An ordered collection (also known as a
sequence). The user of this
interface has precise control over where in the list each element is
inserted. The user can access elements by their integer index (position in
the list), and search for elements in the list.
Unlike sets, lists typically allow duplicate elements. More formally,
lists typically allow pairs of elements e1 and e2
such that e1.equals(e2), and they typically allow multiple
null elements if they allow null elements at all. It is not inconceivable
that someone might wish to implement a list that prohibits duplicates, by
throwing runtime exceptions when the user attempts to insert them, but we
expect this usage to be rare.
The List interface places additional stipulations, beyond those
specified in the Collection interface, on the contracts of the
iterator, add, remove, equals, and
hashCode methods. Declarations for other inherited methods are
also included here for convenience.
The List interface provides four methods for positional (indexed)
access to list elements. Lists (like Java arrays) are zero based. Note
that these operations may execute in time proportional to the index value
for some implementations (the LinkedList class, for
example). Thus, iterating over the elements in a list is typically
preferable to indexing through it if the caller does not know the
implementation.
The List interface provides a special iterator, called a
ListIterator, that allows element insertion and replacement, and
bidirectional access in addition to the normal operations that the
Iterator interface provides. A method is provided to obtain a
list iterator that starts at a specified position in the list.
The List interface provides two methods to search for a specified
object. From a performance standpoint, these methods should be used with
caution. In many implementations they will perform costly linear
searches.
The List interface provides two methods to efficiently insert and
remove multiple elements at an arbitrary point in the list.
Note: While it is permissible for lists to contain themselves as elements,
extreme caution is advised: the equals and hashCode
methods are no longer well defined on a such a list.
Some list implementations have restrictions on the elements that
they may contain. For example, some implementations prohibit null elements,
and some have restrictions on the types of their elements. Attempting to
add an ineligible element throws an unchecked exception, typically
NullPointerException or ClassCastException. Attempting
to query the presence of an ineligible element may throw an exception,
or it may simply return false; some implementations will exhibit the former
behavior and some will exhibit the latter. More generally, attempting an
operation on an ineligible element whose completion would not result in
the insertion of an ineligible element into the list may throw an
exception or it may succeed, at the option of the implementation.
Such exceptions are marked as "optional" in the specification for this
interface.
This interface is a member of the
Java Collections Framework.
Ensures that this collection contains the specified element (optional
operation). Returns
true if this collection changed as a
result of the call. (Returns
false if this collection does
not permit duplicates and already contains the specified element.)
Collections that support this operation may place limitations on what
elements may be added to this collection. In particular, some
collections will refuse to add null elements, and others will
impose restrictions on the type of elements that may be added.
Collection classes should clearly specify in their documentation any
restrictions on what elements may be added.
If a collection refuses to add a particular element for any reason
other than that it already contains the element, it must throw
an exception (rather than returning false). This preserves
the invariant that a collection always contains the specified element
after this call returns.
Inserts the specified element at the specified position in this list
(optional operation). Shifts the element currently at that position
(if any) and any subsequent elements to the right (adds one to their
indices).
Adds all of the elements in the specified collection to this collection
(optional operation). 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 collection, and this collection is
nonempty.)
Inserts all of the elements in the specified collection into this
list at the specified position (optional operation). 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 this list in the order that they are returned by the
specified collection's iterator. The behavior of this operation is
unspecified if the specified collection is modified while the
operation is in progress. (Note that this will occur if the specified
collection is this list, and it's nonempty.)
Removes all of the elements from this collection (optional operation).
This collection will be empty after this method returns unless it
throws an exception.
Returns true if this collection contains the specified
element. More formally, returns true if and only if this
collection contains at least one element e such that
(o==null ? e==null : o.equals(e)).
Returns true if this collection contains all of the elements
in the specified collection.
Compares the specified object with this list for equality. Returns
true if and only if the specified object is also a list, both
lists have the same size, and all corresponding pairs of elements in
the two lists are equal. (Two elements e1 and
e2 are equal if (e1==null ? e2==null :
e1.equals(e2)).) In other words, two lists are defined to be
equal if they contain the same elements in the same order. This
definition ensures that the equals method works properly across
different implementations of the List interface.
Returns the element at the specified position in this list.
Returns the hash code value for this list. The hash code of a list
is defined to be the result of the following calculation:
hashCode = 1;
Iterator i = list.iterator();
while (i.hasNext()) {
Object obj = i.next();
hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
}
This ensures that
list1.equals(list2) implies that
list1.hashCode()==list2.hashCode() for any two lists,
list1 and
list2, as required by the general
contract of
Object.hashCode.
Returns the index in this list of the first occurrence of the specified
element, or -1 if this list does not contain this 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.
Returns true if this collection contains no elements.
Returns an iterator over the elements in this collection. There are no
guarantees concerning the order in which the elements are returned
(unless this collection is an instance of some class that provides a
guarantee).
Returns the index in this list of the last occurrence of the specified
element, or -1 if this list does not contain this 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.
Returns a list iterator of the elements in this list (in proper
sequence).
Returns a list iterator of the elements in this list (in proper
sequence), starting at the specified position in this list. The
specified index indicates the first element that would be returned by
an initial call to the next method. An initial call to
the previous method would return the element with the
specified index minus one.
Removes the element at the specified position in this list (optional
operation). Shifts any subsequent elements to the left (subtracts one
from their indices). Returns the element that was removed from the
list.
Removes a single instance of the specified element from this
collection, if it is present (optional operation). More formally,
removes an element e such that (o==null ? e==null :
o.equals(e)), if this collection contains one or more such
elements. Returns true if this collection contained the specified
element (or equivalently, if this collection changed as a result of the
call).
Removes all this collection's elements that are also contained in the
specified collection (optional operation). After this call returns,
this collection will contain no elements in common with the specified
collection.
Retains only the elements in this collection that are contained in the
specified collection (optional operation). In other words, removes from
this collection all of its elements that are not contained in the
specified collection.
Replaces the element at the specified position in this list with the
specified element (optional operation).
Returns the number of elements in this collection. If this collection
contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
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 supported
by this list.
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 and
lastIndexOf, 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.)
Returns an array containing all of the elements in this collection. If
the collection makes any guarantees as to what order its elements are
returned by its iterator, this method must return the elements in the
same order.
The returned array will be "safe" in that no references to it are
maintained by this collection. (In other words, this method must
allocate a new array even if this collection is backed by an array).
The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based
APIs.
Returns an array containing all of the elements in this collection;
the runtime type of the returned array is that of the specified array.
If the collection 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 collection.
If this collection fits in the specified array with room to spare
(i.e., the array has more elements than this collection), the element
in the array immediately following the end of the collection is set to
null. This is useful in determining the length of this
collection only if the caller knows that this collection does
not contain any null elements.)
If this collection makes any guarantees as to what order its elements
are returned by its iterator, this method must return the elements in
the same order.
Like the toArray method, this method acts as bridge between
array-based and collection-based APIs. Further, this method allows
precise control over the runtime type of the output array, and may,
under certain circumstances, be used to save allocation costs
Suppose l is a List known to contain only strings.
The following code can be used to dump the list into a newly allocated
array of String:
String[] x = (String[]) v.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to
toArray().