java.util
Interface List

All Known Implementing Classes:

AbstractList, LinkedList, Vector, ArrayList

public abstract interface List

extends Collection

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.

Since:

JDK1.2

See Also:

Collection, Set, ArrayList, LinkedList, Vector, Arrays.asList(Object[]), Collections.nCopies(int, Object), Collections.EMPTY_LIST, AbstractList, AbstractSequentialList

Method Summary

void	add(int index, Object element) Inserts the specified element at the specified position in this list (optional operation).
boolean	add(Object o) Appends the specified element to the end of this list (optional operation).
boolean	addAll(Collection 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).
boolean	addAll(int index, Collection c) Inserts all of the elements in the specified collection into this list at the specified position (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.
Object	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 in this list of the first occurrence of the specified element, or -1 if this list does not contain this element.
boolean	isEmpty() Returns true if this list contains no elements.
Iterator	iterator() Returns an iterator over the elements in this list in proper sequence.
int	lastIndexOf(Object o) Returns the index in this list of the last occurrence of the specified element, or -1 if this list does not contain this element.
ListIterator	listIterator() Returns a list iterator of the elements in this list (in proper sequence).
ListIterator	listIterator(int index) Returns a list iterator of the elements in this list (in proper sequence), starting at the specified position in this list.
Object	remove(int index) Removes the element at the specified position in this list (optional operation).
boolean	remove(Object o) Removes the first occurrence in this list of the specified element (optional operation).
boolean	removeAll(Collection c) Removes from this list all the 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).
Object	set(int index, Object 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.
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.
Object[]	toArray(Object[] a) Returns an array containing all of the elements in this list in proper sequence; the runtime type of the returned array is that of the specified array.

Method Detail

size

public int size()

Returns the number of elements in this list. If this list contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Specified by:

size in interface Collection

Returns:

the number of elements in this list.

isEmpty

public boolean isEmpty()

Returns true if this list contains no elements.

Specified by:

isEmpty in interface Collection

Returns:

true if this list contains no elements.

contains

public boolean contains(Object o)

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

Specified by:

contains in interface Collection

Parameters:

o - element whose presence in this list is to be tested.

iterator

public Iterator iterator()

Returns an iterator over the elements in this list in proper sequence.

Specified by:

iterator in interface Collection

Returns:

an iterator over the elements in this list in proper sequence.

toArray

public Object[] toArray()

Returns an array containing all of the elements in this list in proper sequence. Obeys the general contract of the Collection.toArray method.

Specified by:

toArray in interface Collection

Returns:

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

See Also:

Arrays.asList(Object[])

toArray

public Object[] toArray(Object[] a)

Returns an array containing all of the elements in this list in proper sequence; the runtime type of the returned array is that of the specified array. Obeys the general contract of the Collection.toArray(Object[]) method.

Specified by:

toArray in interface Collection

Parameters:

a - the array into which the elements of this list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.

Returns:

an array containing the elements of this list.

Throws:

ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this list.

add

public boolean add(Object o)

Appends the specified element to the end of this list (optional operation).

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.

Specified by:

add in interface Collection

Parameters:

o - element to be appended to this list.

Returns:

true (as per the general contract of the Collection.add method).

Throws:

UnsupportedOperationException - if the add method is not supported by this list.

ClassCastException - if the class of the specified element prevents it from being added to this list.

IllegalArgumentException - if some aspect of this element prevents it from being added to this collection.

remove

public boolean remove(Object o)

Removes the first occurrence in this list of the specified element (optional operation). If this list does not contain the element, it is unchanged. More formally, removes the element with the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))) (if such an element exists).

Specified by:

remove in interface Collection

Parameters:

o - element to be removed from this list, if present.

Returns:

true if this list contained the specified element.

Throws:

UnsupportedOperationException - if the remove method is not supported by this list.

containsAll

public boolean containsAll(Collection c)

Returns true if this list contains all of the elements of the specified collection.

Specified by:

containsAll in interface Collection

Parameters:

c - collection to be checked for containment in this list.

Returns:

true if this list contains all of the elements of the specified collection.

See Also:

contains(Object)

addAll

public boolean addAll(Collection 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). 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.)

Specified by:

addAll in interface Collection

Parameters:

c - collection whose elements are to be added to this list.

Returns:

true if this list changed as a result of the call.

Throws:

UnsupportedOperationException - if the addAll method is not supported by this list.

ClassCastException - if the class of an element in the specified collection prevents it from being added to this list.

IllegalArgumentException - if some aspect of an element in the specified collection prevents it from being added to this list.

See Also:

add(Object)

addAll

public boolean addAll(int index,
                      Collection c)

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.)

Parameters:

index - index at which to insert first element from the specified collection.

c - elements to be inserted into this list.

Returns:

true if this list changed as a result of the call.

Throws:

UnsupportedOperationException - if the addAll method is not supported by this list.

ClassCastException - if the class of one of elements of the specified collection prevents it from being added to this list.

IllegalArgumentException - if some aspect of one of elements of the specified collection prevents it from being added to this list.

IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size()).

removeAll

public boolean removeAll(Collection c)

Removes from this list all the elements that are contained in the specified collection (optional operation).

Specified by:

removeAll in interface Collection

Parameters:

c - collection that defines which elements will be removed from this list.

Returns:

true if this list changed as a result of the call.

Throws:

UnsupportedOperationException - if the removeAll method is not supported by this list.

See Also:

remove(Object), contains(Object)

retainAll

public boolean retainAll(Collection c)

Retains only the elements in this list that are contained in the specified collection (optional operation). In other words, removes from this list all the elements that are not contained in the specified collection.

Specified by:

retainAll in interface Collection

Parameters:

c - collection that defines which elements this set will retain.

Returns:

true if this list changed as a result of the call.

Throws:

UnsupportedOperationException - if the retainAll method is not supported by this list.

See Also:

remove(Object), contains(Object)

clear

public void clear()

Removes all of the elements from this list (optional operation). This list will be empty after this call returns (unless it throws an exception).

Specified by:

clear in interface Collection

Throws:

UnsupportedOperationException - if the clear method is not supported by this list.

equals

public boolean equals(Object o)

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.

Specified by:

equals in interface Collection

Parameters:

o - the object to be compared for equality with this list.

Returns:

true if the specified object is equal to this list.

Overrides:

equals in class Object

hashCode

public int hashCode()

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.

Specified by:

hashCode in interface Collection

Returns:

the hash code value for this list.

Overrides:

hashCode in class Object

See Also:

Object.hashCode(), Object.equals(Object), equals(Object)

get

public Object get(int index)

Returns the element at the specified position in this list.

Parameters:

index - index of element to return.

Returns:

the element at the specified position in this list.

Throws:

IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size()).

set

public Object set(int index,
                  Object element)

Replaces the element at the specified position in this list with the specified element (optional operation).

Parameters:

index - index of element to replace.

element - element to be stored at the specified position.

Returns:

the element previously at the specified position.

Throws:

UnsupportedOperationException - if the set method is not supported by this list.

ClassCastException - if the class of the specified element prevents it from being added to this list.

IllegalArgumentException - if some aspect of the specified element prevents it from being added to this list.

IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size()).

add

public void add(int index,
                Object element)

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).

Parameters:

index - index at which the specified element is to be inserted.

element - element to be inserted.

Throws:

UnsupportedOperationException - if the add method is not supported by this list.

ClassCastException - if the class of the specified element prevents it from being added to this list.

IllegalArgumentException - if some aspect of the specified element prevents it from being added to this list.

IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size()).

remove

public Object remove(int index)

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.

Parameters:

index - the index of the element to removed.

Returns:

the element previously at the specified position.

Throws:

UnsupportedOperationException - if the remove method is not supported by this list.

IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size()).

indexOf

public int indexOf(Object o)

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.

Parameters:

o - element to search for.

Returns:

the index in this list of the first occurrence of the specified element, or -1 if this list does not contain this element.

lastIndexOf

public int lastIndexOf(Object o)

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.

Parameters:

o - element to search for.

Returns:

the index in this list of the last occurrence of the specified element, or -1 if this list does not contain this element.

listIterator

public ListIterator listIterator()

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).

listIterator

public ListIterator listIterator(int index)

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 nextElement method. An initial call to the previousElement method would return the element with the specified index minus one.

Parameters:

index - index of first element to be returned from the list iterator (by a call to the next method).

Returns:

a list iterator of the elements in this list (in proper sequence), starting at the specified position in this list.

Throws:

IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size()).

subList

public List subList(int fromIndex,
                    int toIndex)

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 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 this 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.

toKey - high endpoint (exclusive) of the subList.

Returns:

a view of the specified range within this list.

Throws:

IndexOutOfBoundsException - for an illegal endpoint index value (fromIndex < 0 || toIndex > size || fromIndex > toIndex).