ArrayList源码学习

客官°小女子只卖身不卖艺 2022-03-08 04:56 243阅读 0赞

### 文章目录 ###

*   *   *  ArrayList简介
         *  ArrayList核心源码
         *  源码重点方法分析
         *   *  System.arraycopy()和Arrays.copyOf()方法
             *  ArrayList核心扩容技术
         *  ArrayList经典Demo

### ArrayList简介 ###

ArrayList 的底层是数组队列，相当于动态数组。与 Java 中的数组相比，它的容量能动态增长。在添加大量元素前，应用程序可以使用ensureCapacity操作来增加 ArrayList 实例的容量。这可以减少递增式再分配的数量。

它继承于 AbstractList，实现了 List, RandomAccess, Cloneable, java.io.Serializable 这些接口。

在我们学数据结构的时候就知道了线性表的顺序存储，插入删除元素的时间复杂度为O（n）,求表长以及增加元素，取第 i 元素的时间复杂度为O（1）

ArrayList 继承了AbstractList，实现了List。它是一个数组队列，提供了相关的添加、删除、修改、遍历等功能。

ArrayList 实现了RandomAccess 接口，即提供了随机访问功能。RandomAccess 是 Java 中用来被 List 实现，为 List 提供快速访问功能的。在 ArrayList 中，我们即可以通过元素的序号快速获取元素对象，这就是快速随机访问。

ArrayList 实现了Cloneable 接口，即覆盖了函数 clone()，能被克隆。

ArrayList 实现java.io.Serializable 接口，这意味着ArrayList支持序列化，能通过序列化去传输。

和 Vector 不同，ArrayList 中的操作不是线程安全的！所以，建议在单线程中才使用 ArrayList，而在多线程中可以选择 Vector 或者 CopyOnWriteArrayList。

### ArrayList核心源码 ###

package java.util;
 
 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;
 
 /**
 * Default initial capacity.
 * 默认初始容量大小
 */
 private static final int DEFAULT_CAPACITY = 10;
 
 /**
 * Shared empty array instance used for empty instances.
 * 空数组（用于空实例）
 */
 private static final Object[] EMPTY_ELEMENTDATA = {};
 
 /**
 * 用于默认大小空实例的共享空数组实例
 * 我们把它从EMPTY_ELEMENTDATA数组中区分出来，以知道在添加第一个元素时容量需要增加多少
 */
 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
 
 /**
 * 保存ArrayList数据的数组
 */
 transient Object[] elementData; // non-private to simplify nested class access
 
 /**
 * ArrayList 所包含的元素个数
 * @serial
 */
 private int size;
 
 /**
 * 带初始容量参数的构造函数。（用户自己指定容量）
 */
 public ArrayList(int initialCapacity) {
 if (initialCapacity > 0) {
 this.elementData = new Object[initialCapacity];
 } else if (initialCapacity == 0) {
 this.elementData = EMPTY_ELEMENTDATA;
 } else {
 throw new IllegalArgumentException("Illegal Capacity: "+
 initialCapacity);
 }
 }
 
 /**
 * Constructs an empty list with an initial capacity of ten.
 * 默认构造函数， DEFAULTCAPACITY_EMPTY_ELEMENTDATA为0.初始值为10
 * 也就是说初始其实是空数组，当添加一个元素的时候数组容量变为10
 */
 public ArrayList() {
 this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
 }
 
 /**
 * 构造一个包含指定集合的元素的列表
 */
 public ArrayList(Collection<? extends E> c) {
 elementData = c.toArray();
 if ((size = elementData.length) != 0) {
 // c.toArray might (incorrectly) not return Object[] (see 6260652)
 if (elementData.getClass() != Object[].class)
 elementData = Arrays.copyOf(elementData, size, Object[].class);
 } else {
 // replace with empty array.
 this.elementData = EMPTY_ELEMENTDATA;
 }
 }
 
 /**
 * 修改这个ArrayList实例的容量是列表的当前大小
 * 可以使用此操作来最小化ArrayList实例的存储
 */
 public void trimToSize() {
 modCount++;
 if (size < elementData.length) {
 elementData = (size == 0)
 ? EMPTY_ELEMENTDATA
 : Arrays.copyOf(elementData, size);
 }
 }
 
 	//下面是ArrayList的扩容机制
 	//ArrayList的扩容机制提高了性能，如果每次只扩充一个
 	//那么频繁的插入会导致频繁的拷贝，降低性能，而ArrayList的扩容机制避免了这种情况
 	
 /**
 * 如有必要，增加此ArrayList实例的容量，以确保它至少能容纳元素的数量
 * @param minCatacity 所需的最小容量
 */
 public void ensureCapacity(int minCapacity) {
 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;
 
 if (minCapacity > minExpand) {
 ensureExplicitCapacity(minCapacity);
 }
 }
 
 	//得到最小扩容量
 private void ensureCapacityInternal(int minCapacity) {
 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
 minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
 }
 
 ensureExplicitCapacity(minCapacity);
 }
 
 	//判断是否需要扩容
 private void ensureExplicitCapacity(int minCapacity) {
 modCount++;
 
 // overflow-conscious code
 if (minCapacity - elementData.length > 0)
 //调用grow方法进行扩容，调用此方法代表已经开始扩容了
 grow(minCapacity);
 }
 
 /**
 * 要分配的最大数组大小
 */
 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
 
 /**
 * ArrayList扩容的核心方法
 * @param minCapacity the desired minimum capacity
 */
 private void grow(int minCapacity) {
 // oldCapacity为旧容量，newCapacity为新容量
 int oldCapacity = elementData.length;
 //将oldCapacity 右移一位，其效果相当于oldCapacity /2，
 //我们知道位运算的速度远远快于整除运算，整句运算式的结果就是将新容量更新为旧容量的1.5倍
 int newCapacity = oldCapacity + (oldCapacity >> 1);
 //然后检查新容量是否大于最小需要容量，若还是小于最小需要容量，那么就把最小需要容量当作数组的新容量，
 if (newCapacity - minCapacity < 0)
 newCapacity = minCapacity;
 //再检查新容量是否超出了ArrayList所定义的最大容量，
 //若超出了，则调用hugeCapacity()来比较minCapacity和 MAX_ARRAY_SIZE，
 //如果minCapacity大于最大容量，则新容量则为ArrayList定义的最大容量，否则，新容量大小则为 minCapacity。 
 
 if (newCapacity - MAX_ARRAY_SIZE > 0)
 newCapacity = hugeCapacity(minCapacity);
 // minCapacity is usually close to size, so this is a win:
 elementData = Arrays.copyOf(elementData, newCapacity);
 }
 
 	//比较minCapacity和MAX_ARRAY_SIZE
 private static int hugeCapacity(int minCapacity) {
 if (minCapacity < 0) // overflow
 throw new OutOfMemoryError();
 return (minCapacity > MAX_ARRAY_SIZE) ?
 Integer.MAX_VALUE :
 MAX_ARRAY_SIZE;
 }
 
 /**
 * 返回此列表中的元素数
 */
 public int size() {
 return size;
 }
 
 /**
 * 如果此列表不包含元素，则返回 true
 */
 public boolean isEmpty() {
 return size == 0;
 }
 
 /**
 * 如果此列表包含指定的元素，则返回 true
 */
 public boolean contains(Object o) {
 return indexOf(o) >= 0;
 }
 
 /**
 * 返回此列表中指定元素的首次出现的索引，如果此列表不包含此元素，则为-1
 */
 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;
 }
 
 /**
 * 返回此列表中指定元素的最后一次出现的索引，如果此列表不包含元素，则返回-1
 */
 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;
 }
 
 /**
 * 返回此ArrayList实例的拷贝
 *		
 * @return a clone of this <tt>ArrayList</tt> instance
 */
 public Object clone() {
 try {
 ArrayList<?> v = (ArrayList<?>) super.clone();
 v.elementData = Arrays.copyOf(elementData, size);
 v.modCount = 0;
 return v;
 } catch (CloneNotSupportedException e) {
 // this shouldn't happen, since we are Cloneable
 throw new InternalError(e);
 }
 }
 
 /**
 * List转数组
 * 以正确的顺序（从第一个到最后一个元素）返回一个包含此列表中所有元素的数组。 
 */
 public Object[] toArray() {
 return Arrays.copyOf(elementData, size);
 }
 
 /**
 * 以正确的顺序返回一个包含此列表中所有元素的数组（从第一个到最后一个元素）; 
 * 返回的数组的运行时类型是指定数组的运行时类型。 如果列表适合指定的数组，则返回其中。 
 * 否则，将为指定数组的运行时类型和此列表的大小分配一个新数组。 
 * 如果列表适用于指定的数组，其余空间（即数组的列表数量多于此元素），则紧跟在集合结束后的数组中的元素设置为null 。
 *（这仅在调用者知道列表不包含任何空元素的情况下才能确定列表的长度。）
 */
 @SuppressWarnings("unchecked")
 public <T> T[] toArray(T[] a) {
 if (a.length < size)
 // Make a new array of a's runtime type, but my contents:
 return (T[]) Arrays.copyOf(elementData, size, a.getClass());
 System.arraycopy(elementData, 0, a, 0, size);
 if (a.length > size)
 a[size] = null;
 return a;
 }
 
 // Positional Access Operations
 
 @SuppressWarnings("unchecked")
 E elementData(int index) {
 return (E) elementData[index];
 }
 
 /**
 * 返回此列表中指定位置的元素。
 */
 public E get(int index) {
 rangeCheck(index);
 
 return elementData(index);
 }
 
 /**
 * 用指定的元素替换此列表中指定位置的元素。 
 */
 public E set(int index, E element) {
 	 //对index进行界限检查
 rangeCheck(index);
 
 E oldValue = elementData(index);
 elementData[index] = element;
 //返回原来在这个位置的元素
 return oldValue;
 }
 
 /**
 * 将特定的元素追加到此列表的末尾
 */
 public boolean add(E e) {
 ensureCapacityInternal(size + 1); // Increments modCount!!
 //这里看到ArrayList添加元素的实质就相当于为数组赋值
 elementData[size++] = e;
 return true;
 }
 
 /**
 * 在此列表中的指定位置插入指定的元素。 
 * 先调用 rangeCheckForAdd 对index进行界限检查；然后调用 ensureCapacityInternal 方法保证capacity足够大；
 * 再将从index开始之后的所有成员后移一个位置；将element插入index位置；最后size加1。
 */
 public void add(int index, E element) {
 rangeCheckForAdd(index);
 
 ensureCapacityInternal(size + 1); // Increments modCount!!
 System.arraycopy(elementData, index, elementData, index + 1,
 size - index);
 elementData[index] = element;
 size++;
 }
 
 /**
 * 删除该列表中指定位置的元素。 将任何后续元素移动到左侧（从其索引中减去一个元素）。
 */
 public E remove(int index) {
 rangeCheck(index);
 
 modCount++;
 E oldValue = elementData(index);
 
 int numMoved = size - index - 1;
 if (numMoved > 0)
 System.arraycopy(elementData, index+1, elementData, index,
 numMoved);
 elementData[--size] = null; // clear to let GC do its work
 
 //返回从列表中删除的元素
 return oldValue;
 }
 
 /**
 * 从列表中删除指定元素的第一个出现位置的元素（如果存在）。 如果列表不包含该元素，则它不会更改。
 */
 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，用的较少，用时再深究
 */
 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; // clear to let GC do its work
 }
 
 /**
 * 从列表中删除所有元素
 */
 public void clear() {
 modCount++;
 
 // 把数组中所有元素的值设为null（？不是释放 应该是后面用垃圾回收机制回收内存）
 for (int i = 0; i < size; i++)
 elementData[i] = null;
 
 size = 0;
 }
 
 /**
 * 按指定集合的Iterator返回的顺序将指定集合中的所有元素追加到此列表的末尾。
 */
 public boolean addAll(Collection<? extends E> c) {
 Object[] a = c.toArray();
 int numNew = a.length;
 ensureCapacityInternal(size + numNew); // Increments modCount
 System.arraycopy(a, 0, elementData, size, numNew);
 size += numNew;
 return numNew != 0;
 }
 
 /**
 * 将指定集合中的所有元素插入到此列表中，从指定的位置开始。
 */
 public boolean addAll(int index, Collection<? extends E> c) {
 rangeCheckForAdd(index);
 
 Object[] a = c.toArray();
 int numNew = a.length;
 ensureCapacityInternal(size + numNew); // Increments modCount
 
 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;
 }
 
 /**
 * 从此列表中删除所有索引为fromIndex （含）和toIndex之间的元素。
 * 将任何后续元素移动到左侧（减少其索引）。 
 */
 protected void removeRange(int fromIndex, int toIndex) {
 modCount++;
 int numMoved = size - toIndex;
 System.arraycopy(elementData, toIndex, elementData, fromIndex,
 numMoved);
 
 // clear to let GC do its work
 int newSize = size - (toIndex-fromIndex);
 for (int i = newSize; i < size; i++) {
 elementData[i] = null;
 }
 size = newSize;
 }
 
 /**
 * 检查给定的索引是否在范围内。
 */
 private void rangeCheck(int index) {
 if (index >= size)
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 /**
 * A version of rangeCheck used by add and addAll.
 * add和addAll使用的rangeCheck的一个版本
 */
 private void rangeCheckForAdd(int index) {
 if (index > size || index < 0)
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 /**
 * 返回IndexOutOfBoundsException细节信息
 */
 private String outOfBoundsMsg(int index) {
 return "Index: "+index+", Size: "+size;
 }
 
 /**
 * 从此列表中删除指定集合中包含的所有元素。 
 */
 public boolean removeAll(Collection<?> c) {
 Objects.requireNonNull(c);
 return batchRemove(c, false);
 }
 
 /**
 * 仅保留此列表中包含在指定集合中的元素。 （取交集）
 * 即 从此列表中删除其中不包含在指定集合中的所有元素。
 * @see Collection#contains(Object)
 */
 public boolean retainAll(Collection<?> c) {
 Objects.requireNonNull(c);
 return batchRemove(c, true);
 }
 
 private boolean batchRemove(Collection<?> c, boolean complement) {
 final Object[] elementData = this.elementData;
 int r = 0, w = 0;
 boolean modified = false;
 try {
 for (; r < size; r++)
 if (c.contains(elementData[r]) == complement)
 elementData[w++] = elementData[r];
 } finally {
 // Preserve behavioral compatibility with AbstractCollection,
 // even if c.contains() throws.
 if (r != size) {
 System.arraycopy(elementData, r,
 elementData, w,
 size - r);
 w += size - r;
 }
 if (w != size) {
 // clear to let GC do its work
 for (int i = w; i < size; i++)
 elementData[i] = null;
 modCount += size - w;
 size = w;
 modified = true;
 }
 }
 return modified;
 }
 
 /**
 * Save the state of the <tt>ArrayList</tt> instance to a stream (that
 * is, serialize it).
 *
 * @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 stuff
 int expectedModCount = modCount;
 s.defaultWriteObject();
 
 // Write out size as capacity for behavioural compatibility with clone()
 s.writeInt(size);
 
 // Write out all elements in the proper order.
 for (int i=0; i<size; i++) {
 s.writeObject(elementData[i]);
 }
 
 if (modCount != expectedModCount) {
 throw new ConcurrentModificationException();
 }
 }
 
 /**
 * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
 * deserialize it).
 */
 private void readObject(java.io.ObjectInputStream s)
 throws java.io.IOException, ClassNotFoundException {
 elementData = EMPTY_ELEMENTDATA;
 
 // Read in size, and any hidden stuff
 s.defaultReadObject();
 
 // Read in capacity
 s.readInt(); // ignored
 
 if (size > 0) {
 // be like clone(), allocate array based upon size not capacity
 ensureCapacityInternal(size);
 
 Object[] a = elementData;
 // Read in all elements in the proper order.
 for (int i=0; i<size; i++) {
 a[i] = s.readObject();
 }
 }
 }
 
 /**
 * 从列表中的指定位置开始，返回列表中的元素（按正确顺序）的列表迭代器。
 */
 public ListIterator<E> listIterator(int index) {
 if (index < 0 || index > size)
 throw new IndexOutOfBoundsException("Index: "+index);
 return new ListItr(index);
 }
 
 /**
 * 返回列表中的列表迭代器（按适当的顺序）。 
 *
 * The returned list iterator is <a href="#fail-fast">fail-fast</a>.
 *
 * @see #listIterator(int)
 */
 public ListIterator<E> listIterator() {
 return new ListItr(0);
 }
 
 /**
 * 以正确的顺序返回该列表中的元素的迭代器。 
 *
 */
 public Iterator<E> iterator() {
 return new Itr();
 }
 
 /**
 * An optimized version of AbstractList.Itr
 */
 private class Itr implements Iterator<E> {
 int cursor; // index of next element to return
 int lastRet = -1; // index of last element returned; -1 if no such
 int expectedModCount = modCount;
 
 public boolean hasNext() {
 return cursor != size;
 }
 
 @SuppressWarnings("unchecked")
 public E next() {
 checkForComodification();
 int i = cursor;
 if (i >= size)
 throw new NoSuchElementException();
 Object[] elementData = ArrayList.this.elementData;
 if (i >= elementData.length)
 throw new ConcurrentModificationException();
 cursor = i + 1;
 return (E) elementData[lastRet = i];
 }
 
 public void remove() {
 if (lastRet < 0)
 throw new IllegalStateException();
 checkForComodification();
 
 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 traffic
 cursor = i;
 lastRet = i - 1;
 checkForComodification();
 }
 
 final void checkForComodification() {
 if (modCount != expectedModCount)
 throw new ConcurrentModificationException();
 }
 }
 
 /**
 * An optimized version of AbstractList.ListItr
 */
 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();
 }
 }
 }
 
 /**
 * Returns a view of the portion of this list between the specified
 * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If
 * {@code fromIndex} and {@code toIndex} are equal, the returned list is
 * empty.) The returned list is backed by this list, so non-structural
 * changes in the returned list are reflected in this list, and vice-versa.
 * The returned list supports all of the optional list operations.
 *
 * This method eliminates the need for explicit range operations (of
 * the sort that commonly exist for arrays). Any operation that expects
 * a list can be used as a range operation by passing a subList view
 * instead of a whole list. For example, the following idiom
 * removes a range of elements from a list:
 * <pre>
 * list.subList(from, to).clear();
 * </pre>
 * Similar idioms may be constructed for {@link #indexOf(Object)} and
 * {@link #lastIndexOf(Object)}, and all of the algorithms in the
 * {@link Collections} class can be applied to a subList.
 *
 * The semantics of the list returned by this method become undefined if
 * the backing list (i.e., this list) is structurally modified in
 * any way other than via the returned list. (Structural modifications are
 * those that change the size of this list, or otherwise perturb it in such
 * a fashion that iterations in progress may yield incorrect results.)
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 * @throws IllegalArgumentException {@inheritDoc}
 */
 public List<E> subList(int fromIndex, int toIndex) {
 subListRangeCheck(fromIndex, toIndex, size);
 return new SubList(this, 0, fromIndex, toIndex);
 }
 
 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 + ")");
 }
 
 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 traffic
 lastRet = 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();
 }
 };
 }
 
 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);
 }
 }
 
 @Override
 public 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();
 }
 }
 
 /**
 * Creates a <a href="Spliterator.html#binding">late-binding</a>
 * and fail-fast {@link Spliterator} over the elements in this
 * list.
 *
 * The {@code Spliterator} reports {@link Spliterator#SIZED},
 * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}.
 * Overriding implementations should document the reporting of additional
 * characteristic values.
 *
 * @return a {@code Spliterator} over the elements in this list
 * @since 1.8
 */
 @Override
 public 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> {
 
 private final ArrayList<E> list;
 private int index; // current index, modified on advance/split
 private int fence; // -1 until used; then one past last index
 private 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 traversed
 this.index = origin;
 this.fence = fence;
 this.expectedModCount = expectedModCount;
 }
 
 private int getFence() { // initialize fence to size on first use
 int 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 small
 new 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 loop
 ArrayList<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;
 }
 else
 mc = 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;
 }
 }
 
 @Override
 public 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 unmodified
 int 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 elements
 final 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++;
 }
 }

### 源码重点方法分析 ###

#### System.arraycopy()和Arrays.copyOf()方法 ####

在ArrayList的源码中我们发现这两个数组复制的方法都被使用到了。如在add中用到了arraycopy()方法实现让数组从index+1的位置开始复制原数组从index开始到最后的元素再在index位置插入新元素

/**
         * 在此列表中的指定位置插入指定的元素。 
         *先调用 rangeCheckForAdd 对index进行界限检查；然后调用 ensureCapacityInternal 方法保证capacity足够大；
         *再将从index开始之后的所有成员后移一个位置；将element插入index位置；最后size加1。
         */
        public void add(int index, E element) {
            rangeCheckForAdd(index);
    
            ensureCapacityInternal(size + 1);  // Increments modCount!!
            //arraycopy()方法实现数组自己复制自己
            //elementData:源数组;index:源数组中的起始位置;elementData：目标数组；index + 1：目标数组中的起始位置； size - index：要复制的数组元素的数量；
            System.arraycopy(elementData, index, elementData, index + 1, size - index);
            elementData[index] = element;
            size++;
        }

在toArray()方法中用到了copyOf()方法

/**
         *以正确的顺序（从第一个到最后一个元素）返回一个包含此列表中所有元素的数组。 
         *返回的数组将是“安全的”，因为该列表不保留对它的引用。 （换句话说，这个方法必须分配一个新的数组）。
         *因此，调用者可以自由地修改返回的数组。 此方法充当基于阵列和基于集合的API之间的桥梁。
         */
        public Object[] toArray() {
        //elementData：要复制的数组；size：要复制的长度
            return Arrays.copyOf(elementData, size);
        }

**联系:** 看两者源代码可以发现`copyOf()`内部调用了`System.arraycopy()`方法。  
**区别:**  
1.arraycopy()需要目标数组，将原数组拷贝到你自己定义的数组里，而且可以选择拷贝的起点和长度以及放入新数组中的位置  
2.copyOf()是系统自动在内部新建一个数组，并返回该数组。

#### ArrayList核心扩容技术 ####

//下面是ArrayList的扩容机制
    //ArrayList的扩容机制提高了性能，如果每次只扩充一个，
    //那么频繁的插入会导致频繁的拷贝，降低性能，而ArrayList的扩容机制避免了这种情况。
        /**
         * 如有必要，增加此ArrayList实例的容量，以确保它至少能容纳元素的数量
         * @param   minCapacity   所需的最小容量
         */
        public void ensureCapacity(int minCapacity) {
            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;
    
            if (minCapacity > minExpand) {
                ensureExplicitCapacity(minCapacity);
            }
        }
       //得到最小扩容量
        private void ensureCapacityInternal(int minCapacity) {
            if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
                  // 获取默认的容量和传入参数的较大值
                minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
            }
    
            ensureExplicitCapacity(minCapacity);
        }
      //判断是否需要扩容,上面两个方法都要调用
        private void ensureExplicitCapacity(int minCapacity) {
            modCount++;
    
            // 如果说minCapacity也就是所需的最小容量大于保存ArrayList数据的数组的长度的话，就需要调用grow(minCapacity)方法扩容。
            //这个minCapacity到底为多少呢？举个例子在添加元素(add)方法中这个minCapacity的大小就为现在数组的长度加1
            if (minCapacity - elementData.length > 0)
                //调用grow方法进行扩容，调用此方法代表已经开始扩容了
                grow(minCapacity);
        }

/**
 * ArrayList扩容的核心方法。
 */
 private void grow(int minCapacity) {
 //elementData为保存ArrayList数据的数组
 ///elementData.length求数组长度elementData.size是求数组中的元素个数
 // oldCapacity为旧容量，newCapacity为新容量
 int oldCapacity = elementData.length;
 //将oldCapacity 右移一位，其效果相当于oldCapacity /2，
 //我们知道位运算的速度远远快于整除运算，整句运算式的结果就是将新容量更新为旧容量的1.5倍，
 int newCapacity = oldCapacity + (oldCapacity >> 1);
 //然后检查新容量是否大于最小需要容量，若还是小于最小需要容量，那么就把最小需要容量当作数组的新容量，
 if (newCapacity - minCapacity < 0)
 newCapacity = minCapacity;
 //再检查新容量是否超出了ArrayList所定义的最大容量，
 //若超出了，则调用hugeCapacity()来比较minCapacity和 MAX_ARRAY_SIZE，
 //如果minCapacity大于最大容量，则新容量则为ArrayList定义的最大容量，否则，新容量大小则为 minCapacity。 
 if (newCapacity - MAX_ARRAY_SIZE > 0)
 newCapacity = hugeCapacity(minCapacity);
 // minCapacity is usually close to size, so this is a win:
 elementData = Arrays.copyOf(elementData, newCapacity);
 }

扩容机制代码中的已经做了详细的注释解释。 
另外值得注意的是大家很容易忽略的一个运算符：**移位运算符** 　　 
**简介**：移位运算符就是在二进制的基础上对数字进行平移。按照平移的方向和填充数字的规则分为三种:<<(左移)、>>(带符号右移)和>>>(无符号右移)。 　　 
**作用**：对于大数据的2进制运算,位移运算符比那些普通运算符的运算要快很多,因为程序仅仅移动一下而已,不去计算,这样提高了效率,节省了资源 　　比如这里：`int newCapacity = oldCapacity + (oldCapacity >> 1)`; 右移一位相当于除2，右移n位相当于除以 2 的 n 次方。这里 oldCapacity 明显右移了1位所以相当于oldCapacity /2。

另外需要注意的是：

>  *  java 中的length 属性是针对数组说的,比如说你声明了一个数组,想知道这个数组的长度则用到了 length 这个属性.

>  *  java 中的length()方法是针对字 符串String说的,如果想看这个字符串的长度则用到 length()这个方法.

>  *  java 中的size()方法是针对泛型集合说的,如果想看这个泛型有多少个元素,就调用此方法来查看!

### ArrayList经典Demo ###

package list;
 import java.util.ArrayList;
 import java.util.Iterator;
 
 public class ArrayListDemo {
 
 public static void main(String[] srgs){
 ArrayList<Integer> arrayList = new ArrayList<Integer>();
 
 System.out.printf("Before add:arrayList.size() = %d\n",arrayList.size());
 
 arrayList.add(1);
 arrayList.add(3);
 arrayList.add(5);
 arrayList.add(7);
 arrayList.add(9);
 System.out.printf("After add:arrayList.size() = %d\n",arrayList.size());
 
 System.out.println("Printing elements of arrayList");
 // 三种遍历方式打印元素
 // 第一种：通过迭代器遍历
 System.out.print("通过迭代器遍历:");
 Iterator<Integer> it = arrayList.iterator();
 while(it.hasNext()){
 System.out.print(it.next() + " ");
 }
 System.out.println();
 
 // 第二种：通过索引值遍历
 System.out.print("通过索引值遍历:");
 for(int i = 0; i < arrayList.size(); i++){
 System.out.print(arrayList.get(i) + " ");
 }
 System.out.println();
 
 // 第三种：for循环遍历
 System.out.print("for循环遍历:");
 for(Integer number : arrayList){
 System.out.print(number + " ");
 }
 
 // toArray用法
 // 第一种方式(最常用)
 Integer[] integer = arrayList.toArray(new Integer[0]);
 
 // 第二种方式(容易理解)
 Integer[] integer1 = new Integer[arrayList.size()];
 arrayList.toArray(integer1);
 
 // 抛出异常，java不支持向下转型
 //Integer[] integer2 = new Integer[arrayList.size()];
 //integer2 = arrayList.toArray();
 System.out.println();
 
 // 在指定位置添加元素
 arrayList.add(2,2);
 // 删除指定位置上的元素
 arrayList.remove(2); 
 // 删除指定元素
 arrayList.remove((Object)3);
 // 判断arrayList是否包含5
 System.out.println("ArrayList contains 5 is: " + arrayList.contains(5));
 
 // 清空ArrayList
 arrayList.clear();
 // 判断ArrayList是否为空
 System.out.println("ArrayList is empty: " + arrayList.isEmpty());
 }
 }