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 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.
| Method Summary | ||
boolean |
add(E e) Appends the specified element to the end of this list (optional
operation). |
|
void |
add(int index, E element) Inserts the specified element at the specified position in this list
(optional operation). |
|
boolean |
addAll(int index, Collection<? extends E> c) Inserts all of the elements in the specified collection into this
list at the specified position (optional operation). |
|
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 (optional operation). |
|
void |
clear() Removes all of the elements from this list (optional operation). |
|
boolean |
contains(Object o) Returns true if this list contains the specified element. |
|
boolean |
containsAll(Collection<?> c) Returns true if this list contains all of the elements of the
specified collection. |
|
boolean |
equals(Object o) Compares the specified object with this list for equality. |
|
| get(int index) Returns the element at the specified position in this list. |
||
int |
hashCode() Returns the hash code value for 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() 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() Returns a list iterator over the elements in this list (in proper
sequence). |
||
| listIterator(int index) Returns a list iterator of the elements in this list (in proper
sequence), starting at the specified position in this list. |
||
| remove(int index) Removes the element at the specified position in this list (optional
operation). |
||
boolean |
remove(Object o) Removes the first occurrence of the specified element from this list,
if it is present (optional operation). |
|
boolean |
removeAll(Collection<?> c) Removes from this list all of its elements that are contained in the
specified collection (optional operation). |
|
boolean |
retainAll(Collection<?> c) Retains only the elements in this list that are contained in the
specified collection (optional operation). |
|
| set(int index, E element) Replaces the element at the specified position in this list with the
specified element (optional operation). |
||
int |
size() Returns the number of elements in this list. |
|
| 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[] |
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. |
|
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 even if this list 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.
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 list 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.)
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 x 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[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to
toArray().Lists that support this operation may place limitations on what elements may be added to this list. In particular, some lists will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. List classes should clearly specify in their documentation any restrictions on what elements may be added.
int hashCode = 1;
Iterator<E> i = list.iterator();
while (i.hasNext()) {
E 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().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.)