By Doug Lowe

The ArrayList package (Package: java.util) in Java lets you create and maintain a special type of collection object: an array list. An array list is similar to an array but averts many of the most common problems of working with arrays, specifically the following:

  • An array list automatically resizes itself whenever necessary.

  • An array list lets you insert elements into the middle of the collection.

  • An array list lets you delete items.

Constructors

Constructor Explanation
ArrayList() Creates an array list with an initial capacity of ten
elements.
ArrayList(int capacity) Creates an array list with the specified initial capacity.
ArrayList(Collection c) Creates an array list and copies all the elements from the
specified collection into the new array list.

Methods

Method Explanation
add(Object element) Adds the specified object to the array list. If you specified a
type when you created the array list, the object must be of the
correct type.
add(int index, Object element) Adds the specified object to the array list at the specified
index position. If you specified a type when you created the array
list, the object must be of the correct type.
addAll(Collection c) Adds all the elements of the specified collection to this array
list.
addAll(int index, Collection c) Adds all the elements of the specified collection to this array
list at the specified index position.
clear() Deletes all elements from the array list.
clone() Returns a shallow copy of the array list. The elements
contained in the copy are the same object instances as the elements
in the original.
contains(Object elem) Returns a Boolean value that indicates whether the specified
object is in the array list.
containsAll(Collection c) Returns a Boolean value that indicates whether this array list
contains all the objects that are in the specified collection.
ensureCapacity(int minCapacity) Increases the array list’s capacity to the specified
value. (If the capacity is already greater than the specified
value, this method does nothing.)
get(int index) Returns the object at the specified position in the list.
indexOf(Object elem) Returns the index position of the first occurrence of the
specified object in the array list. If the object isn’t in
the list, it returns -1.
isEmpty() Returns a Boolean value that indicates whether the array list
is empty.
iterator() Returns an iterator for the array list.
lastIndexOf(Object elem) Returns the index position of the last occurrence of the
specified object in the array list. If the object isn’t in
the list, it returns -1.
remove(int index) Removes the object at the specified index and returns the
element that was removed.
remove(Object elem) Removes an object from the list. Note that more than one
element refers to the object; this method removes only one of them.
It returns a Boolean value that indicates whether the object was in
the list.
remove(int fromIndex, int
toIndex)
Removes all objects whose index values are between the values
specified. Note that the elements at the fromIndex and toIndex
positions are not themselves removed.
removeAll(Collection c) Removes all the objects in the specified collection from this
array list.
retainAll(Collection c) Removes all the objects that are not in the specified
collection from this array list.
set(int index, Object elem) Sets the specified element to the specified object. The element
that was previously at that position is returned as the
method’s return value.
size() Returns the number of elements in the list.
toArray() Returns the elements of the array list as an array of objects
(Object[]).
toArray(type[] array) Returns the elements of the array list as an array whose type
is the same as the array passed via the parameter.