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Added in API level 1

List

public interface List
implements Collection<E>

java.util.List<E>
Known Indirect Subclasses


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 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.

See also:

Summary

Public methods

abstract boolean add(E e)

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

abstract void add(int index, E element)

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

abstract 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).

abstract 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).

abstract void clear()

Removes all of the elements from this list (optional operation).

abstract boolean contains(Object o)

Returns true if this list contains the specified element.

abstract boolean containsAll(Collection<?> c)

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

abstract boolean equals(Object o)

Compares the specified object with this list for equality.

abstract E get(int index)

Returns the element at the specified position in this list.

abstract int hashCode()

Returns the hash code value for this list.

abstract 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.

abstract boolean isEmpty()

Returns true if this list contains no elements.

abstract Iterator<E> iterator()

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

abstract 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.

abstract ListIterator<E> listIterator(int index)

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

abstract ListIterator<E> listIterator()

Returns a list iterator over the elements in this list (in proper sequence).

abstract E remove(int index)

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

abstract boolean remove(Object o)

Removes the first occurrence of the specified element from this list, if it is present (optional operation).

abstract boolean removeAll(Collection<?> c)

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

default void replaceAll(UnaryOperator<E> operator)

Replaces each element of this list with the result of applying the operator to that element.

abstract boolean retainAll(Collection<?> c)

Retains only the elements in this list that are contained in the specified collection (optional operation).

abstract E set(int index, E element)

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

abstract int size()

Returns the number of elements in this list.

default void sort(Comparator<? super E> c)

Sorts this list using the supplied Comparator to compare elements.

default Spliterator<E> spliterator()

Creates a Spliterator over the elements in this list.

abstract List<E> subList(int fromIndex, int toIndex)

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.

abstract Object[] toArray()

Returns an array containing all of the elements in this list in proper sequence (from first to last element).

abstract <T> 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.

Inherited methods

From interface java.util.Collection
From interface java.lang.Iterable

Public methods

add

Added in API level 1
boolean add (E e)

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.

Parameters
e E: element to be appended to this list
Returns
boolean true (as specified by add(E))
Throws
UnsupportedOperationException if the add operation is not supported by this list
ClassCastException if the class of the specified element prevents it from being added to this list
NullPointerException if the specified element is null and this list does not permit null elements
IllegalArgumentException if some property of this element prevents it from being added to this list

add

Added in API level 1
void add (int index, 
                E 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 int: index at which the specified element is to be inserted
element E: element to be inserted
Throws
UnsupportedOperationException if the add operation is not supported by this list
ClassCastException if the class of the specified element prevents it from being added to this list
NullPointerException if the specified element is null and this list does not permit null elements
IllegalArgumentException if some property of the specified element prevents it from being added to this list
IndexOutOfBoundsException if the index is out of range (index < 0 || index > size())

addAll

Added in API level 1
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). The behavior of this operation is undefined 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
c Collection: collection containing elements to be added to this list
Returns
boolean true if this list changed as a result of the call
Throws
UnsupportedOperationException if the addAll operation is not supported by this list
ClassCastException if the class of an element of the specified collection prevents it from being added to this list
NullPointerException if the specified collection contains one or more null elements and this list does not permit null elements, or if the specified collection is null
IllegalArgumentException if some property of an element of the specified collection prevents it from being added to this list

See also:

addAll

Added in API level 1
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). 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 undefined 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 int: index at which to insert the first element from the specified collection
c Collection: collection containing elements to be added to this list
Returns
boolean true if this list changed as a result of the call
Throws
UnsupportedOperationException if the addAll operation is not supported by this list
ClassCastException if the class of an element of the specified collection prevents it from being added to this list
NullPointerException if the specified collection contains one or more null elements and this list does not permit null elements, or if the specified collection is null
IllegalArgumentException if some property of an element of the specified collection prevents it from being added to this list
IndexOutOfBoundsException if the index is out of range (index < 0 || index > size())

clear

Added in API level 1
void clear ()

Removes all of the elements from this list (optional operation). The list will be empty after this call returns.

Throws
UnsupportedOperationException if the clear operation is not supported by this list

contains

Added in API level 1
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)).

Parameters
o Object: element whose presence in this list is to be tested
Returns
boolean true if this list contains the specified element
Throws
ClassCastException if the type of the specified element is incompatible with this list (optional)
NullPointerException if the specified element is null and this list does not permit null elements (optional)

containsAll

Added in API level 1
boolean containsAll (Collection<?> c)

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

Parameters
c Collection: collection to be checked for containment in this list
Returns
boolean true if this list contains all of the elements of the specified collection
Throws
ClassCastException if the types of one or more elements in the specified collection are incompatible with this list (optional)
NullPointerException if the specified collection contains one or more null elements and this list does not permit null elements (optional), or if the specified collection is null

See also:

equals

Added in API level 1
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.

Parameters
o Object: the object to be compared for equality with this list
Returns
boolean true if the specified object is equal to this list

get

Added in API level 1
E get (int index)

Returns the element at the specified position in this list.

Parameters
index int: index of the element to return
Returns
E the element at the specified position in this list
Throws
IndexOutOfBoundsException if the index is out of range (index < 0 || index >= size())

hashCode

Added in API level 1
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:

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

Returns
int the hash code value for this list

See also:

indexOf

Added in API level 1
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. 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 Object: element to search for
Returns
int the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element
Throws
ClassCastException if the type of the specified element is incompatible with this list (optional)
NullPointerException if the specified element is null and this list does not permit null elements (optional)

isEmpty

Added in API level 1
boolean isEmpty ()

Returns true if this list contains no elements.

Returns
boolean true if this list contains no elements

iterator

Added in API level 1
Iterator<E> iterator ()

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

Returns
Iterator<E> an iterator over the elements in this list in proper sequence

lastIndexOf

Added in API level 1
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. 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 Object: element to search for
Returns
int the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element
Throws
ClassCastException if the type of the specified element is incompatible with this list (optional)
NullPointerException if the specified element is null and this list does not permit null elements (optional)

listIterator

Added in API level 1
ListIterator<E> listIterator (int index)

Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list. The specified index indicates the first element that would be returned by an initial call to next. An initial call to previous would return the element with the specified index minus one.

Parameters
index int: index of the first element to be returned from the list iterator (by a call to next)
Returns
ListIterator<E> a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list
Throws
IndexOutOfBoundsException if the index is out of range (index < 0 || index > size())

listIterator

Added in API level 1
ListIterator<E> listIterator ()

Returns a list iterator over the elements in this list (in proper sequence).

Returns
ListIterator<E> a list iterator over the elements in this list (in proper sequence)

remove

Added in API level 1
E 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 int: the index of the element to be removed
Returns
E the element previously at the specified position
Throws
UnsupportedOperationException if the remove operation is not supported by this list
IndexOutOfBoundsException if the index is out of range (index < 0 || index >= size())

remove

Added in API level 1
boolean remove (Object o)

Removes the first occurrence of the specified element from this list, if it is present (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). Returns true if this list contained the specified element (or equivalently, if this list changed as a result of the call).

Parameters
o Object: element to be removed from this list, if present
Returns
boolean true if this list contained the specified element
Throws
ClassCastException if the type of the specified element is incompatible with this list (optional)
NullPointerException if the specified element is null and this list does not permit null elements (optional)
UnsupportedOperationException if the remove operation is not supported by this list

removeAll

Added in API level 1
boolean removeAll (Collection<?> c)

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

Parameters
c Collection: collection containing elements to be removed from this list
Returns
boolean true if this list changed as a result of the call
Throws
UnsupportedOperationException if the removeAll operation is not supported by this list
ClassCastException if the class of an element of this list is incompatible with the specified collection (optional)
NullPointerException if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null

See also:

replaceAll

Added in API level 24
void replaceAll (UnaryOperator<E> operator)

Replaces each element of this list with the result of applying the operator to that element. Errors or runtime exceptions thrown by the operator are relayed to the caller.

Implementation Requirements:
  • The default implementation is equivalent to, for this list:
    final ListIterator<E> li = list.listIterator();
         while (li.hasNext()) {
             li.set(operator.apply(li.next()));
         }
     
    If the list's list-iterator does not support the set operation then an UnsupportedOperationException will be thrown when replacing the first element.
Parameters
operator UnaryOperator: the operator to apply to each element
Throws
UnsupportedOperationException if this list is unmodifiable. Implementations may throw this exception if an element cannot be replaced or if, in general, modification is not supported
NullPointerException if the specified operator is null or if the operator result is a null value and this list does not permit null elements (optional)

retainAll

Added in API level 1
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 of its elements that are not contained in the specified collection.

Parameters
c Collection: collection containing elements to be retained in this list
Returns
boolean true if this list changed as a result of the call
Throws
UnsupportedOperationException if the retainAll operation is not supported by this list
ClassCastException if the class of an element of this list is incompatible with the specified collection (optional)
NullPointerException if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null

See also:

set

Added in API level 1
E set (int index, 
                E element)

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

Parameters
index int: index of the element to replace
element E: element to be stored at the specified position
Returns
E the element previously at the specified position
Throws
UnsupportedOperationException if the set operation is not supported by this list
ClassCastException if the class of the specified element prevents it from being added to this list
NullPointerException if the specified element is null and this list does not permit null elements
IllegalArgumentException if some property of the specified element prevents it from being added to this list
IndexOutOfBoundsException if the index is out of range (index < 0 || index >= size())

size

Added in API level 1
int size ()

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

Returns
int the number of elements in this list

sort

Added in API level 24
void sort (Comparator<? super E> c)

Sorts this list using the supplied Comparator to compare elements.

Implementation Requirements:
  • The default implementation is equivalent to, for this list:
    Collections.sort(list, c)
Parameters
c Comparator: the Comparator used to compare list elements. A null value indicates that the elements' natural ordering should be used
Throws
ClassCastException if the list contains elements that are not mutually comparable using the specified comparator
UnsupportedOperationException if the list's list-iterator does not support the set operation
IllegalArgumentException (optional) if the comparator is found to violate the Comparator contract

spliterator

Spliterator<E> spliterator ()

Creates a Spliterator over the elements in this list.

The Spliterator reports SIZED and ORDERED. Implementations should document the reporting of additional characteristic values.

Implementation Requirements:
  • The default implementation creates a late-binding spliterator from the list's Iterator. The spliterator inherits the fail-fast properties of the list's iterator.
Implementation Note:
  • The created Spliterator additionally reports SUBSIZED.
Returns
Spliterator<E> a Spliterator over the elements in this list

subList

Added in API level 1
List<E> 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 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.)

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

toArray

Added in API level 1
Object[] toArray ()

Returns an array containing all of the elements in this list in proper sequence (from first to last element).

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.

Returns
Object[] an array containing all of the elements in this list in proper sequence

See also:

toArray

Added in API level 1
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. If the list 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 list.

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().

Parameters
a T: 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
T[] 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
NullPointerException if the specified array is null
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