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
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.
List interface places additional stipulations, beyond those specified in the
Collection interface, on the contracts of the
hashCode methods. Declarations for other inherited methods are also included here for convenience.
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.
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.
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.
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
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
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.
List.copyOf static factory methods provide a convenient way to create unmodifiable lists. The
List instances created by these methods have the following characteristics:
- They are unmodifiable. Elements cannot be added, removed, or replaced. Calling any mutator method on the List will always cause
UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the List's contents to appear to change.
- They disallow
null elements. Attempts to create them with
null elements result in
- They are serializable if all elements are serializable.
- The order of elements in the list is the same as the order of the provided arguments, or of the elements in the provided array.
- They are value-based. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones. Therefore, identity-sensitive operations on these instances (reference equality (
==), identity hash code, and synchronization) are unreliable and should be avoided.
- They are serialized as specified on the Serialized Form page.
This interface is a member of the Java Collections Framework .