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Added in API level 1

ArrayList

public class ArrayList
extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable

java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.AbstractList<E>
       ↳ java.util.ArrayList<E>
Known Direct Subclasses


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:

   List list = Collections.synchronizedList(new ArrayList(...));

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.

See also:

Summary

Inherited fields

From class java.util.AbstractList

Public constructors

ArrayList(int initialCapacity)

Constructs an empty list with the specified initial capacity.

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.

Public methods

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.

void clear()

Removes all of the elements from this list.

Object clone()

Returns a shallow copy of this ArrayList instance.

boolean contains(Object o)

Returns true if this list contains the specified element.

void 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.

void forEach(Consumer<? super E> action)
E get(int index)

Returns the element at the specified position in this list.

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(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(int index)

Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.

ListIterator<E> listIterator()

Returns a list iterator over the elements in this list (in proper sequence).

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 removeIf(Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate.

void replaceAll(UnaryOperator<E> operator)

Replaces each element of this list with the result of applying the operator to that element.

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.

void sort(Comparator<? super E> c)

Sorts this list using the supplied Comparator to compare elements.

Spliterator<E> spliterator()

Creates a late-binding and fail-fast Spliterator over the elements in this list.

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

Returns a view of the portion of this list between the specified 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.

void trimToSize()

Trims the capacity of this ArrayList instance to be the list's current size.

Protected methods

void removeRange(int fromIndex, int toIndex)

Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive.

Inherited methods

From class java.util.AbstractList
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.util.List
From interface java.util.Collection
From interface java.lang.Iterable

Public constructors

ArrayList

Added in API level 1
ArrayList (int initialCapacity)

Constructs an empty list with the specified initial capacity.

Parameters
initialCapacity int: the initial capacity of the list
Throws
IllegalArgumentException if the specified initial capacity is negative

ArrayList

Added in API level 1
ArrayList ()

Constructs an empty list with an initial capacity of ten.

ArrayList

Added in API level 1
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.

Parameters
c Collection: the collection whose elements are to be placed into this list
Throws
NullPointerException if the specified collection is null

Public methods

add

Added in API level 1
boolean add (E e)

Appends the specified element to the end of this list.

Parameters
e E: element to be appended to this list
Returns
boolean true (as specified by add(E))

add

Added in API level 1
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).

Parameters
index int: index at which the specified element is to be inserted
element E: element to be inserted
Throws
IndexOutOfBoundsException

addAll

Added in API level 1
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. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this list, and this list is nonempty.)

Parameters
c Collection: collection containing elements to be added to this list
Returns
boolean true if this list changed as a result of the call
Throws
NullPointerException if the specified collection is null

addAll

Added in API level 1
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 the list in the order that they are returned by the specified collection's iterator.

Parameters
index int: index at which to insert the first element from the specified collection
c Collection: collection containing elements to be added to this list
Returns
boolean true if this list changed as a result of the call
Throws
IndexOutOfBoundsException
NullPointerException if the specified collection is null

clear

Added in API level 1
void clear ()

Removes all of the elements from this list. The list will be empty after this call returns.

clone

Added in API level 1
Object clone ()

Returns a shallow copy of this ArrayList instance. (The elements themselves are not copied.)

Returns
Object a clone of this ArrayList instance

contains

Added in API level 1
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)).

Parameters
o Object: element whose presence in this list is to be tested
Returns
boolean true if this list contains the specified element

ensureCapacity

Added in API level 1
void 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.

Parameters
minCapacity int: the desired minimum capacity

forEach

Added in API level 24
void forEach (Consumer<? super E> action)

Parameters
action Consumer

get

Added in API level 1
E get (int index)

Returns the element at the specified position in this list.

Parameters
index int: index of the element to return
Returns
E the element at the specified position in this list
Throws
IndexOutOfBoundsException

indexOf

Added in API level 1
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.

Parameters
o Object: element to search for
Returns
int the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element

isEmpty

Added in API level 1
boolean isEmpty ()

Returns true if this list contains no elements.

Returns
boolean true if this list contains no elements

iterator

Added in API level 1
Iterator<E> iterator ()

Returns an iterator over the elements in this list in proper sequence.

The returned iterator is fail-fast.

Returns
Iterator<E> an iterator over the elements in this list in proper sequence

lastIndexOf

Added in API level 1
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.

Parameters
o Object: element to search for
Returns
int the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element

listIterator

Added in API level 1
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 list iterator is fail-fast.

Parameters
index int: index of the first element to be returned from the list iterator (by a call to next)
Returns
ListIterator<E> a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list
Throws
IndexOutOfBoundsException

listIterator

Added in API level 1
ListIterator<E> listIterator ()

Returns a list iterator over the elements in this list (in proper sequence).

The returned list iterator is fail-fast.

Returns
ListIterator<E> a list iterator over the elements in this list (in proper sequence)

See also:

remove

Added in API level 1
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).

Parameters
index int: the index of the element to be removed
Returns
E the element that was removed from the list
Throws
IndexOutOfBoundsException

remove

Added in API level 1
boolean remove (Object o)

Removes the first occurrence of the specified element from this list, if it is present. If the 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).

Parameters
o Object: element to be removed from this list, if present
Returns
boolean true if this list contained the specified element

removeAll

Added in API level 1
boolean removeAll (Collection<?> c)

Removes from this list all of its elements that are contained in the specified collection.

Parameters
c Collection: collection containing elements to be removed from this list
Returns
boolean 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:

removeIf

Added in API level 24
boolean removeIf (Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.

Parameters
filter Predicate: a predicate which returns true for elements to be removed
Returns
boolean true if any elements were removed

replaceAll

Added in API level 24
void replaceAll (UnaryOperator<E> operator)

Replaces each element of this list with the result of applying the operator to that element. Errors or runtime exceptions thrown by the operator are relayed to the caller.

Parameters
operator UnaryOperator: the operator to apply to each element

retainAll

Added in API level 1
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.

Parameters
c Collection: collection containing elements to be retained in this list
Returns
boolean 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:

set

Added in API level 1
E set (int index, 
                E element)

Replaces the element at the specified position in this list with the specified element.

Parameters
index int: index of the element to replace
element E: element to be stored at the specified position
Returns
E the element previously at the specified position
Throws
IndexOutOfBoundsException

size

Added in API level 1
int size ()

Returns the number of elements in this list.

Returns
int the number of elements in this list

sort

Added in API level 24
void sort (Comparator<? super E> c)

Sorts this list using the supplied Comparator to compare elements.

Parameters
c Comparator: the Comparator used to compare list elements. A null value indicates that the elements' natural ordering should be used

spliterator

Added in API level 24
Spliterator<E> spliterator ()

Creates a late-binding and fail-fast Spliterator over the elements in this list.

The Spliterator reports SIZED, SUBSIZED, and ORDERED. Overriding implementations should document the reporting of additional characteristic values.

Returns
Spliterator<E> a Spliterator over the elements in this list

subList

Added in API level 1
List<E> 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.)

Parameters
fromIndex int: low endpoint (inclusive) of the subList
toIndex int: high endpoint (exclusive) of the subList
Returns
List<E> a view of the specified range within this list
Throws
IndexOutOfBoundsException
IllegalArgumentException

toArray

Added in API level 1
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.

Returns
Object[] an array containing all of the elements in this list in proper sequence

toArray

Added in API level 1
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 the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of the list only if the caller knows that the list does not contain any null elements.)

Parameters
a T: 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
T[] an array containing the elements of the 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

trimToSize

Added in API level 1
void trimToSize ()

Trims the capacity of this ArrayList instance to be the list's current size. An application can use this operation to minimize the storage of an ArrayList instance.

Protected methods

removeRange

Added in API level 1
void removeRange (int fromIndex, 
                int toIndex)

Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive. Shifts any succeeding elements to the left (reduces their index). This call shortens the list by (toIndex - fromIndex) elements. (If toIndex==fromIndex, this operation has no effect.)

Parameters
fromIndex int: index of first element to be removed
toIndex int: index after last element to be removed
Throws
IndexOutOfBoundsException if fromIndex or toIndex is out of range (fromIndex < 0 || fromIndex >= size() || toIndex > size() || toIndex < fromIndex)
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