List

public interface List implements Collection<E>

Known Indirect Subclasses AbstractList<E>, AbstractSequentialList<E>, ArrayList<E>, CopyOnWriteArrayList<E>, LinkedList<E>, Stack<E>, Vector<E> AbstractList<E> This class provides a skeletal implementation of the List interface to minimize the effort required to implement this interface backed by a "random access" data store (such as an array).  AbstractSequentialList<E> This class provides a skeletal implementation of the List interface to minimize the effort required to implement this interface backed by a "sequential access" data store (such as a linked list).  ArrayList<E> Resizable-array implementation of the List interface.  CopyOnWriteArrayList<E> A thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.  LinkedList<E> Doubly-linked list implementation of the List and Deque interfaces.  Stack<E> The Stack class represents a last-in-first-out (LIFO) stack of objects.  Vector<E> The Vector class implements a growable array of objects.

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.

Immutable List Static Factory Methods

The ERROR(List.of()/List#of(Object...) List.of()) static factory methods provide a convenient way to create immutable lists. The List instances created by these methods have the following characteristics:

• They are structurally immutable. Elements cannot be added, removed, or replaced. Calling any mutator method 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 NullPointerException.
• 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.

See Also

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

Public Method Summary

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).
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.
void	sort(Comparator<? super E> c) Sorts this list according to the order induced by the specified Comparator.
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 Method Summary

From interface java.util.Collection

abstract boolean	add(E e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns true if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns true if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this collection.
Stream<E>	parallelStream() Returns a possibly parallel Stream with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).
boolean	removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<E>	spliterator() Creates a Spliterator over the elements in this collection.
Stream<E>	stream() Returns a sequential Stream with this collection as its source.
abstract <T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
abstract Iterator<E>	iterator() Returns an iterator over elements of type T.
Spliterator<E>	spliterator() Creates a Spliterator over the elements described by this Iterable.

Public Methods

public abstract 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	element to be appended to this list

Returns

• true (as specified by Collection.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

public abstract 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	index at which the specified element is to be inserted
element	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())

public 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). 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 containing elements to be added to this list

Returns

• 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

• add(Object)

public 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). 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	index at which to insert the first element from the specified collection
c	collection containing elements to be added to this list

Returns

• 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())

public abstract 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

public abstract 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 Objects.equals(o, e).

Parameters

o	element whose presence in this list is to be tested

Returns

• 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)

public abstract boolean containsAll (Collection<?> c)

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

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

• contains(Object)

public abstract 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 Objects.equals(e1, 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	the object to be compared for equality with this list

Returns

• true if the specified object is equal to this list

public abstract E get (int index)

Returns the element at the specified position in this list.

Parameters

index	index of the 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())

public abstract 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 Object.hashCode().

Returns

• the hash code value for this list

See Also

• Object.equals(Object)
• equals(Object)

public 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. More formally, returns the lowest index i such that Objects.equals(o, get(i)), or -1 if there is no such index.

Parameters

o	element to search for

Returns

• 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)

public abstract boolean isEmpty ()

Returns true if this list contains no elements.

Returns

• true if this list contains no elements

public abstract Iterator<E> iterator ()

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

Returns

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

public 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. More formally, returns the highest index i such that Objects.equals(o, get(i)), or -1 if there is no such index.

Parameters

o	element to search for

Returns

• 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)

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

Returns

• 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())

public abstract ListIterator<E> listIterator ()

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

Returns

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

public abstract 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	the index of the element to be removed

Returns

• 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())

public abstract 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 Objects.equals(o, 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	element to be removed from this list, if present

Returns

• 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

public abstract boolean removeAll (Collection<?> c)

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

Parameters

c	collection containing elements to be removed from this list

Returns

• 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

• remove(Object)
• contains(Object)

public 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.

Parameters

operator	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)

public abstract 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 containing elements to be retained in this list

Returns

• 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

• remove(Object)
• contains(Object)

public abstract E set (int index, E element)

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

Parameters

index	index of the 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 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())

public abstract 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

• the number of elements in this list

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

Sorts this list according to the order induced by the specified Comparator.

All elements in this list must be mutually comparable using the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

If the specified comparator is null then all elements in this list must implement the Comparable interface and the elements' {@linkplain Comparable natural ordering} should be used.

This list must be modifiable, but need not be resizable.

For apps running on and targeting Android versions greater than Nougat (API level > 25), Collections.sort(List) delegates to this method. Such apps must not call Collections.sort(List) from this method. Instead, prefer not overriding this method at all. If you must override it, consider this implementation:

 @Override
 public void sort(Comparator<? super E> c) {
   Object[] elements = toArray();
   Arrays.sort(elements, c);
   ListIterator<E> iterator = (ListIterator<Object>) listIterator();
   for (Object element : elements) {
     iterator.next();
     iterator.set((E) element);
   }
 }
 

Parameters

c	the Comparator used to compare list elements. A null value indicates that the elements' {@linkplain Comparable 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

public Spliterator<E> spliterator ()

Creates a Spliterator over the elements in this list.

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

Returns

• a Spliterator over the elements in this list

public 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. (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	low endpoint (inclusive) of the subList
toIndex	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)

public abstract 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

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

See Also

• Arrays.asList(Object[])

public abstract 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	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
NullPointerException	if the specified array is null