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AbstractList

public abstract class AbstractList
extends AbstractCollection<E> implements List<E>

java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.AbstractList<E>
Known Direct Subclasses
Known Indirect Subclasses


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.

Summary

Fields

protected int modCount

The number of times this list has been structurally modified.

Protected constructors

AbstractList()

Sole constructor.

Public methods

void add(int index, E element)

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

This implementation always throws an UnsupportedOperationException.

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).

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).

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.

This implementation first gets a list iterator (with listIterator()).

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.

This implementation first gets a list iterator that points to the end of the list (with listIterator(size())).

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.

This implementation returns a straightforward implementation of the ListIterator interface that extends the implementation of the Iterator interface returned by the iterator() method.

ListIterator<E> listIterator()

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

This implementation returns listIterator(0).

E remove(int index)

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

This implementation always throws an UnsupportedOperationException.

E set(int index, E element)

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

This implementation always throws an UnsupportedOperationException.

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

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

This implementation returns a list that subclasses AbstractList.

Protected methods

void removeRange(int fromIndex, int toIndex)

Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive.

Inherited methods

From class java.util.AbstractCollection
From class java.lang.Object
From interface java.util.Collection
From interface java.util.List
From interface java.lang.Iterable

Fields

modCount

Added in API level 1
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.

Protected constructors

AbstractList

Added in API level 1
AbstractList ()

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

Public methods

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).

This implementation always throws an UnsupportedOperationException.

Parameters
index int: index at which the specified element is to be inserted
element E: element to be inserted
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException
IndexOutOfBoundsException

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.

This implementation calls add(size(), e).

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

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

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.)

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 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
ClassCastException
NullPointerException
IllegalArgumentException
IndexOutOfBoundsException

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.

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

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 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 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

hashCode

Added in API level 1
int hashCode ()

Returns the hash code value for this list.

This implementation uses exactly the code that is used to define the list hash function in the documentation for the hashCode() method.

Returns
int the hash code value for this list

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.

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 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
NullPointerException

iterator

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

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

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
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.

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 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
NullPointerException

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.

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 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

listIterator

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

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

This implementation returns listIterator(0).

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

See also:

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.

This implementation always throws an UnsupportedOperationException.

Parameters
index int: the index of the element to be removed
Returns
E the element previously at the specified position
Throws
UnsupportedOperationException
IndexOutOfBoundsException

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).

This implementation always throws an UnsupportedOperationException.

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
ClassCastException
NullPointerException
IllegalArgumentException
IndexOutOfBoundsException

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.)

This implementation returns a list that subclasses AbstractList. The subclass stores, in private fields, the offset of the subList within the backing list, 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 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 if an endpoint index value is out of range (fromIndex < 0 || toIndex > size)
IllegalArgumentException if the endpoint indices are out of order (fromIndex > toIndex)

Protected methods

removeRange

Added in API level 1
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.

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 int: index of first element to be removed
toIndex int: index after last element to be removed
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