java.util

## Class ArrayList<E>

- java.lang.Object
- - java.util.AbstractCollection<E>
  - - java.util.AbstractList<E>
    - - java.util.ArrayList<E>

- All Implemented Interfaces:
    Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess

      public class ArrayList<E>
      extends AbstractList<E>
      implements List<E>, 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 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.

  Since:
    1.2
  Author:
    Josh Bloch, Neal Gafter
  See Also:
    `Collection`, `List`, `LinkedList`, `Vector`, Serialized Form

- - ### Field Summary

    - ### Fields inherited from class java.util.AbstractList

      `modCount`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `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.
    `ArrayList(int initialCapacity)` Constructs an empty list with the specified initial capacity.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `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.
    `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()` Returns a list iterator over the elements in this list (in proper sequence).
    `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.
    `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.
    `protected void` | `removeRange(int fromIndex, int toIndex)` Removes from this list all of the elements whose index is between `fromIndex`, inclusive, and `toIndex`, exclusive.
    `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.
    `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.

    - ### Methods inherited from class java.util.AbstractList

      `equals, hashCode`

    - ### Methods inherited from class java.util.AbstractCollection

      `containsAll, toString`

    - ### Methods inherited from class java.lang.Object

      `finalize, getClass, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.List

      `containsAll, equals, hashCode, replaceAll, sort, spliterator`

    - ### Methods inherited from interface java.util.Collection

      `parallelStream, removeIf, stream`

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### 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<? 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` - 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<E>`
      Specified by:
        `size` in interface `List<E>`
      Specified by:
        `size` in class `AbstractCollection<E>`
      Returns:
        the number of elements in this list

    - #### isEmpty

          public boolean isEmpty()

      Returns `true` if this list contains no elements.
      Specified by:
        `isEmpty` in interface `Collection<E>`
      Specified by:
        `isEmpty` in interface `List<E>`
      Overrides:
        `isEmpty` in class `AbstractCollection<E>`
      Returns:
        `true` if this list contains no elements

    - #### 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))`.
      Specified by:
        `contains` in interface `Collection<E>`
      Specified by:
        `contains` in interface `List<E>`
      Overrides:
        `contains` in class `AbstractCollection<E>`
      Parameters:
        `o` - element whose presence in this list is to be tested
      Returns:
        `true` if this list contains the specified element

    - #### 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.
      Specified by:
        `indexOf` in interface `List<E>`
      Overrides:
        `indexOf` in class `AbstractList<E>`
      Parameters:
        `o` - element to search for
      Returns:
        the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element

    - #### 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.
      Specified by:
        `lastIndexOf` in interface `List<E>`
      Overrides:
        `lastIndexOf` in class `AbstractList<E>`
      Parameters:
        `o` - element to search for
      Returns:
        the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element

    - #### 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
      See Also:
        `Cloneable`

    - #### 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.

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

    - #### toArray

          public <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. 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<E>`
      Specified by:
        `toArray` in interface `List<E>`
      Overrides:
        `toArray` in class `AbstractCollection<E>`
      Type Parameters:
        `T` - the runtime type of the array to contain the collection
      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 the specified array is not a supertype of the runtime type of every element in this list
        `NullPointerException` - if the specified array is null

    - #### get

          public E get(int index)

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

    - #### set

          public E set(int index,
                       E element)

      Replaces the element at the specified position in this list with the specified element.
      Specified by:
        `set` in interface `List<E>`
      Overrides:
        `set` in class `AbstractList<E>`
      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
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index >= size()`)

    - #### add

          public boolean add(E e)

      Appends the specified element to the end of this list.
      Specified by:
        `add` in interface `Collection<E>`
      Specified by:
        `add` in interface `List<E>`
      Overrides:
        `add` in class `AbstractList<E>`
      Parameters:
        `e` - element to be appended to this list
      Returns:
        `true` (as specified by `Collection.add(E)`)

    - #### add

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

    - #### remove

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

    - #### 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).
      Specified by:
        `remove` in interface `Collection<E>`
      Specified by:
        `remove` in interface `List<E>`
      Overrides:
        `remove` in class `AbstractCollection<E>`
      Parameters:
        `o` - element to be removed from this list, if present
      Returns:
        `true` if this list contained the specified element

    - #### 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<E>`
      Specified by:
        `clear` in interface `List<E>`
      Overrides:
        `clear` in class `AbstractList<E>`

    - #### addAll

          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.)
      Specified by:
        `addAll` in interface `Collection<E>`
      Specified by:
        `addAll` in interface `List<E>`
      Overrides:
        `addAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - collection containing elements to be added to 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<? 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.
      Specified by:
        `addAll` in interface `List<E>`
      Overrides:
        `addAll` in class `AbstractList<E>`
      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
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)
        `NullPointerException` - if the specified collection is null

    - #### 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<E>`
      Parameters:
        `fromIndex` - index of first element to be removed
        `toIndex` - 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`)

    - #### removeAll

          public boolean removeAll(Collection<?> c)

      Removes from this list all of its elements that are contained in the specified collection.
      Specified by:
        `removeAll` in interface `Collection<E>`
      Specified by:
        `removeAll` in interface `List<E>`
      Overrides:
        `removeAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - collection containing elements to be removed from this list
      Returns:
        `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:
        `Collection.contains(Object)`

    - #### retainAll

          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.
      Specified by:
        `retainAll` in interface `Collection<E>`
      Specified by:
        `retainAll` in interface `List<E>`
      Overrides:
        `retainAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - collection containing elements to be retained in this list
      Returns:
        `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:
        `Collection.contains(Object)`

    - #### listIterator

          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.

      Specified by:
        `listIterator` in interface `List<E>`
      Overrides:
        `listIterator` in class `AbstractList<E>`
      Parameters:
        `index` - index of the first element to be returned from the list iterator (by a call to `next`)
      Returns:
        a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)

    - #### listIterator

          public ListIterator<E> listIterator()

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

      The returned list iterator is fail-fast.

      Specified by:
        `listIterator` in interface `List<E>`
      Overrides:
        `listIterator` in class `AbstractList<E>`
      Returns:
        a list iterator over the elements in this list (in proper sequence)
      See Also:
        `listIterator(int)`

    - #### iterator

          public Iterator<E> iterator()

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

      The returned iterator is fail-fast.

      Specified by:
        `iterator` in interface `Iterable<E>`
      Specified by:
        `iterator` in interface `Collection<E>`
      Specified by:
        `iterator` in interface `List<E>`
      Overrides:
        `iterator` in class `AbstractList<E>`
      Returns:
        an iterator over the elements in this list in proper sequence

    - #### subList

          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(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.)

      Specified by:
        `subList` in interface `List<E>`
      Overrides:
        `subList` in class `AbstractList<E>`
      Parameters:
        `fromIndex` - low endpoint (inclusive) of the subList
        `toIndex` - high endpoint (exclusive) of the subList
      Returns:
        a view of the specified range within this list
      Throws:
        `IndexOutOfBoundsException` - if an endpoint index value is out of range `(fromIndex < 0 || toIndex > size)`
        `IllegalArgumentException` - if the endpoint indices are out of order `(fromIndex > toIndex)`

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