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Java's ArrayList Class

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.
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