public CopyOnWriteArrayList() 
{
        setArray(new Object[0]);
}

Creates an empty list.

 public CopyOnWriteArrayList(Collection<? extends E> c) 
{
        Object[] elements = c.toArray();
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elements.getClass() != Object[].class)
            elements = Arrays.copyOf(elements, elements.length, Object[].class);
        setArray(elements);
}

Creates a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

Parameters:

c - the collection of initially held elements

Throws:

NullPointerException - if the specified collection is null

 public CopyOnWriteArrayList(E[] toCopyIn) 
{
        setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));
}

Creates a list holding a copy of the given array.

Parameters:

toCopyIn - the array (a copy of this array is used as the internal array)

Throws:

NullPointerException - if the specified array is null

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

Appends the specified element to the end of this list.

 public  void add(int index,
    E element) 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (index  > len || index <  0)
                throw new IndexOutOfBoundsException("Index: "+index+
                                                    ", Size: "+len);
            Object[] newElements;
            int numMoved = len - index;
            if (numMoved == 0)
                newElements = Arrays.copyOf(elements, len + 1);
            else {
                newElements = new Object[len + 1];
                System.arraycopy(elements, 0, newElements, 0, index);
                System.arraycopy(elements, index, newElements, index + 1,
                                 numMoved);
            }
            newElements[index] = element;
            setArray(newElements);
        } finally {
            lock.unlock();
        }
}

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).

 public boolean addAll(Collection<? extends E> c) 
{
        Object[] cs = c.toArray();
        if (cs.length == 0)
            return false;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + cs.length);
            System.arraycopy(cs, 0, newElements, len, cs.length);
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
}

Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's iterator.

 public boolean addAll(int index,
    Collection<? extends E> c) 
{
        Object[] cs = c.toArray();
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (index  > len || index <  0)
                throw new IndexOutOfBoundsException("Index: "+index+
                                                    ", Size: "+len);
            if (cs.length == 0)
                return false;
            int numMoved = len - index;
            Object[] newElements;
            if (numMoved == 0)
                newElements = Arrays.copyOf(elements, len + cs.length);
            else {
                newElements = new Object[len + cs.length];
                System.arraycopy(elements, 0, newElements, 0, index);
                System.arraycopy(elements, index,
                                 newElements, index + cs.length,
                                 numMoved);
            }
            System.arraycopy(cs, 0, newElements, index, cs.length);
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
}

Inserts all of the elements in the specified collection into this list, starting at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in this list in the order that they are returned by the specified collection's iterator.

 public int addAllAbsent(Collection<? extends E> c) 
{
        Object[] cs = c.toArray();
        if (cs.length == 0)
            return 0;
        Object[] uniq = new Object[cs.length];
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            int added = 0;
            for (int i = 0; i <  cs.length; ++i) { // scan for duplicates
                Object e = cs[i];
                if (indexOf(e, elements, 0, len) <  0 &&
                    indexOf(e, uniq, 0, added) <  0)
                    uniq[added++] = e;
            }
            if (added  > 0) {
                Object[] newElements = Arrays.copyOf(elements, len + added);
                System.arraycopy(uniq, 0, newElements, len, added);
                setArray(newElements);
            }
            return added;
        } finally {
            lock.unlock();
        }
}

Appends all of the elements in the specified collection that are not already contained in this list, to the end of this list, in the order that they are returned by the specified collection's iterator.

 public boolean addIfAbsent(E e) 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // Copy while checking if already present.
            // This wins in the most common case where it is not present
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = new Object[len + 1];
            for (int i = 0; i <  len; ++i) {
                if (eq(e, elements[i]))
                    return false; // exit, throwing away copy
                else
                    newElements[i] = elements[i];
            }
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
}

Append the element if not present.

 public  void clear() 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            setArray(new Object[0]);
        } finally {
            lock.unlock();
        }
}

Removes all of the elements from this list. The list will be empty after this call returns.

 public Object clone() 
{
        try {
            CopyOnWriteArrayList c = (CopyOnWriteArrayList)(super.clone());
            c.resetLock();
            return c;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError();
        }
}

Returns a shallow copy of this list. (The elements themselves are not copied.)

 public boolean contains(Object o) 
{
        Object[] elements = getArray();
        return indexOf(o, elements, 0, elements.length)  >= 0;
}

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

 public boolean containsAll(Collection<?> c) 
{
        Object[] elements = getArray();
        int len = elements.length;
        for (Object e : c) {
            if (indexOf(e, elements, 0, len) <  0)
                return false;
        }
        return true;
}

Returns true if this list contains all of the elements of the specified collection.

 public boolean equals(Object o) 
{
        if (o == this)
            return true;
        if (!(o instanceof List))
            return false;
        List< ? > list = (List< ? >)(o);
        Iterator< ? > it = list.iterator();
        Object[] elements = getArray();
        int len = elements.length;
        for (int i = 0; i <  len; ++i)
            if (!it.hasNext() || !eq(elements[i], it.next()))
                return false;
        if (it.hasNext())
            return false;
        return true;
}

Compares the specified object with this list for equality. Returns {@code true} if the specified object is the same object as this object, or if it is also a List and the sequence of elements returned by an {@linkplain List#iterator() iterator} over the specified list is the same as the sequence returned by an iterator over this list. The two sequences are considered to be the same if they have the same length and corresponding elements at the same position in the sequence are equal. Two elements {@code e1} and {@code e2} are considered equal if {@code (e1==null ? e2==null : e1.equals(e2))}.

 public E get(int index) 
{
        return get(getArray(), index);
}

{@inheritDoc}

 final Object[] getArray() 
{
        return array;
}

Gets the array. Non-private so as to also be accessible from CopyOnWriteArraySet class.

 public int hashCode() 
{
        int hashCode = 1;
        Object[] elements = getArray();
        int len = elements.length;
        for (int i = 0; i <  len; ++i) {
            Object obj = elements[i];
            hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
        }
        return hashCode;
}

Returns the hash code value for this list.

This implementation uses the definition in List#hashCode .

 public int indexOf(Object o) 
{
        Object[] elements = getArray();
        return indexOf(o, elements, 0, elements.length);
}

{@inheritDoc}

 public int indexOf(E e,
    int index) 
{
        Object[] elements = getArray();
        return indexOf(e, elements, index, elements.length);
}

Returns the index of the first occurrence of the specified element in this list, searching forwards from index, or returns -1 if the element is not found. More formally, returns the lowest index i such that (i >= index && (e==null ? get(i)==null : e.equals(get(i)))), or -1 if there is no such index.

 public boolean isEmpty() 
{
        return size() == 0;
}

Returns true if this list contains no elements.

 public Iterator<E> iterator() 
{
        return new COWIterator< E >(getArray(), 0);
}

Returns an iterator over the elements in this list in proper sequence.

The returned iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove method.

 public int lastIndexOf(Object o) 
{
        Object[] elements = getArray();
        return lastIndexOf(o, elements, elements.length - 1);
}

{@inheritDoc}

 public int lastIndexOf(E e,
    int index) 
{
        Object[] elements = getArray();
        return lastIndexOf(e, elements, index);
}

Returns the index of the last occurrence of the specified element in this list, searching backwards from index, or returns -1 if the element is not found. More formally, returns the highest index i such that (i <= index && (e==null ? get(i)==null : e.equals(get(i)))), or -1 if there is no such index.

 public ListIterator<E> listIterator() 
{
        return new COWIterator< E >(getArray(), 0);
}

{@inheritDoc}

The returned iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove, set or add methods.

 public ListIterator<E> listIterator(int index) 
{
        Object[] elements = getArray();
        int len = elements.length;
        if (index< 0 || index >len)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new COWIterator< E >(elements, index);
}

{@inheritDoc}

The returned iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove, set or add methods.

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

Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices). Returns the element that was removed from the list.

 public boolean remove(Object o) 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (len != 0) {
                // Copy while searching for element to remove
                // This wins in the normal case of element being present
                int newlen = len - 1;
                Object[] newElements = new Object[newlen];
                for (int i = 0; i <  newlen; ++i) {
                    if (eq(o, elements[i])) {
                        // found one;  copy remaining and exit
                        for (int k = i + 1; k <  len; ++k)
                            newElements[k-1] = elements[k];
                        setArray(newElements);
                        return true;
                    } else
                        newElements[i] = elements[i];
                }
                // special handling for last cell
                if (eq(o, elements[newlen])) {
                    setArray(newElements);
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
}

Removes the first occurrence of the specified element from this list, if it is present. If this list does not contain the element, it is unchanged. More formally, removes the element with the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))) (if such an element exists). Returns true if this list contained the specified element (or equivalently, if this list changed as a result of the call).

 public boolean removeAll(Collection<?> c) 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (len != 0) {
                // temp array holds those elements we know we want to keep
                int newlen = 0;
                Object[] temp = new Object[len];
                for (int i = 0; i <  len; ++i) {
                    Object element = elements[i];
                    if (!c.contains(element))
                        temp[newlen++] = element;
                }
                if (newlen != len) {
                    setArray(Arrays.copyOf(temp, newlen));
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
}

Removes from this list all of its elements that are contained in the specified collection. This is a particularly expensive operation in this class because of the need for an internal temporary array.

 public boolean retainAll(Collection<?> c) 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (len != 0) {
                // temp array holds those elements we know we want to keep
                int newlen = 0;
                Object[] temp = new Object[len];
                for (int i = 0; i <  len; ++i) {
                    Object element = elements[i];
                    if (c.contains(element))
                        temp[newlen++] = element;
                }
                if (newlen != len) {
                    setArray(Arrays.copyOf(temp, newlen));
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
}

Retains only the elements in this list that are contained in the specified collection. In other words, removes from this list all of its elements that are not contained in the specified collection.

 public E set(int index,
    E element) 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            E oldValue = get(elements, index);
            if (oldValue != element) {
                int len = elements.length;
                Object[] newElements = Arrays.copyOf(elements, len);
                newElements[index] = element;
                setArray(newElements);
            } else {
                // Not quite a no-op; ensures volatile write semantics
                setArray(elements);
            }
            return oldValue;
        } finally {
            lock.unlock();
        }
}

Replaces the element at the specified position in this list with the specified element.

 final  void setArray(Object[] a) 
{
        array = a;
}

Sets the array.

 public int size() 
{
        return getArray().length;
}

Returns the number of elements in this list.

 public List<E> subList(int fromIndex,
    int toIndex) 
{
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (fromIndex <  0 || toIndex  > len || fromIndex  > toIndex)
                throw new IndexOutOfBoundsException();
            return new COWSubList< E >(this, fromIndex, toIndex);
        } finally {
            lock.unlock();
        }
}

Returns a view of the portion of this list between fromIndex, inclusive, and toIndex, exclusive. The returned list is backed by this list, so changes in the returned list are reflected in this list.

The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is modified in any way other than via the returned list.

 public Object[] toArray() 
{
        Object[] elements = getArray();
        return Arrays.copyOf(elements, elements.length);
}

Returns an array containing all of the elements in this list in proper sequence (from first to last element).

The returned array will be "safe" in that no references to it are maintained by this list. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

 public T[] toArray(T[] a) 
{
        Object[] elements = getArray();
        int len = elements.length;
        if (a.length <  len)
            return (T[]) Arrays.copyOf(elements, len, a.getClass());
        else {
            System.arraycopy(elements, 0, a, 0, len);
            if (a.length  > len)
                a[len] = null;
            return a;
        }
}

Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list.

If this list fits in the specified array with room to spare (i.e., the array has more elements than this list), the element in the array immediately following the end of the list is set to null. (This is useful in determining the length of this list only if the caller knows that this list does not contain any null elements.)

Like the #toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a list known to contain only strings. The following code can be used to dump the list into a newly allocated array of String:

    String[] y = x.toArray(new String[0]);

Note that toArray(new Object[0]) is identical in function to toArray().

 public String toString() 
{
        return Arrays.toString(getArray());
}

Returns a string representation of this list. The string representation consists of the string representations of the list's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String#valueOf(Object) .