accept((E) elementData[i++]);}// update once at end of iteration to reduce heap write trafficcursor = i;lastRet = i - 1;checkForComodification();}// 检查数组是否被修改final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}}/*** ListIterator迭代器实现*/private class ListItr extends Itr implements ListIterator<E> {ListItr(int index) {super();cursor = index;}public boolean hasPrevious() {return cursor != 0;}public int nextIndex() {return cursor;}public int previousIndex() {return cursor - 1;}@SuppressWarnings("unchecked")public E previous() {checkForComodification();int i = cursor - 1;if (i < 0)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length)throw new ConcurrentModificationException();cursor = i;return (E) elementData[lastRet = i];}public void set(E e) {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.set(lastRet, e);} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}public void add(E e) {checkForComodification();try {int i = cursor;ArrayList.this.add(i, e);cursor = i + 1;lastRet = -1;expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}}/*** * 该方法返回的是父list的一个视图，fromIndex（包含）到toIndex（不包含）。fromIndex=toIndex 表示子list为空*/public List<E> subList(int fromIndex, int toIndex) {subListRangeCheck(fromIndex, toIndex, size);return new SubList(this, 0, fromIndex, toIndex);}/*** 安全检查* * @param fromIndex* @param toIndex* @param size*/static void subListRangeCheck(int fromIndex, int toIndex, int size) {if (fromIndex < 0)throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);if (toIndex > size)throw new IndexOutOfBoundsException("toIndex = " + toIndex);if (fromIndex > toIndex)throw new IllegalArgumentException("fromIndex(" + fromIndex +") > toIndex(" + toIndex + ")");}/*** 子数组*/private class SubList extends AbstractList<E> implements RandomAccess {private final AbstractList<E> parent;private final int parentOffset;private final int offset;int size;SubList(AbstractList<E> parent,int offset, int fromIndex, int toIndex) {this.parent = parent;this.parentOffset = fromIndex;this.offset = offset + fromIndex;this.size = toIndex - fromIndex;this.modCount = ArrayList.this.modCount;}public E set(int index, E e) {rangeCheck(index);checkForComodification();E oldValue = ArrayList.this.elementData(offset + index);ArrayList.this.elementData[offset + index] = e;return oldValue;}public E get(int index) {rangeCheck(index);checkForComodification();return ArrayList.this.elementData(offset + index);}public int size() {checkForComodification();return this.size;}public void add(int index, E e) {rangeCheckForAdd(index);checkForComodification();parent.add(parentOffset + index, e);this.modCount = parent.modCount;this.size++;}public E remove(int index) {rangeCheck(index);checkForComodification();E result = parent.remove(parentOffset + index);this.modCount = parent.modCount;this.size--;return result;}protected void removeRange(int fromIndex, int toIndex) {checkForComodification();parent.removeRange(parentOffset + fromIndex,parentOffset + toIndex);this.modCount = parent.modCount;this.size -= toIndex - fromIndex;}public boolean addAll(Collection<? extends E> c) {return addAll(this.size, c);}public boolean addAll(int index, Collection<? extends E> c) {rangeCheckForAdd(index);int cSize = c.size();if (cSize == 0)return false;checkForComodification();parent.addAll(parentOffset + index, c);this.modCount = parent.modCount;this.size += cSize;return true;}public Iterator<E> iterator() {return listIterator();}public ListIterator<E> listIterator(final int index) {checkForComodification();rangeCheckForAdd(index);final int offset = this.offset;return new ListIterator<E>() {int cursor = index;int lastRet = -1;int expectedModCount = ArrayList.this.modCount;public boolean hasNext() {return cursor != SubList.this.size;}@SuppressWarnings("unchecked")public E next() {checkForComodification();int i = cursor;if (i >= SubList.this.size)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (offset + i >= elementData.length)throw new ConcurrentModificationException();cursor = i + 1;return (E) elementData[offset + (lastRet = i)];}public boolean hasPrevious() {return cursor != 0;}@SuppressWarnings("unchecked")public E previous() {checkForComodification();int i = cursor - 1;if (i < 0)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (offset + i >= elementData.length)throw new ConcurrentModificationException();cursor = i;return (E) elementData[offset + (lastRet = i)];}@SuppressWarnings("unchecked")public void forEachRemaining(Consumer<? super E> consumer) {Objects.requireNonNull(consumer);final int size = SubList.this.size;int i = cursor;if (i >= size) {return;}final Object[] elementData = ArrayList.this.elementData;if (offset + i >= elementData.length) {throw new ConcurrentModificationException();}while (i != size && modCount == expectedModCount) {consumer.accept((E) elementData[offset + (i++)]);}// update once at end of iteration to reduce heap write// trafficlastRet = cursor = i;checkForComodification();}public int nextIndex() {return cursor;}public int previousIndex() {return cursor - 1;}public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {SubList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = ArrayList.this.modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}public void set(E e) {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.set(offset + lastRet, e);} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}public void add(E e) {checkForComodification();try {int i = cursor;SubList.this.add(i, e);cursor = i + 1;lastRet = -1;expectedModCount = ArrayList.this.modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}final void checkForComodification() {if (expectedModCount != ArrayList.this.modCount)throw new ConcurrentModificationException();}};}/*** 返回指定范围的子数组* * @param fromIndex* @param toIndex* @return*/public List<E> subList(int fromIndex, int toIndex) {subListRangeCheck(fromIndex, toIndex, size);return new SubList(this, offset, fromIndex, toIndex);}private void rangeCheck(int index) {if (index < 0 || index >= this.size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}private void rangeCheckForAdd(int index) {if (index < 0 || index > this.size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}private String outOfBoundsMsg(int index) {return "Index: " + index + ", Size: " + this.size;}private void checkForComodification() {if (ArrayList.this.modCount != this.modCount)throw new ConcurrentModificationException();}public Spliterator<E> spliterator() {checkForComodification();return new ArrayListSpliterator<E>(ArrayList.this, offset,offset + this.size, this.modCount);}}/*** Java 8 lambda 使用流遍历数组*/@Overridepublic void forEach(Consumer<? super E> action) {Objects.requireNonNull(action);final int expectedModCount = modCount;@SuppressWarnings("unchecked") final E[] elementData = (E[]) this.elementData;final int size = this.size;for (int i = 0; modCount == expectedModCount && i < size; i++) {action.accept(elementData[i]);}if (modCount != expectedModCount) {throw new ConcurrentModificationException();}}/*** 为了并行遍历元素而设计的一个迭代器* @return a {@code Spliterator} over the elements in this list* @since 1.8*/@Overridepublic Spliterator<E> spliterator() {return new ArrayListSpliterator<>(this, 0, -1, 0);}/*** Index-based split-by-two, lazily initialized Spliterator*/static final class ArrayListSpliterator<E> implements Spliterator<E> {/**
//forEach（）private final ArrayList<E> list;private int index; // current index, modified on advance/splitprivate int fence; // -1 until used; then one past last indexprivate int expectedModCount; // initialized when fence set/*** Create new spliterator covering the given range*/ArrayListSpliterator(ArrayList<E> list, int origin, int fence,int expectedModCount) {this.list = list; // OK if null unless traversedthis.index = origin;this.fence = fence;this.expectedModCount = expectedModCount;}private int getFence() { // initialize fence to size on first useint hi; // (a specialized variant appears in method forEach)ArrayList<E> lst;if ((hi = fence) < 0) {if ((lst = list) == null)hi = fence = 0;else {expectedModCount = lst.modCount;hi = fence = lst.size;}}return hi;}public ArrayListSpliterator<E> trySplit() {int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;return (lo >= mid) ? null : // divide range in half unless too smallnew ArrayListSpliterator<E>(list, lo, index = mid,expectedModCount);}public boolean tryAdvance(Consumer<? super E> action) {if (action == null)throw new NullPointerException();int hi = getFence(), i = index;if (i < hi) {index = i + 1;@SuppressWarnings("unchecked") E e = (E) list.elementData[i];action.accept(e);if (list.modCount != expectedModCount)throw new ConcurrentModificationException();return true;}return false;}public void forEachRemaining(Consumer<? super E> action) {int i, hi, mc; // hoist accesses and checks from loopArrayList<E> lst;Object[] a;if (action == null)throw new NullPointerException();if ((lst = list) != null && (a = lst.elementData) != null) {if ((hi = fence) < 0) {mc = lst.modCount;hi = lst.size;} elsemc = expectedModCount;if ((i = index) >= 0 && (index = hi) <= a.length) {for (; i < hi; ++i) {@SuppressWarnings("unchecked") E e = (E) a[i];action.accept(e);}if (lst.modCount == mc)return;}}throw new ConcurrentModificationException();}public long estimateSize() {return (long) (getFence() - index);}public int characteristics() {return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;}}@Overridepublic boolean removeIf(Predicate<? super E> filter) {Objects.requireNonNull(filter);// figure out which elements are to be removed// any exception thrown from the filter predicate at this stage// will leave the collection unmodifiedint removeCount = 0;final BitSet removeSet = new BitSet(size);final int expectedModCount = modCount;final int size = this.size;for (int i = 0; modCount == expectedModCount && i < size; i++) {@SuppressWarnings("unchecked") final E element = (E) elementData[i];if (filter.test(element)) {removeSet.set(i);removeCount++;}}if (modCount != expectedModCount) {throw new ConcurrentModificationException();}// shift surviving elements left over the spaces left by removed// elementsfinal boolean anyToRemove = removeCount > 0;if (anyToRemove) {final int newSize = size - removeCount;for (int i = 0, j = 0; (i < size) && (j < newSize); i++, j++) {i = removeSet.nextClearBit(i);elementData[j] = elementData[i];}for (int k = newSize; k < size; k++) {elementData[k] = null;  // Let gc do its work}this.size = newSize;if (modCount != expectedModCount) {throw new ConcurrentModificationException();}modCount++;}return anyToRemove;}@Override@SuppressWarnings("unchecked")public void replaceAll(UnaryOperator<E> operator) {Objects.requireNonNull(operator);final int expectedModCount = modCount;final int size = this.size;for (int i = 0; modCount == expectedModCount && i < size; i++) {elementData[i] = operator.apply((E) elementData[i]);}if (modCount != expectedModCount) {throw new ConcurrentModificationException();}modCount++;}@Override@SuppressWarnings("unchecked")public void sort(Comparator<? super E> c) {final int expectedModCount = modCount;Arrays.sort((E[]) elementData, 0, size, c);if (modCount != expectedModCount) {throw new ConcurrentModificationException();}modCount++;}
}

LinkedList

java.lang.Objectjava.util.AbstractCollection<E>java.util.AbstractList<E>java.util.AbstractSequentialList<E>java.util.LinkedList<E>

List 接口的链接列表实现。实现所有可选的列表操作，并且允许所有元素（包括 null）。

除了实现 List 接口外，LinkedList 类还为在列表的开头及结尾 get、remove 和 insert 元素提供了统一的命名方法。这些操作允许将链接列表用作堆栈、队列或双端队列。

此类实现 Deque 接口，为 add、poll 提供先进先出队列操作，以及其他堆栈和双端队列操作。

所有操作都是按照双重链接列表的需要执行的。在列表中编索引的操作将从开头或结尾遍历列表（从靠近指定索引的一端）。

操作效率：大部分效率都是o(n)的

同步：

注意，此实现不是同步的。如果多个线程同时访问一个链接列表，而其中至少一个线程从结构上修改了该列表，则它必须保持外部同步。（结构修改指添加或删除一个或多个元素的任何操作；仅设置元素的值不是结构修改。）这一般通过对自然封装该列表的对象进行同步操作来完成。如果不存在这样的对象，则应该使用 Collections.synchronizedList 方法来“包装”该列表。最好在创建时完成这一操作，以防止对列表进行意外的不同步访问，如下所示：

   List list = Collections.synchronizedList(new LinkedList(...));

iterator ：

此类的 iterator 和 listIterator 方法返回的迭代器是快速失败 的：在迭代器创建之后，如果从结构上对列表进行修改，除非通过迭代器自身的 remove 或 add 方法，其他任何时间任何方式的修改，迭代器都将抛出 ConcurrentModificationException。因此，面对并发的修改，迭代器很快就会完全失败，而不冒将来不确定的时间任意发生不确定行为的风险。

注意，迭代器的快速失败行为不能得到保证，一般来说，存在不同步的并发修改时，不可能作出任何硬性保证。快速失败迭代器尽最大努力抛出 ConcurrentModificationException。因此，编写依赖于此异常的程序的方式是错误的，正确做法是：迭代器的快速失败行为应该仅用于检测程序错误。

源码解析

package java.util;
import java.util.function.Consumer;
public class LinkedList<E>extends AbstractSequentialList<E>implements List<E>, Deque<E>, Cloneable, java.io.Serializable {/*** 元素数量*/transient int size = 0;/*** 首结点引用*/transient Node<E> first;/*** 尾节点引用*/transient Node<E> last;/*** 无参构造方法*/public LinkedList() {}/*** 通过一个集合初始化，元素顺序有这个集合的迭代器返回顺序决定* * @param c*            其元素将被放入此列表中的集合* @throws NullPointerException*             如果指定的集合是空的*/public LinkedList(Collection<? extends E> c) {// 调用无参构造this();// 添加所有的元素addAll(c);}/*** 头插，内部使用*/private void linkFirst(E e) {// 当前首结点final Node<E> f = first;// 构建新节点final Node<E> newNode = new Node<>(null, e, f);// 将newNode作为首节点first = newNode;// 如果原首节点为null，即原链表为null，则链表尾节点也设置为newNodeif (f == null)last = newNode;else                // 否则，原首节点的prev设置为newNodef.prev = newNode;size++;             // 长度+1modCount++; // 修改次数+1}/*** 尾插*/void linkLast(E e) {// 获取尾结点final Node<E> l = last;// 构建新节点final Node<E> newNode = new Node<>(l, e, null);// newNode作为尾节点last = newNode;// 如果原尾节点为null，即原链表为null，则链表首节点也设置为newNodeif (l == null)first = newNode;else    // 否则，原尾节点的next设置为newNodel.next = newNode;size++;modCount++;}/*** 在非空节点succ前插入节点值e*/void linkBefore(E e, Node<E> succ) {final Node<E> pred = succ.prev;// 构建新节点final Node<E> newNode = new Node<>(pred, e, succ);// 设置newNode为succ的前节点succ.prev = newNode;// 如果succ.prev为null，即succ为首节点，则将newNode设置为首节点if (pred == null)first = newNode;else        // 如果succ不是首节点pred.next = newNode;size++;modCount++;}/*** 删除首结点，返回存储的元素，内部使用*/private E unlinkFirst(Node<E> f) {// 获取首结点存储的元素final E element = f.item;// 获取首结点的后继结点final Node<E> next = f.next;// 删除首结点f.item = null;f.next = null; // 便于垃圾回收// 首结点后继结点设为首结点first = next;// 如果原来首结点的后继结点为空，则尾结点设为null// 否则，原来首结点的后继结点的前驱结点设为nullif (next == null)last = null;elsenext.prev = null;size--;modCount++;// 返回原来首结点存储的元素return element;}/*** 删除尾结点，返回存储的元素，内部使用*/private E unlinkLast(Node<E> l) {// 获取尾结点存储的元素final E element = l.item;// 获取尾结点的前驱结点final Node<E> prev = l.prev;// 删除尾结点l.item = null;l.prev = null; // help GC// 原来尾结点的前驱结点设为尾结点last = prev;// 如果原来尾结点的前驱结点为空，则首结点设为null// 否则，原来尾结点的前驱结点的后继结点设为nullif (prev == null)first = null;elseprev.next = null;size--;modCount++;// 返回原来尾结点存储的元素return element;}/*** 删除指定非空结点，返回存储的元素*/E unlink(Node<E> x) {// 获取指定非空结点存储的元素final E element = x.item;// 获取指定非空结点的后继结点final Node<E> next = x.next;// 获取指定非空结点的前驱结点final Node<E> prev = x.prev;/*** 如果指定非空结点的前驱结点为空，则指定非空结点的后继结点设为首结点 * 否则，指定非空结点的后继结点设为指定非空结点的前驱结点的后继结点，* 指定非空结点的前驱结点设为null*/if (prev == null) {first = next;} else {prev.next = next;x.prev = null;}/*** 如果指定非空结点的后继结点为空，则指定非空结点的前驱结点设为尾结点 * 否则，指定非空结点的前驱结点设为指定非空结点的后继结点的前驱结点，* 指定非空结点的后继结点设为null*/if (next == null) {last = prev;} else {next.prev = prev;x.next = null;}// 指定非空结点存储的元素设为nullx.item = null;size--;modCount++;// 返回指定非空结点存储的元素return element;}/*** 获取首结点存储的元素* * @return 首结点存储的元素* @throws NoSuchElementException*             如果链表为空*/public E getFirst() {// 获取首结点引用final Node<E> f = first;// 如果首结点为空，则抛出无该元素异常if (f == null)throw new NoSuchElementException();// 返回首结点存储的元素return f.item;}/*** 获取尾结点存储的元素* * @return 尾结点存储的元素* @throws NoSuchElementException*             如果链表为空*/public E getLast() {// 获取尾结点引用final Node<E> l = last;// 如果尾结点为空，则抛出无该元素异常if (l == null)throw new NoSuchElementException();// 返回尾结点存储的元素return l.item;}/*** 删除首结点，返回存储的元素* * @return 首结点存储的元素* @throws NoSuchElementException*             如果链表为空*/public E removeFirst() {// 获取首结点引用final Node<E> f = first;// 如果首结点为空，则抛出无该元素异常if (f == null)throw new NoSuchElementException();// 删除首结点，返回存储的元素return unlinkFirst(f);}/*** 删除尾结点，返回存储的元素* * @return 尾结点存储的元素* @throws NoSuchElementException*             如果链表为空*/public E removeLast() {// 获取尾结点引用final Node<E> l = last;// 如果尾结点为空，则抛出无该元素异常if (l == null)throw new NoSuchElementException();// 删除尾结点，返回存储的元素return unlinkLast(l);}/*** 头部插入指定元素* * @param e*            要添加的元素*/public void addFirst(E e) {// 通过头插法来插入指定元素linkFirst(e);}/*** 尾部插入指定元素，该方法等价于add()* * @param e*            the element to add*/public void addLast(E e) {linkLast(e);}/*** 判断是否包含指定元素* * @param o*            判断链表是否包含的元素* @return {@code true} 如果链表包含指定的元素*/public boolean contains(Object o) {// 返回指定元素的索引位置，不存在就返回-1，然后比较返回bool值return indexOf(o) != -1;}/*** 获取元素数量* * @return 元素数量*/public int size() {return size;}/*** 插入指定元素，返回操作结果,默认添加到末尾作为最后一个元素* * @param e*            要添加到此链表中的元素* @return {@code true} (as specified by {@link Collection#add})*/public boolean add(E e) {// 通过尾插法来插入指定元素linkLast(e);return true;}/*** 删除指定元素，默认从first节点开始，删除第一次出现的那个元素* * @param o*            要从该列表中删除的元素（如果存在）* @return {@code true} 如果这个列表包含指定的元素*/public boolean remove(Object o) {// 会根据是否为null分开处理。若值不是null，会用到对象的equals()方法if (o == null) {// 遍历链表，查找到指定元素后删除该结点，返回truefor (Node<E> x = first; x != null; x = x.next) {if (x.item == null) {unlink(x);return true;}}} else {for (Node<E> x = first; x != null; x = x.next) {if (o.equals(x.item)) {unlink(x);return true;}}}// 查找失败return false;}/*** 将集合插入到链表尾部* * @param c*            包含要添加到此链表中的元素的集合* @return {@code true} 如果该链表因添加而改变* @throws NullPointerException*             如果指定的集合是空的*/public boolean addAll(Collection<? extends E> c) {return addAll(size, c);}/*** 将集合从指定位置开始插入* * @param index*            在哪个索引处前插入指定集合中的第一个元素* @param c*            包含要添加到此链表中的元素的集合* @return {@code true} 如果该链表因添加而改变* @throws IndexOutOfBoundsException*             {@inheritDoc}* @throws NullPointerException*             如果指定的集合是空的*/public boolean addAll(int index, Collection<? extends E> c) {// 检查索引是否正确（0<=index<=size）checkPositionIndex(index);// 得到元素数组Object[] a = c.toArray();// 得到元素个数int numNew = a.length;// 没有元素，返回falseif (numNew == 0)return false;// succ指向当前需要插入节点的位置，pred指向其前一个节点Node<E> pred, succ;// 末尾开始添加，当前节点后一个节点为null，前一个节点为尾节点if (index == size) {succ = null;pred = last;} else {    //当前位置的节点为指定位置的节点，前一个节点为要添加的节点的前一个节点succ = node(index);pred = succ.prev;}// 遍历数组并添加到列表中for (Object o : a) {@SuppressWarnings("unchecked") E e = (E) o;// “封装”一个新节点Node<E> newNode = new Node<>(pred, e, null);// 原链表为null，新插入的节点作为链表首节点if (pred == null)first = newNode;elsepred.next = newNode;    // 存在前节点，前节点向后指向新加的节点pred = newNode; // pred指针向后移动}// 从最后开始添加，则新节点成为尾节点if (succ == null) {last = pred;} else {pred.next = succ;   // 新节点向后指向之前得到的后续第一个节点succ.prev = pred;   // 后续的第一个节点也应改为向前指向最后一个添加的节点}size += numNew;modCount++;return true;}/*** 删除所有元素*/public void clear() {// 遍历链表，删除所有结点,方便gc回收垃圾for (Node<E> x = first; x != null; ) {Node<E> next = x.next;x.item = null;x.next = null;x.prev = null;x = next;}// 首尾结点置空first = last = null;// 元素数量置0size = 0;modCount++;}// 位置访问操作/*** 获取指定位置的元素* * @param index*            要返回的元素的索引* @return 该链表中指定位置的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public E get(int index) {// 判断指定位置是否合法checkElementIndex(index);// 返回指定位置的元素return node(index).item;}/*** 修改指定位置的元素，返回之前元素* * @param index*            要替换的元素的索引* @param element*            要存储在指定位置的元素* @return 之前在指定位置的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public E set(int index, E element) {// 判断指定位置是否合法checkElementIndex(index);// 获取指定位置的结点Node<E> x = node(index);// 获取该结点存储的元素E oldVal = x.item;// 修改该结点存储的元素x.item = element;// 返回该结点存储的之前的元素return oldVal;}/*** 在指定位置前插入指定元素* 头插法* @param index*            指定元素将被插入的索引* @param element*            要插入的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public void add(int index, E element) {// 判断指定位置是否合法checkPositionIndex(index);// 如果指定位置在尾部，则通过尾插法来插入指定元素if (index == size)linkLast(element);else        // 如果指定位置不是尾部，则添加到指定位置前linkBefore(element, node(index));}/*** 删除指定位置的元素，返回之前元素* * @param index*            要删除的元素的索引* @return 之前在指定位置的元素* @throws IndexOutOfBoundsException*             {@inheritDoc}*/public E remove(int index) {// 判断指定位置是否合法checkElementIndex(index);// 删除指定位置的结点，返回之前元素return unlink(node(index));}/*** 判断指定位置是否合法*/private boolean isElementIndex(int index) {return index >= 0 && index < size;}/*** 判断迭代器遍历时或插入元素时指定位置是否合法*/private boolean isPositionIndex(int index) {return index >= 0 && index <= size;}/*** 获取越界异常信息*/private String outOfBoundsMsg(int index) {return "Index: " + index + ", Size: " + size;}/*** 判断指定位置是否合法* * @param index*/private void checkElementIndex(int index) {if (!isElementIndex(index))throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}/*** 判断指定位置是否合法* * @param index*/private void checkPositionIndex(int index) {if (!isPositionIndex(index))throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}/*** 获取指定下标的结点*/Node<E> node(int index) {// 如果指定下标<一半元素数量，则从首结点开始遍历// 否则，从尾结点开始遍历if (index < (size >> 1)) {Node<E> x = first;for (int i = 0; i < index; i++)x = x.next;return x;} else {Node<E> x = last;for (int i = size - 1; i > index; i--)x = x.prev;return x;}}// 查询操作/*** 获取首次出现指定元素的位置 -1表示不存在* 同样是根据是否为null进行区分* @param o*            要查找的元素* @return the index of the first occurrence of the specified element in*         this list, or -1 if this list does not contain the element*/public int indexOf(Object o) {int index = 0;if (o == null) {// 遍历链表，顺序查找指定元素for (Node<E> x = first; x != null; x = x.next) {if (x.item == null)return index;index++;}} else {for (Node<E> x = first; x != null; x = x.next) {if (o.equals(x.item))return index;index++;}}return -1;}/*** 获取逆序下首次出现指定元素的位置 -1表示不存在* * @param o*            要查找的元素* @return the index of the last occurrence of the specified element in this*         list, or -1 if this list does not contain the element*/public int lastIndexOf(Object o) {int index = size;if (o == null) {// 遍历链表，逆序查找指定元素for (Node<E> x = last; x != null; x = x.prev) {index--;if (x.item == null)return index;}} else {for (Node<E> x = last; x != null; x = x.prev) {index--;if (o.equals(x.item))return index;}}return -1;}// 队列操作/*** 出队（从前端），获得第一个元素，不存在会返回null，不会删除元素（节点） 获取首元素* * @return the head of this list, or {@code null} 如果链表为空* @since 1.5*/public E peek() {final Node<E> f = first;// 如果首结点为空，则返回null// 否则，返回首结点存储的元素return (f == null) ? null : f.item;}/*** 出队（从前端），不删除元素，若为null会抛出异常而不是返回null 获取首元素* * @return the head of this list* @throws NoSuchElementException*             如果链表为空* @since 1.5*/public E element() {// 返回首结点存储的元素return getFirst();}/*** 出队（从前端），如果不存在会返回null，存在的话会返回值并移除这个元素（节点） 获取并删除首元素* * @return the head of this list, or {@code null} 如果链表为空* @since 1.5*/public E poll() {// 获取首结点引用final Node<E> f = first;// 如果首结点为空，则返回null// 否则，删除首结点，返回首结点存储的元素return (f == null) ? null : unlinkFirst(f);}/*** 出队（从前端），如果不存在会抛出异常而不是返回null，存在的话会返回值并移除这个元素（节点） 获取并删除首元素* * @return the head of this list* @throws NoSuchElementException*             如果链表为空* @since 1.5*/public E remove() {// 删除首结点，返回首结点存储的元素return removeFirst();}/*** 入队（从后端），始终返回true* * 链表不会溢出* @param e*            the element to add* @return {@code true} (as specified by {@link Queue#offer})* @since 1.5*/public boolean offer(E e) {// 通过尾插法插入指定元素，返回操作结果return add(e);}// 双端队列操作/*** 入队（从前端），始终返回true* * @param e*            要插入的元素* @return {@code true} (as specified by {@link Deque#offerFirst})* @since 1.6*/public boolean offerFirst(E e) {// 通过头插法来插入指定元素addFirst(e);return true;}/*** 入队（从后端），始终返回true* * @param e*            要插入的元素* @return {@code true} (as specified by {@link Deque#offerLast})* @since 1.6*/public boolean offerLast(E e) {// 通过尾插法来插入指定元素addLast(e);return true;}/*** 出队（从前端），获得第一个元素，不存在会返回null，不会删除元素（节点）* * @return the first element of this list, or {@code null} 如果链表为空* @since 1.6*/public E peekFirst() {// 获取首结点引用final Node<E> f = first;// 如果首结点为空，则返回null// 否则，返回首结点存储的元素return (f == null) ? null : f.item;}/*** 出队（从后端），获得最后一个元素，不存在会返回null，不会删除元素（节点）* * @return the last element of this list, or {@code null} 如果链表为空* @since 1.6*/public E peekLast() {// 获取尾结点引用final Node<E> l = last;// 如果尾结点为空，则返回null// 否则，返回尾结点存储的元素return (l == null) ? null : l.item;}/*** 出队（从前端），获得第一个元素，不存在会返回null，会删除元素（节点）* * @return the first element of this list, or {@code null} if this list is*         empty* @since 1.6*/public E pollFirst() {// 获取首结点引用final Node<E> f = first;// 如果首结点为空，则返回null// 否则，删除首结点，返回首结点存储的元素return (f == null) ? null : unlinkFirst(f);}/*** 出队（从后端），获得最后一个元素，不存在会返回null，会删除元素（节点）* * @return the last element of this list, or {@code null} if this list is*         empty* @since 1.6*/public E pollLast() {// 获取尾结点引用final Node<E> l = last;// 如果尾结点为空，则返回null// 否则，删除尾结点，返回尾结点存储的元素return (l == null) ? null : unlinkLast(l);}/*** 入栈，从前面添加* * @param e*            the element to push* @since 1.6*/public void push(E e) {// 通过头插法来插入指定元素addFirst(e);}/*** 出栈，返回栈顶元素，从前面移除（会删除）* * @return the element at the front of this list (which is the top of the*         stack represented by this list)* @throws NoSuchElementException*             如果链表为空* @since 1.6*/public E pop() {// 删除首结点，返回首结点存储的元素return removeFirst();}/*** 删除顺序下首次出现的指定元素，返回操作结果* * @param o*            要从该列表中删除的元素（如果存在）* @return {@code true} 如果链表包含指定的元素* @since 1.6*/public boolean removeFirstOccurrence(Object o) {// 删除顺序下首次出现的指定元素对应的结点，返回操作结果return remove(o);}/*** 删除逆序下首次出现的指定元素，返回操作结果* * @param o*            要从该列表中删除的元素（如果存在）* @return {@code true} 如果链表包含指定的元素* @since 1.6*/public boolean removeLastOccurrence(Object o) {// 由于LinkedList中允许存放null，因此下面通过两种情况来分别处理if (o == null) {// 遍历链表，从尾结点开始查找指定元素// 如果查找成功，删除该结点，返回truefor (Node<E> x = last; x != null; x = x.prev) {if (x.item == null) {unlink(x);return true;}}} else {for (Node<E> x = last; x != null; x = x.prev) {if (o.equals(x.item)) {unlink(x);return true;}}}// 查找失败return false;}/*** Returns a list-iterator of the elements in this list (in proper* sequence), starting at the specified position in the list. Obeys the* general contract of {@code List.listIterator(int)}.* <p>* <p>* The list-iterator is <i>fail-fast</i>: if the list is structurally* modified at any time after the Iterator is created, in any way except* through the list-iterator's own {@code remove} or {@code add} methods,* the list-iterator will throw a {@code ConcurrentModificationException}.* Thus, in the face of concurrent modification, the iterator fails quickly* and cleanly, rather than risking arbitrary, non-deterministic behavior at* an undetermined time in the future.* * @param index*            index of the first element to be returned from the*            list-iterator (by a call to {@code next})* @return a ListIterator of the elements in this list (in proper sequence),*         starting at the specified position in the list* @throws IndexOutOfBoundsException*             {@inheritDoc}* @see List#listIterator(int)*/public ListIterator<E> listIterator(int index) {checkPositionIndex(index);return new ListItr(index);}private class ListItr implements ListIterator<E> {private Node<E> lastReturned;private Node<E> next;private int nextIndex;private int expectedModCount = modCount;ListItr(int index) {// assert isPositionIndex(index);next = (index == size) ? null : node(index);nextIndex = index;}public boolean hasNext() {return nextIndex < size;}public E next() {checkForComodification();if (!hasNext())throw new NoSuchElementException();lastReturned = next;next = next.next;nextIndex++;return lastReturned.item;}public boolean hasPrevious() {return nextIndex > 0;}public E previous() {checkForComodification();if (!hasPrevious())throw new NoSuchElementException();lastReturned = next = (next == null) ? last : next.prev;nextIndex--;return lastReturned.item;}public int nextIndex() {return nextIndex;}public int previousIndex() {return nextIndex - 1;}public void remove() {checkForComodification();if (lastReturned == null)throw new IllegalStateException();Node<E> lastNext = lastReturned.next;unlink(lastReturned);if (next == lastReturned)next = lastNext;elsenextIndex--;lastReturned = null;expectedModCount++;}public void set(E e) {if (lastReturned == null)throw new IllegalStateException();checkForComodification();lastReturned.item = e;}public void add(E e) {checkForComodification();lastReturned = null;if (next == null)linkLast(e);elselinkBefore(e, next);nextIndex++;expectedModCount++;}public void forEachRemaining(Consumer<? super E> action) {Objects.requireNonNull(action);while (modCount == expectedModCount && nextIndex < size) {action.accept(next.item);lastReturned = next;next = next.next;nextIndex++;}checkForComodification();}final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}}/*** 节点的数据结构，包含前后节点的引用和当前节点* * @param <E>*/private static class Node<E> {// 存储的元素E item;// 后继结点Node<E> next;// 前驱结点Node<E> prev;// 前驱结点、存储的元素和后继结点作为参数的构造方法Node(Node<E> prev, E element, Node<E> next) {this.item = element;this.next = next;this.prev = prev;}}/*** 返回迭代器* * @since 1.6*/public Iterator<E> descendingIterator() {return new DescendingIterator();}/*** 因为采用链表实现，所以迭代器很简单*/private class DescendingIterator implements Iterator<E> {private final ListItr itr = new ListItr(size());public boolean hasNext() {return itr.hasPrevious();}public E next() {return itr.previous();}public void remove() {itr.remove();}}/*** 父类克隆方法*/@SuppressWarnings("unchecked")private LinkedList<E> superClone() {try {return (LinkedList<E>) super.clone();} catch (CloneNotSupportedException e) {throw new InternalError(e);}}/*** 克隆，浅拷贝* * 浅拷贝时，若存储的是对象的引用，拷贝时，对象本身的改变将表现到副本中，而深拷贝不会。* @return a shallow copy of this {@code LinkedList} instance*/public Object clone() {LinkedList<E> clone = superClone();// 链表初始化clone.first = clone.last = null;clone.size = 0;clone.modCount = 0;// 插入结点for (Node<E> x = first; x != null; x = x.next)clone.add(x.item);// 返回克隆后的对象引用return clone;}/*** 返回新的数组，数组含有列表中所有元素*/public Object[] toArray() {Object[] result = new Object[size];int i = 0;for (Node<E> x = first; x != null; x = x.next)result[i++] = x.item;return result;}/*** Returns an array containing all of the elements in this list in proper* sequence (from first to last element); the runtime type of the returned* array is that of the specified array. If the list fits in the specified* array, it is returned therein. Otherwise, a new array is allocated with* the runtime type of the specified array and the size of this list.* <p>* <p>* If the list fits in the specified array with room to spare (i.e., the* array has more elements than the list), the element in the array* immediately following the end of the list is set to {@code null}. (This* is useful in determining the length of the list <i>only</i> if the* caller knows that the list does not contain any null elements.)* <p>* <p>* Like the {@link #toArray()} method, this method acts as bridge between* array-based and collection-based APIs. Further, this method allows* precise control over the runtime type of the output array, and may, under* certain circumstances, be used to save allocation costs.* <p>* <p>* Suppose {@code x} is a list known to contain only strings. The following* code can be used to dump the list into a newly allocated array of* {@code String}:* <p>* * <pre>*     String[] y = x.toArray(new String[0]);* </pre>* * <p>* Note that {@code toArray(new Object[0])} is identical in function to* {@code toArray()}.* * @param a*            the array into which the elements of the list are to be*            stored, if it is big enough; otherwise, a new array of the*            same runtime type is allocated for this purpose.* @return an array containing the elements of the list* @throws ArrayStoreException*             if the runtime type of the specified array is not a supertype*             of the runtime type of every element in this list* @throws NullPointerException*             if the specified array is null*/@SuppressWarnings("unchecked")public <T> T[] toArray(T[] a) {if (a.length < size)a = (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), size);int i = 0;Object[] result = a;for (Node<E> x = first; x != null; x = x.next)result[i++] = x.item;if (a.length > size)a[size] = null;return a;}private static final long serialVersionUID = 876323262645176354L;/*** 序列化*/private void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException {// 默认序列化s.defaultWriteObject();// 写入元素数量s.writeInt(size);// 遍历链表，写入所有元素for (Node<E> x = first; x != null; x = x.next)s.writeObject(x.item);}/*** 反序列化*/@SuppressWarnings("unchecked")private void readObject(java.io.ObjectInputStream s)throws java.io.IOException, ClassNotFoundException {// 默认反序列化s.defaultReadObject();// 读取元素数量int size = s.readInt();// 遍历链表，读取所有元素并尾部插入for (int i = 0; i < size; i++)linkLast((E) s.readObject());}/*** Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>* and <em>fail-fast</em> {@link Spliterator} over the elements in this* list.* <p>* <p>* The {@code Spliterator} reports {@link Spliterator#SIZED} and* {@link Spliterator#ORDERED}. Overriding implementations should document* the reporting of additional characteristic values.* * @return a {@code Spliterator} over the elements in this list* @implNote The {@code Spliterator} additionally reports*           {@link Spliterator#SUBSIZED} and implements {@code trySplit} to*           permit limited parallelism..* @since 1.8*/@Overridepublic Spliterator<E> spliterator() {return new LLSpliterator<E>(this, -1, 0);}/*** A customized variant of Spliterators.IteratorSpliterator*/static final class LLSpliterator<E> implements Spliterator<E> {static final int BATCH_UNIT = 1 << 10;  // batch array size incrementstatic final int MAX_BATCH = 1 << 25;  // max batch array size;final LinkedList<E> list; // null OK unless traversedNode<E> current;      // current node; null until initializedint est;              // size estimate; -1 until first neededint expectedModCount; // initialized when est setint batch;            // batch size for splitsLLSpliterator(LinkedList<E> list, int est, int expectedModCount) {this.list = list;this.est = est;this.expectedModCount = expectedModCount;}final int getEst() {int s; // force initializationfinal LinkedList<E> lst;if ((s = est) < 0) {if ((lst = list) == null)s = est = 0;else {expectedModCount = lst.modCount;current = lst.first;s = est = lst.size;}}return s;}public long estimateSize() {return (long) getEst();}public Spliterator<E> trySplit() {Node<E> p;int s = getEst();if (s > 1 && (p = current) != null) {int n = batch + BATCH_UNIT;if (n > s)n = s;if (n > MAX_BATCH)n = MAX_BATCH;Object[] a = new Object[n];int j = 0;do {a[j++] = p.item;} while ((p = p.next) != null && j < n);current = p;batch = j;est = s - j;return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);}return null;}public void forEachRemaining(Consumer<? super E> action) {Node<E> p;int n;if (action == null) throw new NullPointerException();if ((n = getEst()) > 0 && (p = current) != null) {current = null;est = 0;do {E e = p.item;p = p.next;action.accept(e);} while (p != null && --n > 0);}if (list.modCount != expectedModCount)throw new ConcurrentModificationException();}public boolean tryAdvance(Consumer<? super E> action) {Node<E> p;if (action == null) throw new NullPointerException();if (getEst() > 0 && (p = current) != null) {--est;E e = p.item;current = p.next;action.accept(e);if (list.modCount != expectedModCount)throw new ConcurrentModificationException();return true;}return false;}public int characteristics() {return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;}}}

Vector

基于synchronized实现的线程安全的ArrayList，但在插入元素时容量扩充的机制和ArrayList稍有不同，并可通过传入capacityIncrement来控制容量的扩充。

成员变量

protected Object[] elementData;
protected int elementCount;
protected int capacityIncrement;

构造方法

public Vector(int initialCapacity) {this(initialCapacity, 0);
}
public Vector() {this(10);
}
public Vector(int initialCapacity, int capacityIncrement) {super();if (initialCapacity < 0)throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);this.elementData = new Object[initialCapacity];this.capacityIncrement = capacityIncrement;
}

添加

public synchronized boolean add(E e) {modCount++;ensureCapacityHelper(elementCount + 1);elementData[elementCount++] = e;return true;
}

删除

public boolean remove(Object o) {return removeElement(o);
}public synchronized boolean removeElement(Object obj) {modCount++;int i = indexOf(obj);if (i >= 0) {removeElementAt(i);return true;}return false;
}

扩容

private void grow(int minCapacity) {// overflow-conscious codeint oldCapacity = elementData.length;int newCapacity = oldCapacity + ((capacityIncrement > 0) ?capacityIncrement : oldCapacity);if (newCapacity - minCapacity < 0)newCapacity = minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);elementData = Arrays.copyOf(elementData, newCapacity);
}

获取

public synchronized E get(int index) {if (index >= elementCount)throw new ArrayIndexOutOfBoundsException(index);return elementData(index);
}

更新

public synchronized E set(int index, E element) {if (index >= elementCount)throw new ArrayIndexOutOfBoundsException(index);E oldValue = elementData(index);elementData[index] = element;return oldValue;
}

包含

public boolean contains(Object o) {return indexOf(o, 0) >= 0;
}public synchronized int indexOf(Object o, int index) {if (o == null) {for (int i = index ; i < elementCount ; i++)if (elementData[i]==null)return i;} else {for (int i = index ; i < elementCount ; i++)if (o.equals(elementData[i]))return i;}return -1;
}

CopyOnWriteArrayList

是一个线程安全、并且在读操作时无锁的ArrayList。

很多时候，我们的系统应对的都是读多写少的并发场景。CopyOnWriteArrayList容器允许并发读，读操作是无锁的，性能较高。至于写操作，比如向容器中添加一个元素，则首先将当前容器复制一份，然后在新副本上执行写操作，结束之后再将原容器的引用指向新容器。

优点

1）采用读写分离方式，读的效率非常高

2）CopyOnWriteArrayList的迭代器是基于创建时的数据快照的，故数组的增删改不会影响到迭代器

缺点

1）内存占用高，每次执行写操作都要将原容器拷贝一份，数据量大时，对内存压力较大，可能会引起频繁GC

2）只能保证数据的最终一致性，不能保证数据的实时一致性。写和读分别作用在新老不同容器上，在写操作执行过程中，读不会阻塞但读取到的却是老容器的数据。

成员变量

/** The lock protecting all mutators */
final transient ReentrantLock lock = new ReentrantLock();

/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;

构造方法

public CopyOnWriteArrayList() {setArray(new Object[0]);
}final void setArray(Object[] a) {array = a;
}

添加（有锁，锁内重新创建数组）

final Object[] getArray() {return array;}public boolean add(E e) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;Object[] newElements = Arrays.copyOf(elements, len + 1);newElements[len] = e;setArray(newElements);return true;} finally {lock.unlock();}
}

存在则添加（有锁，锁内重新创建数组）

先保存一份数组snapshot，如果snapshot中存在，则直接返回。

如果不存在，那么加锁，获取当前数组current，比较snapshot与current，遍历它们共同长度内的元素，如果发现current中某一个元素等于e，那么直接返回（当然current与snapshot相同就不必看了）；

之后再遍历current单独的部分，如果发现current中某一个元素等于e，那么直接返回；

此时可以去创建一个长度+1的新数组，将e加入。

public boolean addIfAbsent(E e) {Object[] snapshot = getArray();return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :addIfAbsent(e, snapshot);
}private boolean addIfAbsent(E e, Object[] snapshot) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] current = getArray();int len = current.length;if (snapshot != current) {// Optimize for lost race to another addXXX operationint common = Math.min(snapshot.length, len);for (int i = 0; i < common; i++)//如果snapshot与current元素不同但current与e相同，那么直接返回（扫描0到common）if (current[i] != snapshot[i] && eq(e, current[i]))return false;// 如果current中存在e，那么直接返回（扫描commen到len）if (indexOf(e, current, common, len) >= 0)return false;}Object[] newElements = Arrays.copyOf(current, len + 1);newElements[len] = e;setArray(newElements);return true;} finally {lock.unlock();}
}

删除（有锁，锁内重新创建数组）

public E remove(int index) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;E oldValue = get(elements, index);int numMoved = len - index - 1;if (numMoved == 0)setArray(Arrays.copyOf(elements, len - 1));else {Object[] newElements = new Object[len - 1];System.arraycopy(elements, 0, newElements, 0, index);System.arraycopy(elements, index + 1, newElements, index,numMoved);setArray(newElements);}return oldValue;} finally {lock.unlock();}
}

获取（无锁）

public E get(int index) {return get(getArray(), index);
}private E get(Object[] a, int index) {return (E) a[index];
}

更新（有锁，锁内重新创建数组）

public E set(int index, E element) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();E oldValue = get(elements, index);if (oldValue != element) {int len = elements.length;Object[] newElements = Arrays.copyOf(elements, len);newElements[index] = element;setArray(newElements);} else {// 为了保持“volatile”的语义，任何一个读操作都应该是一个写操作的结果，也就是读操作看到的数据一定是某个写操作的结果（尽管写操作没有改变数据本身）。所以这里即使不设置也没有问题，仅仅是为了一个语义上的补充（就如源码中的注释所言）// Not quite a no-op; ensures volatile write semanticssetArray(elements);}return oldValue;} finally {lock.unlock();}
}

包含（无锁）

public boolean contains(Object o) {Object[] elements = getArray();return indexOf(o, elements, 0, elements.length) >= 0;
}private static int indexOf(Object o, Object[] elements,int index, int fence) {if (o == null) {for (int i = index; i < fence; i++)if (elements[i] == null)return i;} else {for (int i = index; i < fence; i++)if (o.equals(elements[i]))return i;}return -1;
}

List实现类之间的区别

(1) 对于需要快速插入，删除元素，应该使用LinkedList。

(2) 对于需要快速随机访问元素，应该使用ArrayList。

(3) 对于“单线程环境” 或者 “多线程环境，但List仅仅只会被单个线程操作”，此时应该使用非同步的类(如ArrayList)。

对于“多线程环境，且List可能同时被多个线程操作”，此时，应该使用同步的类(如Vector、CopyOnWriteArrayList)。

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