java.util
class ArrayList

java.lang.Object java.util.AbstractCollection java.util.AbstractList java.util.ArrayList

All Implemented Interfaces:

Serializable, Cloneable, Collection, List, RandomAccess

Direct Known Subclasses:

AttributeList, RoleList, RoleUnresolvedList

Most common way to construct:

ArrayList list = new ArrayList();

Based on 161 examples

public class ArrayList
extends AbstractList
implements List, RandomAccess, Cloneable, Serializable

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 {@link Collections#synchronizedList 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 {@link 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.

ArrayList

public ArrayList()

Constructs an empty list with an initial capacity of ten.

ArrayList

public ArrayList(Collection c)

Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

Parameters:

c - the collection whose elements are to be placed into this list

ArrayList

public ArrayList(int initialCapacity)

Constructs an empty list with the specified initial capacity.

Parameters:

initialCapacity - the initial capacity of the list

add

public boolean add(Object e)

Appends the specified element to the end of this list.

Overrides:

add in class AbstractList

Parameters:

e - element to be appended to this list

Returns:

true (as specified by {@link Collection#add})

add

public void add(int index,
                Object 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).

Overrides:

add in class AbstractList

Parameters:

index - index at which the specified element is to be inserted

element - element to be inserted

addAll

public boolean addAll(Collection 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.)

Overrides:

addAll in class AbstractCollection

Parameters:

c - collection containing elements to be added to this list

Returns:

true if this list changed as a result of the call

addAll

public boolean addAll(int index,
                      Collection 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.

Overrides:

addAll in class AbstractList

Parameters:

index - index at which to insert the first element from the specified collection

c - collection containing elements to be added to this list

Returns:

true if this list changed as a result of the call

clear

public void clear()

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

Overrides:

clear in class AbstractList

clone

public Object clone()

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

Overrides:

clone in class Object

Returns:

a clone of this ArrayList instance

contains

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 (o==null ? e==null : o.equals(e)).

Overrides:

contains in class AbstractCollection

Parameters:

o - element whose presence in this list is to be tested

Returns:

true if this list contains the specified element

ensureCapacity

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 - the desired minimum capacity

get

public Object get(int index)

Returns the element at the specified position in this list.

Overrides:

get in class AbstractList

Parameters:

index - index of the element to return

Returns:

the element at the specified position in this list

indexOf

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 (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

Overrides:

indexOf in class AbstractList

Parameters:

o

isEmpty

public boolean isEmpty()

Returns true if this list contains no elements.

Overrides:

isEmpty in class AbstractCollection

Returns:

true if this list contains no elements

lastIndexOf

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 (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

Overrides:

lastIndexOf in class AbstractList

Parameters:

o

remove

public Object 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).

Overrides:

remove in class AbstractList

Parameters:

index - the index of the element to be removed

Returns:

the element that was removed from the list

remove

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 (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).

Overrides:

remove in class AbstractCollection

Parameters:

o - element to be removed from this list, if present

Returns:

true if this list contained the specified element

removeRange

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.)

Overrides:

removeRange in class AbstractList

Parameters:

fromIndex - index of first element to be removed

toIndex - index after last element to be removed

set

public Object set(int index,
                  Object element)

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

Overrides:

set in class AbstractList

Parameters:

index - index of the element to replace

element - element to be stored at the specified position

Returns:

the element previously at the specified position

size

public int size()

Returns the number of elements in this list.

Overrides:

size in class AbstractCollection

Returns:

the number of elements in this list

sort

public void sort()

Use java.util.Collections.sort(List).

toArray

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.

Overrides:

toArray in class AbstractCollection

Returns:

an array containing all of the elements in this list in proper sequence

toArray

public Object[] toArray(Object[] 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.)

Overrides:

toArray in class AbstractCollection

Parameters:

a - 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:

an array containing the elements of the list

trimToSize

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.