ArrayList

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## java.util
Class ArrayList

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

All Implemented Interfaces:
  Serializable, Cloneable, Collection, List, RandomAccess

    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 `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 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.

Since:
  1.2
Version:
  1.25, 02/02/00
Author:
  Josh Bloch
See Also:
  `Collection`, `List`, `LinkedList`, `Vector`, `Collections.synchronizedList(List)`, Serialized Form

Constructor Summary
`ArrayList()` Constructs an empty list with an initial capacity of ten.
`ArrayList(Collection c)` Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
`ArrayList(int initialCapacity)` Constructs an empty list with the specified initial capacity.

Method Summary
`void` | `add(int index, Object element)` Inserts the specified element at the specified position in this list.
`boolean` | `add(Object o)` Appends the specified element to the end of this list.
`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.
`boolean` | `addAll(int index, Collection 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 elem)` 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.
`Object` | `get(int index)` Returns the element at the specified position in this list.
`int` | `indexOf(Object elem)` Searches for the first occurence of the given argument, testing for equality using the `equals` method.
`boolean` | `isEmpty()` Tests if this list has no elements.
`int` | `lastIndexOf(Object elem)` Returns the index of the last occurrence of the specified object in this list.
`Object` | `remove(int index)` Removes the element at the specified position in this list.
`Object` | `set(int index, Object 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.
`Object[]` | `toArray()` Returns an array containing all of the elements in this list in the correct order.
`Object[]` | `toArray(Object[] a)` Returns an array containing all of the elements in this list in the correct order; 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.

Methods inherited from class java.util.AbstractList
`equals, hashCode, iterator, listIterator, listIterator, subList`

Methods inherited from class java.util.AbstractCollection
`containsAll, remove, removeAll, retainAll, toString`

Methods inherited from class java.lang.Object
`getClass, notify, notifyAll, wait, wait, wait`

Methods inherited from interface java.util.List
`containsAll, equals, hashCode, iterator, listIterator, listIterator, remove, removeAll, retainAll, subList`

Constructor Detail

### ArrayList

    public ArrayList(int initialCapacity)

  Constructs an empty list with the specified initial capacity.

  Parameters:
    `initialCapacity` - the initial capacity of the list.
  Throws:
    `IllegalArgumentException` - if the specified initial capacity is negative

### 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. The `ArrayList` instance has an initial capacity of 110% the size of the specified collection.

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

Method Detail

### 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.

### 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.

### size

    public int size()

  Returns the number of elements in this list.

  Specified by:
    `size` in interface `Collection`
  Specified by:
    `size` in interface `List`
  Specified by:
    `size` in class `AbstractCollection`
  Returns:
    the number of elements in this list.

### isEmpty

    public boolean isEmpty()

  Tests if this list has no elements.

  Specified by:
    `isEmpty` in interface `Collection`
  Specified by:
    `isEmpty` in interface `List`
  Overrides:
    `isEmpty` in class `AbstractCollection`
  Returns:
    `true` if this list has no elements; `false` otherwise.

### contains

    public boolean contains(Object elem)

  Returns `true` if this list contains the specified element.

  Specified by:
    `contains` in interface `Collection`
  Specified by:
    `contains` in interface `List`
  Overrides:
    `contains` in class `AbstractCollection`
  Parameters:
    `elem` - element whose presence in this List is to be tested.
  Returns:
    `true` if the specified element is present; `false` otherwise.

### indexOf

    public int indexOf(Object elem)

  Searches for the first occurence of the given argument, testing for equality using the `equals` method.

  Specified by:
    `indexOf` in interface `List`
  Overrides:
    `indexOf` in class `AbstractList`
  Parameters:
    `elem` - an object.
  Returns:
    the index of the first occurrence of the argument in this list; returns `-1` if the object is not found.
  See Also:
    `Object.equals(Object)`

### lastIndexOf

    public int lastIndexOf(Object elem)

  Returns the index of the last occurrence of the specified object in this list.

  Specified by:
    `lastIndexOf` in interface `List`
  Overrides:
    `lastIndexOf` in class `AbstractList`
  Parameters:
    `elem` - the desired element.
  Returns:
    the index of the last occurrence of the specified object in this list; returns -1 if the object is not found.

### clone

    public Object clone()

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

  Returns:
    a clone of this `ArrayList` instance.
  See Also:
    `Cloneable`

### toArray

    public Object[] toArray()

  Returns an array containing all of the elements in this list in the correct order.

  Specified by:
    `toArray` in interface `Collection`
  Specified by:
    `toArray` in interface `List`
  Overrides:
    `toArray` in class `AbstractCollection`
  Returns:
    an array containing all of the elements in this list in the correct order.
  See Also:
    `Arrays.asList(Object[])`

### toArray

    public Object[] toArray(Object[] a)

  Returns an array containing all of the elements in this list in the correct order; 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.

  Specified by:
    `toArray` in interface `Collection`
  Specified by:
    `toArray` in interface `List`
  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.
  Throws:
    `ArrayStoreException` - if the runtime type of a is not a supertype of the runtime type of every element in this list.

### get

    public Object get(int index)

  Returns the element at the specified position in this list.

  Specified by:
    `get` in interface `List`
  Specified by:
    `get` in class `AbstractList`
  Parameters:
    `index` - index of element to return.
  Returns:
    the element at the specified position in this list.
  Throws:
    `IndexOutOfBoundsException` - if index is out of range `(index < 0 || index >= size())`.

### set

    public Object set(int index,
                      Object element)

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

  Specified by:
    `set` in interface `List`
  Overrides:
    `set` in class `AbstractList`
  Parameters:
    `index` - index of element to replace.
    `element` - element to be stored at the specified position.
  Returns:
    the element previously at the specified position.
  Throws:
    `IndexOutOfBoundsException` - if index out of range `(index < 0 || index >= size())`.

### add

    public boolean add(Object o)

  Appends the specified element to the end of this list.

  Specified by:
    `add` in interface `Collection`
  Specified by:
    `add` in interface `List`
  Overrides:
    `add` in class `AbstractList`
  Parameters:
    `o` - element to be appended to this list.
  Returns:
    `true` (as per the general contract of 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).

  Specified by:
    `add` in interface `List`
  Overrides:
    `add` in class `AbstractList`
  Parameters:
    `index` - index at which the specified element is to be inserted.
    `element` - element to be inserted.
  Throws:
    `IndexOutOfBoundsException` - if index is out of range `(index < 0 || index > size())`.

### 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).

  Specified by:
    `remove` in interface `List`
  Overrides:
    `remove` in class `AbstractList`
  Parameters:
    `index` - the index of the element to removed.
  Returns:
    the element that was removed from the list.
  Throws:
    `IndexOutOfBoundsException` - if index out of range `(index < 0 || index >= size())`.

### clear

    public void clear()

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

  Specified by:
    `clear` in interface `Collection`
  Specified by:
    `clear` in interface `List`
  Overrides:
    `clear` in class `AbstractList`

### 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.)

  Specified by:
    `addAll` in interface `Collection`
  Specified by:
    `addAll` in interface `List`
  Overrides:
    `addAll` in class `AbstractCollection`
  Parameters:
    `c` - the elements to be inserted into this list.
  Returns:
    `true` if this list changed as a result of the call.
  Throws:
    `NullPointerException` - if the specified collection is null.
  See Also:
    `AbstractCollection.add(Object)`

### 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.

  Specified by:
    `addAll` in interface `List`
  Overrides:
    `addAll` in class `AbstractList`
  Parameters:
    `index` - index at which to insert first element from the specified collection.
    `c` - elements to be inserted into this list.
  Returns:
    `true` if this list changed as a result of the call.
  Throws:
    `IndexOutOfBoundsException` - if index out of range `(index < 0 || index > size())`.
    `NullPointerException` - if the specified Collection is null.

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