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>


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.

Summary

Inherited fields

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 addFirst(E element)

Adds an element as the first element of this collection (optional operation).

void addLast(E element)

Adds an element as the last element of this collection (optional operation).

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.

boolean equals(Object o)

Compares the specified object with this list for equality.

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

Returns the element at the specified position in this list.

E getFirst()

Gets the first element of this collection.

E getLast()

Gets the last element of this collection.

int hashCode()

Returns the hash code value for 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.

E removeFirst()

Removes and returns the first element of this collection (optional operation).

boolean removeIf(Predicate<? super E> filter)

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

E removeLast()

Removes and returns the last element of this collection (optional operation).

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 according to the order induced by the specified Comparator.

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

Public constructors

ArrayList

Added in API level 1
public 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
public ArrayList ()

Constructs an empty list with an initial capacity of ten.

ArrayList

Added in API level 1
public 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
public 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 Collection#add)

add

Added in API level 1
public 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
public 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
public 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

addFirst

Added in API level 35
public void addFirst (E element)

Adds an element as the first element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the first element in encounter order.

Parameters
element E: the element to be added

addLast

Added in API level 35
public void addLast (E element)

Adds an element as the last element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the last element in encounter order.

Parameters
element E: the element to be added.

clear

Added in API level 1
public 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
public 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
public 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 Objects.equals(o, 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
public 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

equals

Added in API level 1
public boolean equals (Object o)

Compares the specified object with this list for equality. Returns true if and only if the specified object is also a list, both lists have the same size, and all corresponding pairs of elements in the two lists are equal. (Two elements e1 and e2 are equal if (e1==null ? e2==null : e1.equals(e2)).) In other words, two lists are defined to be equal if they contain the same elements in the same order.

Parameters
o Object: the object to be compared for equality with this list

Returns
boolean true if the specified object is equal to this list

forEach

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

Parameters
action Consumer

Throws
NullPointerException

get

Added in API level 1
public 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

getFirst

Added in API level 35
public E getFirst ()

Gets the first element of this collection.

Returns
E the retrieved element

Throws
NoSuchElementException

getLast

Added in API level 35
public E getLast ()

Gets the last element of this collection.

Returns
E the retrieved element

Throws
NoSuchElementException

hashCode

Added in API level 1
public int hashCode ()

Returns the hash code value for this list.

Returns
int the hash code value for this list

indexOf

Added in API level 1
public 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 Objects.equals(o, 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
public 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
public 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
public 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 Objects.equals(o, 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
public 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
public 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
public 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
public 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 Objects.equals(o, 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
public 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

removeFirst

Added in API level 35
public E removeFirst ()

Removes and returns the first element of this collection (optional operation).

Returns
E the removed element

Throws
NoSuchElementException

removeIf

Added in API level 24
public 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

Throws
NullPointerException

removeLast

Added in API level 35
public E removeLast ()

Removes and returns the last element of this collection (optional operation).

Returns
E the removed element

Throws
NoSuchElementException

replaceAll

Added in API level 24
public 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
public 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

set

Added in API level 1
public 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
public 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
public void sort (Comparator<? super E> c)

Sorts this list according to the order induced by the specified Comparator.

All elements in this list must be mutually comparable using the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

If the specified comparator is null then all elements in this list must implement the Comparable interface and the elements' natural ordering should be used.

This list must be modifiable, but need not be resizable.

For apps running on and targeting Android versions greater than Nougat (API level > 25), Collections#sort(List) delegates to this method. Such apps must not call Collections#sort(List) from this method. Instead, prefer not overriding this method at all. If you must override it, consider this implementation:

 @Override
 public void sort(Comparator<? super E> c) {
   Object[] elements = toArray();
   Arrays.sort(elements, c);
   ListIterator<E> iterator = (ListIterator<Object>) listIterator();
   for (Object element : elements) {
     iterator.next();
     iterator.set((E) element);
   }
 }
 

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
public Spliterator<E> spliterator ()

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

The Spliterator reports Spliterator#SIZED, Spliterator#SUBSIZED, and Spliterator#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
public 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(java.lang.Object) and lastIndexOf(java.lang.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
public 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
public 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
public 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
protected 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 || toIndex > size() || toIndex < fromIndex)