public class ArrayList
extends AbstractList implements List , RandomAccess, Cloneable, SerializableResizable-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.synchronizedListmethod. This is best done at creation time, to prevent accidental unsynchronized access to the list:
List list = Collections.synchronizedList(new ArrayList(...));
The iterators returned by this class's
listIteratormethods 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
addmethods, 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
ConcurrentModificationExceptionon 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.
Constructors Constructor and Description
ArrayList()Constructs an empty list with an initial capacity of ten.
ArrayList(Collection extends E> c)Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
ArrayList(int initialCapacity)Constructs an empty list with the specified initial capacity.
Methods Modifier and Type Method and Description
add(E e)Appends the specified element to the end of this list.
add(int index, E element)Inserts the specified element at the specified position in this list.
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.
addAll(int index, Collection extends E> c)Inserts all of the elements in the specified collection into this list, starting at the specified position.
clear()Removes all of the elements from this list.
clone()Returns a shallow copy of this ArrayList instance.
contains(Object o)Returns true if this list contains the specified element.
ensureCapacity(int minCapacity)Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
get(int index)Returns the element at the specified position in this list.
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.
isEmpty()Returns true if this list contains no elements.
iterator()Returns an iterator over the elements in this list in proper sequence.
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()Returns a list iterator over the elements in this list (in proper sequence).
listIterator(int index)Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.
remove(int index)Removes the element at the specified position in this list.
remove(Object o)Removes the first occurrence of the specified element from this list, if it is present.
removeAll(Collection> c)Removes from this list all of its elements that are contained in the specified collection.
removeRange(int fromIndex, int toIndex)Removes from this list all of the elements whose index is between
fromIndex, inclusive, and
retainAll(Collection> c)Retains only the elements in this list that are contained in the specified collection.
set(int index, E element)Replaces the element at the specified position in this list with the specified element.
size()Returns the number of elements in this list.
subList(int fromIndex, int toIndex)Returns a view of the portion of this list between the specified
fromIndex, inclusive, and
toArray()Returns an array containing all of the elements in this list in proper sequence (from first to last element).
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.
trimToSize()Trims the capacity of this ArrayList instance to be the list's current size.