ArrayList implementation in Java source code

Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. [This class is roughly equivalent to Vector, except that it is unsynchronized.]

The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O[n] time. All of the other operations run in linear time [roughly speaking]. The constant factor is low compared to that for the LinkedList implementation.

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.

Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. [A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.] This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

• • finalize, getClass, notify, notifyAll, wait, wait, wait
  • containsAll, equals, hashCode

• • public ArrayList[int initialCapacity]

    Constructs an empty list with the specified initial capacity.

    Parameters: initialCapacity - the initial capacity of the list Throws: IllegalArgumentException - if the specified initial capacity is negative
  • public ArrayList[]

    Constructs an empty list with an initial capacity of ten.

  • public ArrayList[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 Specified by: retainAll in interface List Overrides: retainAll in class AbstractCollection 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: Collection.contains[Object]
  • public ListIterator 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 list iterator is fail-fast.

    Specified by: listIterator in interface List Overrides: listIterator in class AbstractList 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[]]
  • public List subList[int fromIndex, int toIndex]

    Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. [If fromIndex and 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.

    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:

    list.subList[from, to].clear[]; Similar idioms may be constructed for indexOf[Object] and lastIndexOf[Object], and all of the algorithms in the Collections class can be applied to a subList.

    The semantics of the list returned by this method become undefined if the backing list [i.e., this list] is structurally modified 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.]

    Specified by: subList in interface List Overrides: subList in class AbstractList 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 - if an endpoint index value is out of range [fromIndex < 0 || toIndex > size] IllegalArgumentException - if the endpoint indices are out of order [fromIndex > toIndex]
  • public void forEach[Consumer