Java 集合体系之 ArrayList 源码分析
前言
ArrayList 的底层我们都知道，是通过数组来实现的，那么其内部又是如何做到可动态扩展的呢？下面就来扒开源码一探究竟。
源码分析
直接上代码，注释写的很清晰了已经：
public class ArrayList extends AbstractList
implements List, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
/**
* 默认的初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Object 类型的空数组实例
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Object 类型的默认大小（10）的空数组实例
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 存储 ArrayList 元素的数组缓冲区
*/
transient Object[] elementData; // non-private to simplify nested class access
// transient: 被修饰的变量不加入序列化
/**
* ArrayList 的大小（包含的元素数）
*/
private int size;
/**
* 构建一个具有初始容量的空集合
*/
public ArrayList(int initialCapacity) {
// 如果初始容量大于 0，则创建一个新的初始容量的数组
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {// 如果初始容量等于 0，则直接使用已经定义好的空数组实例
this.elementData = EMPTY_ELEMENTDATA;
} else {// 如果小于 0，则直接抛出异常
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* 构建一个初始容量为 10 的空集合
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 构建一个包含指定集合元素的集合（按照集合的迭代器返回的顺序）
*/
public ArrayList(Collection c) {
// 将集合转为数组
elementData = c.toArray();
// 如果传递过来的集合有数据
if ((size = elementData.length) != 0) {
// c.toArray 返回的结果有可能并不是 Object[] 类型的 （官方 bug）
// 详见官方 bug 地址：http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6260652
if (elementData.getClass() != Object[].class)
// 指定 Object[] 类型，重新 copy 数组
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// 没有数据，则直接使用原先定义好的空数组
this.elementData = EMPTY_ELEMENTDATA;
}
}
/**
* 修改 ArrayList 实际的容量为 size
* 由于 elementData 的容量是可以被扩展的，而 size 是包含元素的个数
* 所以会出现 size 比 elementData.length 小的情况出现，浪费了空间
* 该方法的作用就是重新返回一个和元素个数相等长度的数组给 elementData
*/
public void trimToSize() {
modCount++;// 集合已被修改的次数 +1
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
/**
* 对 ArrayList 的容量进行扩容
* 如果构造的是一个具有初始容量的集合（无参构造）
* 那么传递的参数（minCapacity）必须大于默认初始容量（10）
*/
public void ensureCapacity(int minCapacity) {
// 判断了构造的是否是具有初始容量的数组
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
? 0
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
// 确保明确的容量
ensureExplicitCapacity(minCapacity);
}
}
private void ensureCapacityInternal(int minCapacity) {
// 如果数组是有默认容量的数组
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
// 那么最小容量则是默认容量和最小容量中最大的一个
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
// 扩容
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;// 集合被修改次数 +1
// 确保最小容量大于数组的容量
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* 要分配数组的最大大小
* 当尝试分配更大的数组容量时，会导致 OutOfMemoryError
* -8 是因为数组自己需要 8byte 来存储元数据
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 增加容量以保证它至少容纳最小容量参数指定元素的数量
*
* @param minCapacity 所需的最小容量
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);// 新容量：原有容量 + 原有容量的一半
if (newCapacity - minCapacity < 0)// 如果新容量小于了最小容量，那么新容量则为最小容量
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)// 如果新容量超出了最大的数组容量
newCapacity = hugeCapacity(minCapacity);// 新容量等于最大的数组容量
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);// 创建新容量的数组（原数据不变，只是容量变大）
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // 当最小容量小于 0，直接抛出 OutOfMemoryError 异常
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?// 最小容量是否大于最大数组容量
Integer.MAX_VALUE :// 是则返回 Integer.MAX_VALUE
MAX_ARRAY_SIZE;// 否则返回最大数组容量
}
/**
* 返回 ArrayList 中存储元素的数量（注意，该 size 并不等于 length）
*/
public int size() {
return size;
}
/**
* 返回 ArrayList 中存储的元素是否是空的
*/
public boolean isEmpty() {
return size == 0;
}
/**
* 如果 ArrayList 中包含了指定的元素，则返回 true，否则返回 false（ null 元素也算）
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
/**
* 返回指定元素（null 也算）在集合（数组）中第一次出现的位置（角标）
* 如果没有找到则会返回 -1
*/
public int indexOf(Object o) {
// 遍历数组进行比较，比较到就直接返回对应位置角标
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;
}
/**
* 同 indexOf()，只不过是反向遍历，所以返回的是指定元素最后一次出现的位置
*/
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 ArrayList 实例的浅拷贝（元素本身不被复制）
* 浅拷贝：两个变量指示内存中的地址不一样，但是变量中的元素指向的是同一个
* 深拷贝：两个变量指示内存中的地址不一样，变量中元素也不是同一个
*/
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);
}
}
/**
* 该方法作为 ArrayList 和数组之间的桥梁
* 以正确的顺序返回一个包含 ArrayList 中所有元素的数组（从第一个到最后一个）
*
* 返回的数组是“安全”的，因为 ArrayList 不保留对它的引用（换句话说，该方法必须分配一个新的数组）
* 因此，调用者可以自由的修改返回的数组
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
/**
* 返回指定数组类型的数组（参数指定了一个具体类型的数组）
* 如果 ArrayList 中的元素数和指定的数组的长度相同
* 那么就会直接将元素 copy 到指定数组中，并返回该数组
*
* 如果指定数组的长度小于了 ArrayList 中的元素
* 那么就会按照指定数组的运行时类型和 ArrayList 大小重新分配一个数组。
*
* 如果指定数组的长度大于了 ArrayList 中的元素
* 那么就会将数组中结束位置的元素设置为 null
*（当 ArrayList 中不包含任何空元素时，这有助于调用者确定 ArrayList 的长度）
*/
@SuppressWarnings("unchecked")
public 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;
}
// 对数组进行位置访问操作，这里拿方法重新包装了一层
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
/**
* 返回 ArrayList 中指定位置的元素
* 如果指定的 index 位置超过了实际存储的元素数
* 那么则会抛出 IndexOutOfBoundsException 异常
*/
public E get(int index) {
rangeCheck(index);// 校验了传递的角标是否大于了实际存储的元素数
return elementData(index);
}
/**
* 用指定的新元素替换指定位置上的旧元素，并返回旧元素
* 如果传递的指定位置超过了元素数则会抛出角标越界异常
*/
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/**
* 将指定的元素添加到 ArrayList 末尾
*/
public boolean add(E e) {
// 如果数组容量不够，则进行扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;// 赋值
return true;
}
/**
* 在 ArrayList 指定的位置插入指定的元素
* 将当前位于该位置的元素（如果有）和随后的任何元素移动到右侧
* 位移是通过 System.arraycopy() 方法来实现的
* 将原数组的指定位置到后面的元素复制到从指定元素 + 1 到元素长度位置
* 以此成功实现位移
*/
public void add(int index, E element) {
rangeCheckForAdd(index);// 校验是否越界
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
/**
* 删除 ArrayList 中指定位置的元素
* 移除后会将任何后续元素移动到左侧
*/
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; // 让 GC 工作起来
return oldValue;
}
/**
* 从 ArrayList 中删除第一个出现的指定元素，删除成功返回 true，否则返回 false
*/
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;
}
/*
* 专用删除方法，不校验角标越界，并且不返回删除的值
*/
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 ArrayList 中删除所有元素
*/
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)// 遍历数组中有元素的部分，然后将元素置为 null，明确让 GC 回收
elementData[i] = null;
size = 0;
}
/**
* 按照指定集合迭代器返回的顺序，追加指定集合内所有的元素到 ArrayList 的末尾
* 追加成功返回 treu，否则返回 false。如果指定集合在操作进行中被修改，则此操作的行为是为定义的
* （这意味着如果指定的集合是当前的 ArrayList，并且此列表非空，则此调用的行为是未定义的）
*/
public boolean addAll(Collection c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // 根据指定集合的大小进行数组扩容
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
/**
* 从指定位置开始，将指定集合中的所有元素插入到此 ArrayList
*/
public boolean addAll(int index, Collection c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)// 先将原有元素往后进行移动
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
// 然后再将指定集合的元素复制到当前数组
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
/**
* 删除 fromIndex 和 toIndex 之间所有的元素（包括 toIndex）
* 移除后，会将后面的元素移到左边，如果 fromIndex 和 toIndex 相等，则不会有效果
*/
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;
}
/**
* 校验给定的索引是否在范围内，如果不在则抛出 IndexOutOfBoundException 异常
* 该方法并不校验索引是否是负数，它始终是在访问数组之前调用，如果索引为负数则抛出
* ArrayIndexOutOfBoundsException 异常
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* 由 add 和 addAll 使用的 rangeCheck 版本
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* 构造 IndexOutOfBoundsException 异常的详细错误信息
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
/**
* 从此 ArrayList 中删除指定集合中的所有元素
*/
public boolean removeAll(Collection c) {
Objects.requireNonNull(c);// 校验指定集合是否为 null
return batchRemove(c, false);
}
/**
* 仅保留此 ArrayList 中包含指定集合中的元素
* 换句话说，从此 ArrayList 中删除不包含在指定集合中的所有元素
*/
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;
}
/**
* 将 ArrayList 实例的状态保存到流中（即序列化它）
*/
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 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 listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
/**
* 返回集合中的集合迭代器（按适当的顺序）
*/
public ListIterator listIterator() {
return new ListItr(0);
}
/**
* 以适当的顺序返回该集合中的元素的迭代器
*/
public Iterator iterator() {
return new Itr();
}
/**
* AbstractList.Itr 的优化版本
*/
private class Itr implements Iterator {
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 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();
}
}
/**
* AbstractList.ListItr 的优化版本
*/
private class ListItr extends Itr implements ListIterator {
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();
}
}
}
/**
* 返回此 ArrayList 中指定的元素，从 fromIndex 到 toIndex（包括 toIndex）
* 如果 fromIndex 和 toIndex 是相等的，那么返回的集合为空
* 返回的集合由此 ArrayList 支持，因此，返回的集合中的非结构性的更改将反映到此 ArrayList 中，反之亦然
*/
public List 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 implements RandomAccess {
private final AbstractList parent;
private final int parentOffset;
private final int offset;
int size;
SubList(AbstractList 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 c) {
return addAll(this.size, c);
}
public boolean addAll(int index, Collection 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 iterator() {
return listIterator();
}
public ListIterator listIterator(final int index) {
checkForComodification();
rangeCheckForAdd(index);
final int offset = this.offset;
return new ListIterator() {
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 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 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 spliterator() {
checkForComodification();
return new ArrayListSpliterator(ArrayList.this, offset,
offset + this.size, this.modCount);
}
}
@Override
public void forEach(Consumer 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 late-binding 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 spliterator() {
return new ArrayListSpliterator<>(this, 0, -1, 0);
}
/** Index-based split-by-two, lazily initialized Spliterator */
static final class ArrayListSpliterator implements Spliterator {
/*
* If ArrayLists were immutable, or structurally immutable (no
* adds, removes, etc), we could implement their spliterators
* with Arrays.spliterator. Instead we detect as much
* interference during traversal as practical without
* sacrificing much performance. We rely primarily on
* modCounts. These are not guaranteed to detect concurrency
* violations, and are sometimes overly conservative about
* within-thread interference, but detect enough problems to
* be worthwhile in practice. To carry this out, we (1) lazily
* initialize fence and expectedModCount until the latest
* point that we need to commit to the state we are checking
* against; thus improving precision. (This doesn't apply to
* SubLists, that create spliterators with current non-lazy
* values). (2) We perform only a single
* ConcurrentModificationException check at the end of forEach
* (the most performance-sensitive method). When using forEach
* (as opposed to iterators), we can normally only detect
* interference after actions, not before. Further
* CME-triggering checks apply to all other possible
* violations of assumptions for example null or too-small
* elementData array given its size(), that could only have
* occurred due to interference. This allows the inner loop
* of forEach to run without any further checks, and
* simplifies lambda-resolution. While this does entail a
* number of checks, note that in the common case of
* list.stream().forEach(a), no checks or other computation
* occur anywhere other than inside forEach itself. The other
* less-often-used methods cannot take advantage of most of
* these streamlinings.
*/
private final ArrayList 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 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 lst;
if ((hi = fence) < 0) {
if ((lst = list) == null)
hi = fence = 0;
else {
expectedModCount = lst.modCount;
hi = fence = lst.size;
}
}
return hi;
}
public ArrayListSpliterator trySplit() {
int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
return (lo >= mid) ? null : // divide range in half unless too small
new ArrayListSpliterator(list, lo, index = mid,
expectedModCount);
}
public boolean tryAdvance(Consumer 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 action) {
int i, hi, mc; // hoist accesses and checks from loop
ArrayList 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 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 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 c) {
final int expectedModCount = modCount;
Arrays.sort((E[]) elementData, 0, size, c);
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
}
总结
通过源码可以看到 ArrayList 内部其实是维护了两个数组的，一个是空数组，一个是带默认容量的数组，其默认容量为 10。 在进行集合操作的时候会进行判断，如果当数组满了，装不下新元素，那么就会进行扩容。扩容是根据原有的容量大小 + 原油容量大小的一半，比如原有的容量为 10，那么扩容后的容量就是：10 + (10 / 2) = 15 了。
另外我们还知道了，ArrayList 也是有最终容量的，其定义的是 Integer.MAX_VALUE - 8，这里减去的是数组自身需要存储元数据的空间大小。
好了，ArrayList 的分析就到这里了，下一篇再来分析分析其他的集合实现类。