本文最后更新于：2020年12月15日 中午
ArrayList探究
简介
- 实现List接口,继承自AbstractList
- 基于数组实现，内存连续
- 便利查找效率高（数组），但插入删除效率低（数组）
Tips
- 查找优先，或者不经常修改时候推荐使用
- 默认容量大小为10，1.5倍扩容
- Arrays.copyOf 和 System.arrayCopy
源码分析
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
//数组初始化大小
private static final int DEFAULT_CAPACITY = 10;
//空对象数组
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//对象数组
transient Object[] elementData; // non-private to simplify nested class access
//已使用大小，容量
private int size;
//根据参数，初始化ArrayList
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);
}
}
//无参初始化
public ArrayList() {
//默认空数组赋值给对象数组
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
//通过给定集合初始化
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
//防止转换失败，重新进行拷贝
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
//去除未使用的空部分，即重新拷贝数组
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
//确保容量足够，不够进行扩容
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;
//给定最小容量>最小扩张
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
//计算容量
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//空集合时候，取给定值和默认大小中较大的一个
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
//直接返回给定的最小容量
return minCapacity;
}
//内部方法，确保容量足够
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
//默认数组最大值
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//对数组进行扩容
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//扩容为之前大小1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//取扩容后的大小和给定值中较大的一个
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//超过最大值时候进行处理
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
//数组扩容拷贝
elementData = Arrays.copyOf(elementData, newCapacity);
}
//极值处理
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
//最大值仍为Integer.MaxValue
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
//获取其中元素（已经使用）的个数
public int size() {
return size;
}
//判断是否为空
public boolean isEmpty() {
return size == 0;
}
//判断是否包含給顶参数
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
public int indexOf(Object o) {
if (o == null) {
//若为空，则找到空对象的下标
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
//不为空，则返回相等的对象下标
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
//不存在则返回-1
return -1;
}
//获取对象最后一次出现的下标
public int lastIndexOf(Object o) {
//倒叙查找
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;
}
//对象的克隆，浅克隆
public Object clone() {
try {
ArrayList<?> v = (ArrayList<?>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
//修改次数置为0
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
}
//返回对象数组
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
//转换为数组
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
//长度小，Arrays.copyOf
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
//长度大System.arrayCopy
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
// 获取给定下标的值（内部方法）
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
public E get(int index) {
//边界校验
rangeCheck(index);
return elementData(index);
}
//设置给定下标的值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
//返回旧值
return oldValue;
}
//添加元素（尾部添加）
public boolean add(E e) {
// 确保空间足够，不够则进行扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
//指定位置插入元素
public void add(int index, E element) {
//边界值校验
rangeCheckForAdd(index);
//确保空间足够，不够则进行扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
//数组拷贝，从index下标开始 拷贝到从index+1下标开始
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
//index位置赋值
elementData[index] = element;
//已使用长度增加
size++;
}
//删除指定位置的元素
public E remove(int index) {
//位置校验
rangeCheck(index);
//修改次数自增
modCount++;
E oldValue = elementData(index);
//计算需要移动的值的数量（删除值后续往前移动）
int numMoved = size - index - 1;
if (numMoved > 0)
//数组拷贝,index+1开始的下标拷贝给index开始的下标
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//尾部元素置空
elementData[--size] = null; // clear to let GC do its work
//返回已删除的旧值
return oldValue;
}
//删除ArrayList中指定元素
public boolean remove(Object o) {
if (o == null) {
//为空时删除空元素
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
//不为空时,equals比较删除相同值元素
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
//未找到则删除失败，返回false
return false;
}
//快速删除方法
private void fastRemove(int index) {
//修改次数自增
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
//数组拷贝，index+1位置开始元素拷贝给index位置开始元素
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//尾部置空，如果是存放对象，此处不在引用，可以让gc有机会回收
elementData[--size] = null; // clear to let GC do its work
}
//清空当前ArrayList
public void clear() {
//修改次数自增
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
//存值部分逐一置为null
elementData[i] = null;
//使用个数置为0
size = 0;
}
//添加整个集合，本质是数组的拷贝
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;
}
//从指定位置开始，添加整个集合
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
//转换为对象数组
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
//进行对象数组拷贝
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
//删除指定区间内的数据
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
//计算出需要拷贝的长度
int numMoved = size - toIndex;
//进行数组拷贝
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
//计算出新的已使用长度
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
//对于已经删除的元素，进行置空处理
elementData[i] = null;
}
size = newSize;
}
//边界值校验
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//新增前边界值校验
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//返回越界信息
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
//批量移除
public boolean removeAll(Collection<?> c) {
//判断是否为空
Objects.requireNonNull(c);
return batchRemove(c, false);
}
//取交集方法
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
//批量移除方法
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)
//根据传值，去交集是两者都存在，直接存入数组（取交集）；其他情况不存在才保存（删除）
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
//从r开始进行进行数组拷贝
System.arraycopy(elementData, r,
elementData, w,
size - r);
//重新计算长度
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
//后续置空
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
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();
}
}
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
int capacity = calculateCapacity(elementData, size);
SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, 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();
}
}
}
//返回迭代器
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
//默认返回从零开始的迭代器
public ListIterator<E> listIterator() {
return new ListItr(0);
}
public Iterator<E> iterator() {
return new Itr();
}
//内部类 实现迭代器接口
private class Itr implements Iterator<E> {
//指向的下个位置
int cursor; // index of next element to return
//末位
int lastRet = -1; // index of last element returned; -1 if no such
//修改次数
int expectedModCount = modCount;
Itr() {}
//是否还有下个节点
public boolean hasNext() {
return cursor != size;
}
//返回指向的下个节点
@SuppressWarnings("unchecked")
public E next() {
//校验修改次数（b笨笨好）
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
//当前指向下一个节点
cursor = i + 1;
//返回当前节点尾部
return (E) elementData[lastRet = i];
}
//删除节点
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
//比较修改次数
checkForComodification();
try {
//删除当前节点
ArrayList.this.remove(lastRet);
//当前指向已删除节点
cursor = lastRet;
//尾部为-1
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
//通过迭代器形式的遍历
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
//边界以及空值遍历
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
//获取当前数组不可变
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.
*
* <p>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.
*
* <p>The semantics of the list returned by this method become undefined if
* the backing list (i.e., this list) is <i>structurally modified</i> 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 <em><a href="Spliterator.html#binding">late-binding</a></em>
* and <em>fail-fast</em> {@link Spliterator} over the elements in this
* list.
*
* <p>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> {
/*
* 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<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++;
}
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