Module java.base
Package java.util

Class AbstractList<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractList<E>
All Implemented Interfaces:
Iterable<E>, Collection<E>, List<E>
Direct Known Subclasses:
AbstractSequentialList, ArrayList, Vector
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<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). For sequential access data (such as a linked list), AbstractSequentialList should be used in preference to this class.

To implement an unmodifiable list, the programmer needs only to extend this class and provide implementations for the get(int) and size() methods.

To implement a modifiable list, the programmer must additionally override the set(int, E) method (which otherwise throws an UnsupportedOperationException). If the list is variable-size the programmer must additionally override the add(int, E) and remove(int) methods.

The programmer should generally provide a void (no argument) and collection constructor, as per the recommendation in the Collection interface specification.

Unlike the other abstract collection implementations, the programmer does not have to provide an iterator implementation; the iterator and list iterator are implemented by this class, on top of the "random access" methods: get(int), set(int, E), add(int, E) and remove(int).

The documentation for each non-abstract method in this class describes its implementation in detail. Each of these methods may be overridden if the collection being implemented admits a more efficient implementation.

This class is a member of the Java Collections Framework.

Since:
1.2

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    protected int
    modCount
    The number of times this list has been structurally modified.

  • Constructor Summary

    Constructors
    Modifier
    Constructor
    Description
    protected
    AbstractList()
    Sole constructor.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    add​(int index, E element)
    Inserts the specified element at the specified position in this list (optional operation).
    boolean
    add​(E e)
    Appends the specified element to the end of this list (optional operation).
    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).
    void
    clear()
    Removes all of the elements from this list (optional operation).
    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.
    int
    hashCode()
    Returns the hash code value for 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.
    Iterator<E>
    iterator()
    Returns an iterator over the elements in this list in proper sequence.
    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()
    Returns a list iterator over the elements in this list (in proper sequence).
    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.
    E
    remove​(int index)
    Removes the element at the specified position in this list (optional operation).
    protected void
    removeRange​(int fromIndex, int toIndex)
    Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive.
    E
    set​(int index, E element)
    Replaces the element at the specified position in this list with the specified element (optional operation).
    List<E>
    subList​(int fromIndex, int toIndex)
    Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.

    Methods declared in class java.util.AbstractCollection

    addAll, contains, containsAll, isEmpty, remove, removeAll, retainAll, toArray, toArray, toString

    Methods declared in class java.lang.Object

    clone, finalize, getClass, notify, notifyAll, wait, wait, wait

    Methods declared in interface java.util.Collection

    parallelStream, removeIf, stream, toArray

    Methods declared in interface java.lang.Iterable

    forEach

    Methods declared in interface java.util.List

    addAll, contains, containsAll, isEmpty, remove, removeAll, replaceAll, retainAll, size, sort, spliterator, toArray, toArray

  • Field Details

    • modCount

      protected transient int modCount
      The number of times this list has been structurally modified. Structural modifications are those that change the size of the list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.

      This field is used by the iterator and list iterator implementation returned by the iterator and listIterator methods. If the value of this field changes unexpectedly, the iterator (or list iterator) will throw a ConcurrentModificationException in response to the next, remove, previous, set or add operations. This provides fail-fast behavior, rather than non-deterministic behavior in the face of concurrent modification during iteration.

      Use of this field by subclasses is optional. If a subclass wishes to provide fail-fast iterators (and list iterators), then it merely has to increment this field in its add(int, E) and remove(int) methods (and any other methods that it overrides that result in structural modifications to the list). A single call to add(int, E) or remove(int) must add no more than one to this field, or the iterators (and list iterators) will throw bogus ConcurrentModificationExceptions. If an implementation does not wish to provide fail-fast iterators, this field may be ignored.

  • Constructor Details

    • AbstractList

      protected AbstractList()
      Sole constructor. (For invocation by subclass constructors, typically implicit.)

  • Method Details

    • add

      public 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.

      Specified by:
      add in interface Collection<E>
      Specified by:
      add in interface List<E>
      Overrides:
      add in class AbstractCollection<E>
      Implementation Requirements:
      This implementation calls add(size(), e).

      Note that this implementation throws an UnsupportedOperationException unless add(int, E) is overridden.

      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

    • get

      public abstract E get​(int index)
      Returns the element at the specified position in this list.
      Specified by:
      get in interface List<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 (optional operation).
      Specified by:
      set in interface List<E>
      Implementation Requirements:
      This implementation always throws an UnsupportedOperationException.
      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())

    • add

      public 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).
      Specified by:
      add in interface List<E>
      Implementation Requirements:
      This implementation always throws an UnsupportedOperationException.
      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())

    • remove

      public 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.
      Specified by:
      remove in interface List<E>
      Implementation Requirements:
      This implementation always throws an UnsupportedOperationException.
      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())

    • 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>
      Implementation Requirements:
      This implementation first gets a list iterator (with listIterator()). Then, it iterates over the list until the specified element is found or the end of the list is reached.
      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)

    • 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>
      Implementation Requirements:
      This implementation first gets a list iterator that points to the end of the list (with listIterator(size())). Then, it iterates backwards over the list until the specified element is found, or the beginning of the list is reached.
      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)

    • clear

      public void clear()
      Removes all of the elements from this list (optional operation). 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 AbstractCollection<E>
      Implementation Requirements:
      This implementation calls removeRange(0, size()).

      Note that this implementation throws an UnsupportedOperationException unless remove(int index) or removeRange(int fromIndex, int toIndex) is overridden.

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

    • addAll

      public 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.)
      Specified by:
      addAll in interface List<E>
      Implementation Requirements:
      This implementation gets an iterator over the specified collection and iterates over it, inserting the elements obtained from the iterator into this list at the appropriate position, one at a time, using add(int, E). Many implementations will override this method for efficiency.

      Note that this implementation throws an UnsupportedOperationException unless add(int, E) is overridden.

      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())

    • iterator

      public Iterator<E> iterator()
      Returns an iterator over the elements in this list in proper sequence.
      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in interface List<E>
      Specified by:
      iterator in class AbstractCollection<E>
      Implementation Requirements:
      This implementation returns a straightforward implementation of the iterator interface, relying on the backing list's size(), get(int), and remove(int) methods.

      Note that the iterator returned by this method will throw an UnsupportedOperationException in response to its remove method unless the list's remove(int) method is overridden.

      This implementation can be made to throw runtime exceptions in the face of concurrent modification, as described in the specification for the (protected) modCount field.

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

    • listIterator

      public ListIterator<E> listIterator()
      Returns a list iterator over the elements in this list (in proper sequence).
      Specified by:
      listIterator in interface List<E>
      Implementation Requirements:
      This implementation returns listIterator(0).
      Returns:
      a list iterator over the elements in this list (in proper sequence)
      See Also:
      listIterator(int)

    • listIterator

      public 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.
      Specified by:
      listIterator in interface List<E>
      Implementation Requirements:
      This implementation returns a straightforward implementation of the ListIterator interface that extends the implementation of the Iterator interface returned by the iterator() method. The ListIterator implementation relies on the backing list's get(int), set(int, E), add(int, E) and remove(int) methods.

      Note that the list iterator returned by this implementation will throw an UnsupportedOperationException in response to its remove, set and add methods unless the list's remove(int), set(int, E), and add(int, E) methods are overridden.

      This implementation can be made to throw runtime exceptions in the face of concurrent modification, as described in the specification for the (protected) modCount field.

      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())

    • subList

      public 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.)

      Specified by:
      subList in interface List<E>
      Implementation Requirements:
      This implementation returns a list that subclasses AbstractList. The subclass stores, in private fields, the size of the subList (which can change over its lifetime), and the expected modCount value of the backing list. There are two variants of the subclass, one of which implements RandomAccess. If this list implements RandomAccess the returned list will be an instance of the subclass that implements RandomAccess.

      The subclass's set(int, E), get(int), add(int, E), remove(int), addAll(int, Collection) and removeRange(int, int) methods all delegate to the corresponding methods on the backing abstract list, after bounds-checking the index and adjusting for the offset. The addAll(Collection c) method merely returns addAll(size, c).

      The listIterator(int) method returns a "wrapper object" over a list iterator on the backing list, which is created with the corresponding method on the backing list. The iterator method merely returns listIterator(), and the size method merely returns the subclass's size field.

      All methods first check to see if the actual modCount of the backing list is equal to its expected value, and throw a ConcurrentModificationException if it is not.

      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 - if an endpoint index value is out of range (fromIndex < 0 || toIndex > size)
      IllegalArgumentException - if the endpoint indices are out of order (fromIndex > toIndex)

    • 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.
      Specified by:
      equals in interface Collection<E>
      Specified by:
      equals in interface List<E>
      Overrides:
      equals in class Object
      Implementation Requirements:
      This implementation first checks if the specified object is this list. If so, it returns true; if not, it checks if the specified object is a list. If not, it returns false; if so, it iterates over both lists, comparing corresponding pairs of elements. If any comparison returns false, this method returns false. If either iterator runs out of elements before the other it returns false (as the lists are of unequal length); otherwise it returns true when the iterations complete.
      Parameters:
      o - the object to be compared for equality with this list
      Returns:
      true if the specified object is equal to this list
      See Also:
      Object.hashCode(), HashMap

    • hashCode

      public int hashCode()
      Returns the hash code value for this list.
      Specified by:
      hashCode in interface Collection<E>
      Specified by:
      hashCode in interface List<E>
      Overrides:
      hashCode in class Object
      Implementation Requirements:
      This implementation uses exactly the code that is used to define the list hash function in the documentation for the List.hashCode() method.
      Returns:
      the hash code value for this list
      See Also:
      Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)

    • removeRange

      protected void removeRange​(int fromIndex, int toIndex)
      Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive. Shifts any succeeding elements to the left (reduces their index). This call shortens the list by (toIndex - fromIndex) elements. (If toIndex==fromIndex, this operation has no effect.)

      This method is called by the clear operation on this list and its subLists. Overriding this method to take advantage of the internals of the list implementation can substantially improve the performance of the clear operation on this list and its subLists.

      Implementation Requirements:
      This implementation gets a list iterator positioned before fromIndex, and repeatedly calls ListIterator.next followed by ListIterator.remove until the entire range has been removed. Note: if ListIterator.remove requires linear time, this implementation requires quadratic time.
      Parameters:
      fromIndex - index of first element to be removed
      toIndex - index after last element to be removed