ArrayList

ArrayList Overview

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

  • Resizable-array implementation of the List interface.implementationListA mutable array of interfaces. (An ArrayList is a legendary dynamic array)
  • Implements all optional list operations, and permits all elements, including null.Implements all optional list operations and allows the addition of all elements includingnull.
  • 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. In addition to implementing the List interface, this class provides methods to manipulate the size of the array internally used to store the List.
  • (This class is roughly equivalent to Vector, except that it is unsynchronized.)This class is roughly equivalent toVectorClass, except that this class is not synchronized (that is, thread unsafe).

  • 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. The size, isEmpty, get, set, iterato, listIterator method run in constant time. The add method runs in apportioned constant time, taking O(n) time to add n elements.

  • All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.All other methods run in linear time (roughly speaking). Constant factor andLinkedListThe implementation is relatively low.

  • 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. Each ArrayList instance has capacity. Capacity is the size of the array used to store elements in the list. It’s always at least as big as the list. When an element is added to an ArrayList, its capacity increases automatically. Aside from the fact that adding an element has a constant amortization time, there is the detail of not having a growth strategy in place.

  • 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. Using the ensureCapacity method before adding a large number of elements, an application can expand the capacity of an instance of an ArrayList. This may reduce the amount of redistribution growth.

  • 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. Note that the implementation class is not synchronized. If multiple threads concurrently access an ArrayList instance, and at least one of those threads structurally modifies the collection, it must be used externally synchronously. (Structurally modified means any operation that adds or removes one or more elements, or that appears to resize the backup array; Simply setting the value of an element is not a structured modification operation. This is usually done by synchronizing some objects that naturally encapsulate the collection.

  • 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(...) );If no such object exists, the collection should be usedCollections.synchronizedListMethod to encapsulate. This is best done at creation time to prevent sudden asynchronous access to the collection,List list = Collections.synchronizedList(new ArrayList(...) );.
  • The iterators returned by this class’s {@link #iterator() iterator} and {@link #listIterator(int) 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 {@link ListIterator#remove() remove} or {@link ListIterator#add(Object) 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. fail-fastThrough this classiterator()andlistIterator(int)The iterator returned isfail-fast. What if the list is structurally modified at any time after the iterator is created, anyway, except through the iterator’s ownremoveoraddMethod, the iterator will throw oneConcurrentModificationExceptionThe exception. Therefore, in the face ofConcurrentModificationExceptionWhen exceptions occur, iterators end quickly and cleanly, rather than risk arbitrary, uncertain behavior at an indefinite 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 {@code 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. Note that the fail-fast behavior of iterators cannot be guaranteed because, in general, it is impossible to make any hard guarantees in the case of unsynchronized concurrent modifications. Fail – fast iterator for the biggest benefit, throw ConcurrentModificationException anomalies. Once again, it would be a mistake to write a program that relied on this exception to ensure correctness: the fail-fast behavior of an iterator should only be used to detect bugs.