Module java.base
Package java.util

Class LinkedList<E>

  • Type Parameters:
    E - the type of elements held in this collection
    All Implemented Interfaces:
    Serializable, Cloneable, Iterable<E>, Collection<E>, Deque<E>, List<E>, Queue<E>

    public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, Serializable
    Doubly-linked list implementation of the List and Deque interfaces. Implements all optional list operations, and permits all elements (including null).

    All of the operations perform as could be expected for a doubly-linked list. Operations that index into the list will traverse the list from the beginning or the end, whichever is closer to the specified index.

    Note that this implementation is not synchronized. If multiple threads access a linked list concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

       List list = Collections.synchronizedList(new LinkedList(...));

    The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the Iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

    Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

    This class is a member of the Java Collections Framework.

    Since:
    1.2
    See Also:
    List, ArrayList, Serialized Form

    • Constructor Summary

      Constructors 
      Constructor Description
      LinkedList​()
      Constructs an empty list.
      LinkedList​(Collection<? extends E> c)
      Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      void add​(int index, E element)
      Inserts the specified element at the specified position in this list.
      boolean add​(E e)
      Appends the specified element to the end of this list.
      boolean addAll​(int index, Collection<? extends E> c)
      Inserts all of the elements in the specified collection into this list, starting at the specified position.
      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.
      void addFirst​(E e)
      Inserts the specified element at the beginning of this list.
      void addLast​(E e)
      Appends the specified element to the end of this list.
      void clear​()
      Removes all of the elements from this list.
      Object clone​()
      Returns a shallow copy of this LinkedList.
      boolean contains​(Object o)
      Returns true if this list contains the specified element.
      Iterator<E> descendingIterator​()
      Returns an iterator over the elements in this deque in reverse sequential order.
      E element​()
      Retrieves, but does not remove, the head (first element) of this list.
      E get​(int index)
      Returns the element at the specified position in this list.
      E getFirst​()
      Returns the first element in this list.
      E getLast​()
      Returns the last element in this list.
      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.
      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.
      ListIterator<E> listIterator​(int index)
      Returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in the list.
      boolean offer​(E e)
      Adds the specified element as the tail (last element) of this list.
      boolean offerFirst​(E e)
      Inserts the specified element at the front of this list.
      boolean offerLast​(E e)
      Inserts the specified element at the end of this list.
      E peek​()
      Retrieves, but does not remove, the head (first element) of this list.
      E peekFirst​()
      Retrieves, but does not remove, the first element of this list, or returns null if this list is empty.
      E peekLast​()
      Retrieves, but does not remove, the last element of this list, or returns null if this list is empty.
      E poll​()
      Retrieves and removes the head (first element) of this list.
      E pollFirst​()
      Retrieves and removes the first element of this list, or returns null if this list is empty.
      E pollLast​()
      Retrieves and removes the last element of this list, or returns null if this list is empty.
      E pop​()
      Pops an element from the stack represented by this list.
      void push​(E e)
      Pushes an element onto the stack represented by this list.
      E remove​()
      Retrieves and removes the head (first element) of this list.
      E remove​(int index)
      Removes the element at the specified position in this list.
      boolean remove​(Object o)
      Removes the first occurrence of the specified element from this list, if it is present.
      E removeFirst​()
      Removes and returns the first element from this list.
      boolean removeFirstOccurrence​(Object o)
      Removes the first occurrence of the specified element in this list (when traversing the list from head to tail).
      E removeLast​()
      Removes and returns the last element from this list.
      boolean removeLastOccurrence​(Object o)
      Removes the last occurrence of the specified element in this list (when traversing the list from head to tail).
      E set​(int index, E element)
      Replaces the element at the specified position in this list with the specified element.
      int size​()
      Returns the number of elements in this list.
      Spliterator<E> spliterator​()
      Creates a late-binding and fail-fast Spliterator over the elements in this list.
      Object[] toArray​()
      Returns an array containing all of the elements in this list in proper sequence (from first to last element).
      <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.

    • Constructor Detail

      • LinkedList

        public LinkedList​()
        Constructs an empty list.

      • LinkedList

        public LinkedList​(Collection<? extends E> c)
        Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
        Parameters:
        c - the collection whose elements are to be placed into this list
        Throws:
        NullPointerException - if the specified collection is null

    • Method Detail

      • getFirst

        public E getFirst​()
        Returns the first element in this list.
        Specified by:
        getFirst in interface Deque<E>
        Returns:
        the first element in this list
        Throws:
        NoSuchElementException - if this list is empty

      • getLast

        public E getLast​()
        Returns the last element in this list.
        Specified by:
        getLast in interface Deque<E>
        Returns:
        the last element in this list
        Throws:
        NoSuchElementException - if this list is empty

      • removeFirst

        public E removeFirst​()
        Removes and returns the first element from this list.
        Specified by:
        removeFirst in interface Deque<E>
        Returns:
        the first element from this list
        Throws:
        NoSuchElementException - if this list is empty

      • removeLast

        public E removeLast​()
        Removes and returns the last element from this list.
        Specified by:
        removeLast in interface Deque<E>
        Returns:
        the last element from this list
        Throws:
        NoSuchElementException - if this list is empty

      • addFirst

        public void addFirst​(E e)
        Inserts the specified element at the beginning of this list.
        Specified by:
        addFirst in interface Deque<E>
        Parameters:
        e - the element to add

      • addLast

        public void addLast​(E e)
        Appends the specified element to the end of this list.

        This method is equivalent to add(E).

        Specified by:
        addLast in interface Deque<E>
        Parameters:
        e - the element to add

      • 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 Objects.equals(o, e).
        Specified by:
        contains in interface Collection<E>
        Specified by:
        contains in interface Deque<E>
        Specified by:
        contains in interface List<E>
        Overrides:
        contains in class AbstractCollection<E>
        Parameters:
        o - element whose presence in this list is to be tested
        Returns:
        true if this list contains the specified element

      • size

        public int size​()
        Returns the number of elements in this list.
        Specified by:
        size in interface Collection<E>
        Specified by:
        size in interface Deque<E>
        Specified by:
        size in interface List<E>
        Specified by:
        size in class AbstractCollection<E>
        Returns:
        the number of elements in this list

      • add

        public boolean add​(E e)
        Appends the specified element to the end of this list.

        This method is equivalent to addLast(E).

        Specified by:
        add in interface Collection<E>
        Specified by:
        add in interface Deque<E>
        Specified by:
        add in interface List<E>
        Specified by:
        add in interface Queue<E>
        Overrides:
        add in class AbstractList<E>
        Parameters:
        e - element to be appended to this list
        Returns:
        true (as specified by Collection.add(E))

      • remove

        public boolean remove​(Object o)
        Removes the first occurrence of the specified element from this list, if it is present. 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).
        Specified by:
        remove in interface Collection<E>
        Specified by:
        remove in interface Deque<E>
        Specified by:
        remove in interface List<E>
        Overrides:
        remove in class AbstractCollection<E>
        Parameters:
        o - element to be removed from this list, if present
        Returns:
        true if this list contained the specified element

      • addAll

        public 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. 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.)
        Specified by:
        addAll in interface Collection<E>
        Specified by:
        addAll in interface Deque<E>
        Specified by:
        addAll in interface List<E>
        Overrides:
        addAll in class AbstractCollection<E>
        Parameters:
        c - collection containing elements to be added to this list
        Returns:
        true if this list changed as a result of the call
        Throws:
        NullPointerException - if the specified collection is null
        See Also:
        AbstractCollection.add(Object)

      • addAll

        public boolean addAll​(int index,
                      Collection<? extends E> c)
        Inserts all of the elements in the specified collection into this list, starting at the specified position. 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 the list in the order that they are returned by the specified collection's iterator.
        Specified by:
        addAll in interface List<E>
        Overrides:
        addAll in class AbstractSequentialList<E>
        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:
        IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())
        NullPointerException - if the specified collection is null

      • clear

        public void clear​()
        Removes all of the elements from this list. The list will be empty after this call returns.
        Specified by:
        clear in interface Collection<E>
        Specified by:
        clear in interface List<E>
        Overrides:
        clear in class AbstractList<E>

      • get

        public E get​(int index)
        Returns the element at the specified position in this list.
        Specified by:
        get in interface List<E>
        Overrides:
        get in class AbstractSequentialList<E>
        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())

      • set

        public E set​(int index,
             E element)
        Replaces the element at the specified position in this list with the specified element.
        Specified by:
        set in interface List<E>
        Overrides:
        set in class AbstractSequentialList<E>
        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:
        IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size())

      • add

        public void add​(int index,
                E element)
        Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).
        Specified by:
        add in interface List<E>
        Overrides:
        add in class AbstractSequentialList<E>
        Parameters:
        index - index at which the specified element is to be inserted
        element - element to be inserted
        Throws:
        IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())

      • remove

        public E remove​(int index)
        Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices). Returns the element that was removed from the list.
        Specified by:
        remove in interface List<E>
        Overrides:
        remove in class AbstractSequentialList<E>
        Parameters:
        index - the index of the element to be removed
        Returns:
        the element previously at the specified position
        Throws:
        IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size())

      • indexOf

        public 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.
        Specified by:
        indexOf in interface List<E>
        Overrides:
        indexOf in class AbstractList<E>
        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

      • lastIndexOf

        public 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.
        Specified by:
        lastIndexOf in interface List<E>
        Overrides:
        lastIndexOf in class AbstractList<E>
        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

      • peek

        public E peek​()
        Retrieves, but does not remove, the head (first element) of this list.
        Specified by:
        peek in interface Deque<E>
        Specified by:
        peek in interface Queue<E>
        Returns:
        the head of this list, or null if this list is empty
        Since:
        1.5

      • element

        public E element​()
        Retrieves, but does not remove, the head (first element) of this list.
        Specified by:
        element in interface Deque<E>
        Specified by:
        element in interface Queue<E>
        Returns:
        the head of this list
        Throws:
        NoSuchElementException - if this list is empty
        Since:
        1.5

      • poll

        public E poll​()
        Retrieves and removes the head (first element) of this list.
        Specified by:
        poll in interface Deque<E>
        Specified by:
        poll in interface Queue<E>
        Returns:
        the head of this list, or null if this list is empty
        Since:
        1.5

      • remove

        public E remove​()
        Retrieves and removes the head (first element) of this list.
        Specified by:
        remove in interface Deque<E>
        Specified by:
        remove in interface Queue<E>
        Returns:
        the head of this list
        Throws:
        NoSuchElementException - if this list is empty
        Since:
        1.5

      • offer

        public boolean offer​(E e)
        Adds the specified element as the tail (last element) of this list.
        Specified by:
        offer in interface Deque<E>
        Specified by:
        offer in interface Queue<E>
        Parameters:
        e - the element to add
        Returns:
        true (as specified by Queue.offer(E))
        Since:
        1.5

      • offerFirst

        public boolean offerFirst​(E e)
        Inserts the specified element at the front of this list.
        Specified by:
        offerFirst in interface Deque<E>
        Parameters:
        e - the element to insert
        Returns:
        true (as specified by Deque.offerFirst(E))
        Since:
        1.6

      • offerLast

        public boolean offerLast​(E e)
        Inserts the specified element at the end of this list.
        Specified by:
        offerLast in interface Deque<E>
        Parameters:
        e - the element to insert
        Returns:
        true (as specified by Deque.offerLast(E))
        Since:
        1.6

      • peekFirst

        public E peekFirst​()
        Retrieves, but does not remove, the first element of this list, or returns null if this list is empty.
        Specified by:
        peekFirst in interface Deque<E>
        Returns:
        the first element of this list, or null if this list is empty
        Since:
        1.6

      • peekLast

        public E peekLast​()
        Retrieves, but does not remove, the last element of this list, or returns null if this list is empty.
        Specified by:
        peekLast in interface Deque<E>
        Returns:
        the last element of this list, or null if this list is empty
        Since:
        1.6

      • pollFirst

        public E pollFirst​()
        Retrieves and removes the first element of this list, or returns null if this list is empty.
        Specified by:
        pollFirst in interface Deque<E>
        Returns:
        the first element of this list, or null if this list is empty
        Since:
        1.6

      • pollLast

        public E pollLast​()
        Retrieves and removes the last element of this list, or returns null if this list is empty.
        Specified by:
        pollLast in interface Deque<E>
        Returns:
        the last element of this list, or null if this list is empty
        Since:
        1.6

      • push

        public void push​(E e)
        Pushes an element onto the stack represented by this list. In other words, inserts the element at the front of this list.

        This method is equivalent to addFirst(E).

        Specified by:
        push in interface Deque<E>
        Parameters:
        e - the element to push
        Since:
        1.6

      • pop

        public E pop​()
        Pops an element from the stack represented by this list. In other words, removes and returns the first element of this list.

        This method is equivalent to removeFirst().

        Specified by:
        pop in interface Deque<E>
        Returns:
        the element at the front of this list (which is the top of the stack represented by this list)
        Throws:
        NoSuchElementException - if this list is empty
        Since:
        1.6

      • removeFirstOccurrence

        public boolean removeFirstOccurrence​(Object o)
        Removes the first occurrence of the specified element in this list (when traversing the list from head to tail). If the list does not contain the element, it is unchanged.
        Specified by:
        removeFirstOccurrence in interface Deque<E>
        Parameters:
        o - element to be removed from this list, if present
        Returns:
        true if the list contained the specified element
        Since:
        1.6

      • removeLastOccurrence

        public boolean removeLastOccurrence​(Object o)
        Removes the last occurrence of the specified element in this list (when traversing the list from head to tail). If the list does not contain the element, it is unchanged.
        Specified by:
        removeLastOccurrence in interface Deque<E>
        Parameters:
        o - element to be removed from this list, if present
        Returns:
        true if the list contained the specified element
        Since:
        1.6

      • listIterator

        public ListIterator<E> listIterator​(int index)
        Returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in the list. Obeys the general contract of List.listIterator(int).

        The list-iterator is fail-fast: if the list is structurally modified at any time after the Iterator is created, in any way except through the list-iterator's own remove or add methods, the list-iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

        Specified by:
        listIterator in interface List<E>
        Specified by:
        listIterator in class AbstractSequentialList<E>
        Parameters:
        index - index of the first element to be returned from the list-iterator (by a call to next)
        Returns:
        a ListIterator of 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())
        See Also:
        List.listIterator(int)

      • descendingIterator

        public Iterator<E> descendingIterator​()
        Description copied from interface: Deque
        Returns an iterator over the elements in this deque in reverse sequential order. The elements will be returned in order from last (tail) to first (head).
        Specified by:
        descendingIterator in interface Deque<E>
        Returns:
        an iterator over the elements in this deque in reverse sequence
        Since:
        1.6

      • clone

        public Object clone​()
        Returns a shallow copy of this LinkedList. (The elements themselves are not cloned.)
        Overrides:
        clone in class Object
        Returns:
        a shallow copy of this LinkedList instance
        See Also:
        Cloneable

      • toArray

        public 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). The caller is thus free to modify the returned array.

        This method acts as bridge between array-based and collection-based APIs.

        Specified by:
        toArray in interface Collection<E>
        Specified by:
        toArray in interface List<E>
        Overrides:
        toArray in class AbstractCollection<E>
        Returns:
        an array containing all of the elements in this list in proper sequence
        See Also:
        Arrays.asList(Object[])

      • toArray

        public <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. 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().
        Specified by:
        toArray in interface Collection<E>
        Specified by:
        toArray in interface List<E>
        Overrides:
        toArray in class AbstractCollection<E>
        Type Parameters:
        T - the runtime type of the array to contain the collection
        Parameters:
        a - the array into which the elements of the 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 the 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