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

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.
author:
   Josh Bloch
author:
   Neal Gafter
version:
   1.58, 06/24/07
See Also:   Collection
See Also:   Set
See Also:   ArrayList
See Also:   LinkedList
See Also:   Vector
See Also:   Arrays.asList(Object[])
See Also:   Collections.nCopies(intObject)
See Also:   Collections.EMPTY_LIST
See Also:   AbstractList
See Also:   AbstractSequentialList
since:
   1.2

Lists that support this operation may place limitations on what elements may be added to this list.

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

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.
Parameters:
  e - element to be appended to this list true (as specified by Collection.add)
throws:
  UnsupportedOperationException - if the add operationis not supported by this list
throws:
  ClassCastException - if the class of the specified elementprevents it from being added to this list
throws:
  NullPointerException - if the specified element is null and thislist does not permit null elements
throws:
  IllegalArgumentException - if some property of this elementprevents it from being added to this list

 int hashCode = 1;
 for (E e : list)
 hashCode = 31*hashCode + (e==null ? 0 : e.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();
 

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.)
Parameters:
  fromIndex - low endpoint (inclusive) of the subList
Parameters:
  toIndex - high endpoint (exclusive) of the subList a view of the specified range within this list
throws:
  IndexOutOfBoundsException - for an illegal endpoint index value(fromIndex < 0 || toIndex > size ||fromIndex > toIndex)

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. an array containing all of the elements in this list in propersequence
See Also:   Arrays.asList(Object[])

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 List.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]);