java.util.concurrent

## Class CopyOnWriteArrayList<E>

- java.lang.Object
- - java.util.concurrent.CopyOnWriteArrayList<E>

- Type Parameters:
    `E` - the type of elements held in this collection
  All Implemented Interfaces:
    Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess

      public class CopyOnWriteArrayList<E>
      extends Object
      implements List<E>, RandomAccess, Cloneable, Serializable

  A thread-safe variant of `ArrayList` in which all mutative operations (`add`, `set`, and so on) are implemented by making a fresh copy of the underlying array.

  This is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads. The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw `ConcurrentModificationException`. The iterator will not reflect additions, removals, or changes to the list since the iterator was created. Element-changing operations on iterators themselves (`remove`, `set`, and `add`) are not supported. These methods throw `UnsupportedOperationException`.

  All elements are permitted, including `null`.

  Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a `CopyOnWriteArrayList` happen-before actions subsequent to the access or removal of that element from the `CopyOnWriteArrayList` in another thread.

  This class is a member of the Java Collections Framework.

  Since:
    1.5
  Author:
    Doug Lea
  See Also:
    Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `CopyOnWriteArrayList()` Creates an empty list.
    `CopyOnWriteArrayList(Collection<? extends E> c)` Creates a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
    `CopyOnWriteArrayList(E[] toCopyIn)` Creates a list holding a copy of the given array.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `add(E e)` Appends the specified element to the end of this list.
    `void` | `add(int index, E element)` Inserts the specified element at the specified position in this list.
    `boolean` | `addAll(Collection<? extends E> c)` 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.
    `boolean` | `addAll(int index, Collection<? extends E> c)` Inserts all of the elements in the specified collection into this list, starting at the specified position.
    `int` | `addAllAbsent(Collection<? extends E> c)` 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.
    `boolean` | `addIfAbsent(E e)` Append the element if not present.
    `void` | `clear()` Removes all of the elements from this list.
    `Object` | `clone()` Returns a shallow copy of this list.
    `boolean` | `contains(Object o)` Returns `true` if this list contains the specified element.
    `boolean` | `containsAll(Collection<?> c)` Returns `true` if this list contains all of the elements of the specified collection.
    `boolean` | `equals(Object o)` Compares the specified object with this list for equality.
    `E` | `get(int index)` Returns the element at the specified position in this list.
    `int` | `hashCode()` Returns the hash code value for this list.
    `int` | `indexOf(E e, int index)` 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.
    `int` | `indexOf(Object o)` Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
    `boolean` | `isEmpty()` Returns `true` if this list contains no elements.
    `Iterator<E>` | `iterator()` Returns an iterator over the elements in this list in proper sequence.
    `int` | `lastIndexOf(E e, int 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.
    `int` | `lastIndexOf(Object o)` Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
    `ListIterator<E>` | `listIterator()` Returns a list iterator over the elements in this list (in proper sequence).
    `ListIterator<E>` | `listIterator(int index)` Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.
    `E` | `remove(int index)` Removes the element at the specified position in this list.
    `boolean` | `remove(Object o)` Removes the first occurrence of the specified element from this list, if it is present.
    `boolean` | `removeAll(Collection<?> c)` Removes from this list all of its elements that are contained in the specified collection.
    `boolean` | `retainAll(Collection<?> c)` Retains only the elements in this list that are contained in the specified collection.
    `E` | `set(int index, E element)` Replaces the element at the specified position in this list with the specified element.
    `int` | `size()` Returns the number of elements in this list.
    `List<E>` | `subList(int fromIndex, int toIndex)` Returns a view of the portion of this list between `fromIndex`, inclusive, and `toIndex`, exclusive.
    `Object[]` | `toArray()` Returns an array containing all of the elements in this list in proper sequence (from first to last element).
    `<T> T[]` | `toArray(T[] 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.
    `String` | `toString()` Returns a string representation of this list.

    - ### Methods inherited from class java.lang.Object

      `finalize, getClass, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.List

      `replaceAll, sort, spliterator`

    - ### Methods inherited from interface java.util.Collection

      `parallelStream, removeIf, stream`

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### Constructor Detail

    - #### CopyOnWriteArrayList

          public CopyOnWriteArrayList()

      Creates an empty list.

    - #### CopyOnWriteArrayList

          public CopyOnWriteArrayList(Collection<? extends E> c)

      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

    - #### CopyOnWriteArrayList

          public CopyOnWriteArrayList(E[] toCopyIn)

      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

  - ### Method Detail

    - #### size

          public int size()

      Returns the number of elements in this list.
      Specified by:
        `size` in interface `Collection<E>`
      Specified by:
        `size` in interface `List<E>`
      Returns:
        the number of elements in this list

    - #### isEmpty

          public boolean isEmpty()

      Returns `true` if this list contains no elements.
      Specified by:
        `isEmpty` in interface `Collection<E>`
      Specified by:
        `isEmpty` in interface `List<E>`
      Returns:
        `true` if this list contains no elements

    - #### contains

          public boolean contains(Object o)

      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))`.
      Specified by:
        `contains` in interface `Collection<E>`
      Specified by:
        `contains` in interface `List<E>`
      Parameters:
        `o` - element whose presence in this list is to be tested
      Returns:
        `true` if this list contains the specified element

    - #### indexOf

          public int indexOf(Object o)

      Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index `i` such that `(o==null ? get(i)==null : o.equals(get(i)))`, or -1 if there is no such index.
      Specified by:
        `indexOf` in interface `List<E>`
      Parameters:
        `o` - element to search for
      Returns:
        the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element

    - #### indexOf

          public int indexOf(E e,
                             int index)

      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.
      Parameters:
        `e` - element to search for
        `index` - index to start searching from
      Returns:
        the index of the first occurrence of the element in this list at position `index` or later in the list; `-1` if the element is not found.
      Throws:
        `IndexOutOfBoundsException` - if the specified index is negative

    - #### lastIndexOf

          public int lastIndexOf(Object o)

      Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the highest index `i` such that `(o==null ? get(i)==null : o.equals(get(i)))`, or -1 if there is no such index.
      Specified by:
        `lastIndexOf` in interface `List<E>`
      Parameters:
        `o` - element to search for
      Returns:
        the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element

    - #### lastIndexOf

          public int lastIndexOf(E e,
                                 int 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.
      Parameters:
        `e` - element to search for
        `index` - index to start searching backwards from
      Returns:
        the index of the last occurrence of the element at position less than or equal to `index` in this list; -1 if the element is not found.
      Throws:
        `IndexOutOfBoundsException` - if the specified index is greater than or equal to the current size of this list

    - #### clone

          public Object clone()

      Returns a shallow copy of this list. (The elements themselves are not copied.)
      Overrides:
        `clone` in class `Object`
      Returns:
        a clone of this list
      See Also:
        `Cloneable`

    - #### toArray

          public Object[] toArray()

      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.

      Specified by:
        `toArray` in interface `Collection<E>`
      Specified by:
        `toArray` in interface `List<E>`
      Returns:
        an array containing all the elements in this list
      See Also:
        `Arrays.asList(Object[])`

    - #### toArray

          public <T> T[] toArray(T[] 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()`.
      Specified by:
        `toArray` in interface `Collection<E>`
      Specified by:
        `toArray` in interface `List<E>`
      Type Parameters:
        `T` - the runtime type of the array to contain the collection
      Parameters:
        `a` - the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
      Returns:
        an array containing all the elements in this list
      Throws:
        `ArrayStoreException` - if the runtime type of the specified array is not a supertype of the runtime type of every element in this list
        `NullPointerException` - if the specified array is null

    - #### get

          public E get(int index)

      Returns the element at the specified position in this list.
      Specified by:
        `get` in interface `List<E>`
      Parameters:
        `index` - index of the element to return
      Returns:
        the element at the specified position in this list
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index >= size()`)

    - #### set

          public E set(int index,
                       E element)

      Replaces the element at the specified position in this list with the specified element.
      Specified by:
        `set` in interface `List<E>`
      Parameters:
        `index` - index of the element to replace
        `element` - element to be stored at the specified position
      Returns:
        the element previously at the specified position
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index >= size()`)

    - #### add

          public boolean add(E e)

      Appends the specified element to the end of this list.
      Specified by:
        `add` in interface `Collection<E>`
      Specified by:
        `add` in interface `List<E>`
      Parameters:
        `e` - element to be appended to this list
      Returns:
        `true` (as specified by `Collection.add(E)`)

    - #### add

          public void add(int index,
                          E element)

      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).
      Specified by:
        `add` in interface `List<E>`
      Parameters:
        `index` - index at which the specified element is to be inserted
        `element` - element to be inserted
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)

    - #### remove

          public E remove(int index)

      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.
      Specified by:
        `remove` in interface `List<E>`
      Parameters:
        `index` - the index of the element to be removed
      Returns:
        the element previously at the specified position
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index >= size()`)

    - #### remove

          public boolean remove(Object o)

      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).
      Specified by:
        `remove` in interface `Collection<E>`
      Specified by:
        `remove` in interface `List<E>`
      Parameters:
        `o` - element to be removed from this list, if present
      Returns:
        `true` if this list contained the specified element

    - #### addIfAbsent

          public boolean addIfAbsent(E e)

      Append the element if not present.
      Parameters:
        `e` - element to be added to this list, if absent
      Returns:
        `true` if the element was added

    - #### containsAll

          public boolean containsAll(Collection<?> c)

      Returns `true` if this list contains all of the elements of the specified collection.
      Specified by:
        `containsAll` in interface `Collection<E>`
      Specified by:
        `containsAll` in interface `List<E>`
      Parameters:
        `c` - collection to be checked for containment in this list
      Returns:
        `true` if this list contains all of the elements of the specified collection
      Throws:
        `NullPointerException` - if the specified collection is null
      See Also:
        `contains(Object)`

    - #### removeAll

          public boolean removeAll(Collection<?> c)

      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.
      Specified by:
        `removeAll` in interface `Collection<E>`
      Specified by:
        `removeAll` in interface `List<E>`
      Parameters:
        `c` - collection containing elements to be removed from this list
      Returns:
        `true` if this list changed as a result of the call
      Throws:
        `ClassCastException` - if the class of an element of this list is incompatible with the specified collection (optional)
        `NullPointerException` - if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:
        `remove(Object)`

    - #### retainAll

          public boolean retainAll(Collection<?> c)

      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.
      Specified by:
        `retainAll` in interface `Collection<E>`
      Specified by:
        `retainAll` in interface `List<E>`
      Parameters:
        `c` - collection containing elements to be retained in this list
      Returns:
        `true` if this list changed as a result of the call
      Throws:
        `ClassCastException` - if the class of an element of this list is incompatible with the specified collection (optional)
        `NullPointerException` - if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:
        `remove(Object)`

    - #### addAllAbsent

          public int addAllAbsent(Collection<? extends E> c)

      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.
      Parameters:
        `c` - collection containing elements to be added to this list
      Returns:
        the number of elements added
      Throws:
        `NullPointerException` - if the specified collection is null
      See Also:
        `addIfAbsent(Object)`

    - #### clear

          public void clear()

      Removes all of the elements from this list. The list will be empty after this call returns.
      Specified by:
        `clear` in interface `Collection<E>`
      Specified by:
        `clear` in interface `List<E>`

    - #### addAll

          public boolean addAll(Collection<? extends E> c)

      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.
      Specified by:
        `addAll` in interface `Collection<E>`
      Specified by:
        `addAll` in interface `List<E>`
      Parameters:
        `c` - collection containing elements to be added to this list
      Returns:
        `true` if this list changed as a result of the call
      Throws:
        `NullPointerException` - if the specified collection is null
      See Also:
        `add(Object)`

    - #### addAll

          public boolean addAll(int index,
                                Collection<? extends E> c)

      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.
      Specified by:
        `addAll` in interface `List<E>`
      Parameters:
        `index` - index at which to insert the first element from the specified collection
        `c` - collection containing elements to be added to this list
      Returns:
        `true` if this list changed as a result of the call
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)
        `NullPointerException` - if the specified collection is null
      See Also:
        `add(int,Object)`

    - #### toString

          public String toString()

      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)`.
      Overrides:
        `toString` in class `Object`
      Returns:
        a string representation of this list

    - #### equals

          public boolean equals(Object o)

      Compares the specified object with this list for equality. Returns `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 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 `e1` and `e2` are considered equal if `(e1==null ? e2==null : e1.equals(e2))`.
      Specified by:
        `equals` in interface `Collection<E>`
      Specified by:
        `equals` in interface `List<E>`
      Overrides:
        `equals` in class `Object`
      Parameters:
        `o` - the object to be compared for equality with this list
      Returns:
        `true` if the specified object is equal to this list
      See Also:
        `Object.hashCode()`, `HashMap`

    - #### hashCode

          public int hashCode()

      Returns the hash code value for this list.

      This implementation uses the definition in `List.hashCode()`.

      Specified by:
        `hashCode` in interface `Collection<E>`
      Specified by:
        `hashCode` in interface `List<E>`
      Overrides:
        `hashCode` in class `Object`
      Returns:
        the hash code value for this list
      See Also:
        `Object.equals(java.lang.Object)`, `System.identityHashCode(java.lang.Object)`

    - #### iterator

          public Iterator<E> iterator()

      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.

      Specified by:
        `iterator` in interface `Iterable<E>`
      Specified by:
        `iterator` in interface `Collection<E>`
      Specified by:
        `iterator` in interface `List<E>`
      Returns:
        an iterator over the elements in this list in proper sequence

    - #### listIterator

          public ListIterator<E> listIterator()

      Returns a list 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`, `set` or `add` methods.

      Specified by:
        `listIterator` in interface `List<E>`
      Returns:
        a list iterator over the elements in this list (in proper sequence)

    - #### listIterator

          public ListIterator<E> listIterator(int index)

      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 `next`. An initial call to `previous` would return the element with the specified index minus one.

      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.

      Specified by:
        `listIterator` in interface `List<E>`
      Parameters:
        `index` - index of the first element to be returned from the list iterator (by a call to `next`)
      Returns:
        a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)

    - #### subList

          public List<E> subList(int fromIndex,
                                 int toIndex)

      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.

      Specified by:
        `subList` in interface `List<E>`
      Parameters:
        `fromIndex` - low endpoint (inclusive) of the subList
        `toIndex` - high endpoint (exclusive) of the subList
      Returns:
        a view of the specified range within this list
      Throws:
        `IndexOutOfBoundsException` - for an illegal endpoint index value (`fromIndex < 0 || toIndex > size || fromIndex > toIndex`)

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