ArrayList源码解析
Posted 夜宿山寺
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这个是基于jdk1.8的一个解析，这是一个可以放可重复元素的一个集合。其实为什么说ArrayList在添加或者删除上的性能比LinkedList慢，因为在添加或者删除的时候会让数组进行一个拷贝操作，特别是删除的时候，需要对数组进行一个复制或者移动，但还好都是用java的本地方法进行的，在效率上比较高，还要提到的就是另外一个集合Vector，这是一个线程安全的ArrayList，在实现上并没有多大差别，只是在方法上面添加了一个 synchronized
package java.util;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
//ArrayList实现机制，他实现了List，Cloneable，可通过clone方法拷贝，Serializable可序列化，RandomAccess，可随机访问
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
private static final long serialVersionUID = 8683452581122892189L;
//设置默认的容量
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = ;
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
* DEFAULT_CAPACITY when the first element is added.
*/
//底层数据其实是采用Object来存储的，并不是通过泛型
transient Object[] elementData; // non-private to simplify nested class access
//list数据量记录
private int size;
//指定容量初始化，如果知道数据量的大小，基于优化最好能够指定容量大小，减少数据的拷贝
public ArrayList(int initialCapacity)
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList()
super();
this.elementData = EMPTY_ELEMENTDATA;
//初始化的时候直接拷贝集合数据
public ArrayList(Collection<? extends E> c)
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
// 将此 ArrayList 实例的容量调整为列表的当前大小,因为ArrayList在分配空间的时候会分配多余的空间，调用这个方法可以使那些多余的空间释放掉
public void trimToSize()
modCount++;
if (size < elementData.length)
elementData = Arrays.copyOf(elementData, size);
/**
* Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
//扩大arrayList容量
public void ensureCapacity(int minCapacity)
//判断该ArrayList是否为空没有使用过
int minExpand = (elementData != EMPTY_ELEMENTDATA)
// any size if real element table
? 0
// larger than default for empty table. It's already supposed to be
// at default size.
: DEFAULT_CAPACITY;
//当扩展容量大于最小扩展容量的时候进行扩展
if (minCapacity > minExpand)
ensureExplicitCapacity(minCapacity);
private void ensureCapacityInternal(int minCapacity)
if (elementData == EMPTY_ELEMENTDATA)
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
ensureExplicitCapacity(minCapacity);
private void ensureExplicitCapacity(int minCapacity)
modCount++;
//扩展的容量必须大于当前元素的长度
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//实现list容量的扩充
private void grow(int minCapacity)
// overflow-conscious code
//首先获取到原数组的长度
int oldCapacity = elementData.length;
//然后将原数组的长度+原数组长度除以2
int newCapacity = oldCapacity + (oldCapacity >> 1);
//判断新容量的长度是否小于最小容量
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//判断新容量是否超过了最大容量
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
//扩从的时候实现一个新数组的拷贝
elementData = Arrays.copyOf(elementData, newCapacity);
private static int hugeCapacity(int minCapacity)
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
public int size()
return size;
public boolean isEmpty()
return size == 0;
//是否包含这个元素
public boolean contains(Object o)
return indexOf(o) >= 0;
//底层实现是采用了equals方法，所以使用这个方法最好复写方法的equals方法
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;
//和上面的方法差不多，只不过这个是返回倒叙的一个下标
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;
//复写Object的clone方法实现深克隆
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);
//这里返回的是一个安全的数组，不跟原来数组无关的一个地址
public Object[] toArray()
return Arrays.copyOf(elementData, size);
@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
//这是一个不安全的访问
@SuppressWarnings("unchecked")
E elementData(int index)
return (E) elementData[index];
//这是一个安全的访问
public E get(int index)
rangeCheck(index);
return elementData(index);
//设置相应的index的值
public E set(int index, E element)
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
//添加元素
public boolean add(E e)
//增加容量当容量不够的时候会自动复制数据到另外的一个集合
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
/**
* Inserts the specified element at the specified position in this
* list. Shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException @inheritDoc
*/
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++;
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
return oldValue;
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;
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
public void clear()
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
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;
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;
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;
/**
* Checks if the given index is in range. If not, throws an appropriate
* runtime exception. This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void rangeCheck(int index)
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
/**
* A version of rangeCheck used by add and addAll.
*/
private void rangeCheckForAdd(int index)
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
/**
* Constructs an IndexOutOfBoundsException detail message.
* Of the many possible refactorings of the error handling code,
* this "outlining" performs best with both server and client VMs.
*/
private String outOfBoundsMsg(int index)
return "Index: "+index+", Size: "+size;
//移除该集合类包含传进来的集合里面的元素
public boolean removeAll(Collection<?> c)
Objects.requireNonNull(c);
return batchRemove(c, false);
//只保留传进来的集合里面的元素
public boolean retainAll(Collection<?> c)
Objects.requireNonNull(c);
return batchRemove(c, true);
//主要是为了retainAll removeAll提供具体的实现
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complement设置包含或者不包含
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();
/**
* Returns a list iterator over the elements in this list (in proper
* sequence), starting at the specified position in the list.
* The specified index indicates the first element that would be
* returned by an initial call to @link ListIterator#next next.
* An initial call to @link ListIterator#previous previous would
* return the element with the specified index minus one.
*
* <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @throws IndexOutOfBoundsException @inheritDoc
*/
public ListIterator<E> listIterator(int index)
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
/**
* Returns a list iterator over the elements in this list (in proper
* sequence).
*
* <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @see #listIterator(int)
*/
public ListIterator<E> listIterator()
return new ListItr(0);
/**
* Returns an iterator over the elements in this list in proper sequence.
*
* <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @return an iterator over the elements in this list in proper sequence
*/
public Iterator<E> iterator()
return new Itr();
///实现自己的迭代器
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;
//实现next方法，其实就是指定一个cursor然后让他不断的增加
@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.
*
* <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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