Java Doc for ArrayList.java in  » 6.0-JDK-Core » Collections-Jar-Zip-Logging-regex » java » util » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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 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 ArrayList.iterator() iterator and ArrayList.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 ListIterator.remove remove or ListIterator.add(Object) 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.
author:
   Josh Bloch
author:
   Neal Gafter
version:
   1.65, 06/16/07
See Also:   Collection
See Also:   List
See Also:   LinkedList
See Also:   Vector
since:
   1.2

The returned array will be "safe" in that no references to it are maintained by this list.

The returned iterator is fail-fast. an iterator over the elements in this list in proper sequence

The returned list iterator is fail-fast.
throws:
  IndexOutOfBoundsException -

The returned list iterator is fail-fast.
See Also:   ArrayList.listIterator(int)

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();
 

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.)
throws:
  IndexOutOfBoundsException -
throws:
  IllegalArgumentException -

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. an array containing all of the elements in this list inproper sequence

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 - the array into which the elements of the list are tobe stored, if it is big enough; otherwise, a new array of thesame runtime type is allocated for this purpose. an array containing the elements of the list
throws:
  ArrayStoreException - if the runtime type of the specified arrayis not a supertype of the runtime type of every element inthis list
throws:
  NullPointerException - if the specified array is null