Java™ Platform
Standard Ed. 6

java.util
Class AbstractList<E>

java.lang.Object
  extended by java.util.AbstractCollection<E>
      extended by 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
protected  int modCount
          The number of times this list has been structurally modified.
 
Constructor Summary
protected AbstractList()
          Sole constructor.
 
Method Summary
 boolean add(E e)
          Appends the specified element to the end of this list (optional operation).
 void add(int index, E element)
          Inserts the specified element at the specified position in 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 of the elements in this list (in proper sequence), starting at the specified position in this 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 inherited from class java.util.AbstractCollection
addAll, contains, containsAll, isEmpty, remove, removeAll, retainAll, size, toArray, toArray, toString
 
Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, wait, wait, wait
 
Methods inherited from interface java.util.List
addAll, contains, containsAll, isEmpty, remove, removeAll, retainAll, size, toArray, toArray
 

Field Detail

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 Detail

AbstractList

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

Method Detail

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.

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

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

Specified by:
add in interface Collection<E>
Specified by:
add in interface List<E>
Overrides:
add in class AbstractCollection<E>
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).

This implementation always throws an UnsupportedOperationException.

Specified by:
set in interface List<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:
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).

This implementation always throws an UnsupportedOperationException.

Specified by:
add in interface List<E>
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.

This implementation always throws an UnsupportedOperationException.

Specified by:
remove in interface List<E>
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 (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.

Specified by:
indexOf in interface List<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
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 (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.

Specified by:
lastIndexOf in interface List<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
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.

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.

Specified by:
clear in interface Collection<E>
Specified by:
clear in interface List<E>
Overrides:
clear in class AbstractCollection<E>
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.)

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.

Specified by:
addAll in interface List<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:
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.

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.

Specified by:
iterator in interface Iterable<E>
Specified by:
iterator in interface Collection<E>
Specified by:
iterator in interface List<E>
Specified by:
iterator in class AbstractCollection<E>
Returns:
an iterator over the elements in this list in proper sequence
See Also:
modCount

listIterator

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

This implementation returns listIterator(0).

Specified by:
listIterator in interface List<E>
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 of the elements in this list (in proper sequence), starting at the specified position in this 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.

Specified by:
listIterator in interface List<E>
Parameters:
index - index of first element to be returned from the list iterator (by a call to the next method)
Returns:
a list iterator of the elements in this list (in proper sequence), starting at the specified position in this list
Throws:
IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())
See Also:
modCount

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.)

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.

Specified by:
subList in interface List<E>
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 - endpoint index value 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.

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.

Specified by:
equals in interface Collection<E>
Specified by:
equals in interface List<E>
Overrides:
equals in class Object
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(), Hashtable

hashCode

public 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 List.hashCode() method.

Specified by:
hashCode in interface Collection<E>
Specified by:
hashCode in interface List<E>
Overrides:
hashCode in class Object
Returns:
the hash code value for this list
See Also:
Object.equals(java.lang.Object), Hashtable

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 ArrayList 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 - index of first element to be removed
toIndex - index after last element to be removed

Java™ Platform
Standard Ed. 6

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