org.apache.commons.collections

Class IteratorUtils


public class IteratorUtils
extends java.lang.Object

Provides static utility methods and decorators for Iterator instances. The implementations are provided in the iterators subpackage.

WARNING: Due to human error certain binary incompatabilities were introduced between Commons Collections 2.1 and 3.0. The class remained source and test compatible, so if you can recompile all your classes and dependencies everything is OK. Those methods which are binary incompatible are marked as such, together with alternate solutions that are binary compatible against versions 2.1.1 and 3.1.

Version:
$Revision: 1.27 $ $Date: 2004/05/26 21:53:46 $
Authors:
Stephen Colebourne
Phil Steitz
Since:
Commons Collections 2.1

Field Summary

static ResettableIterator
EMPTY_ITERATOR
An iterator over no elements.
static ResettableListIterator
EMPTY_LIST_ITERATOR
A list iterator over no elements.
static MapIterator
EMPTY_MAP_ITERATOR
A map iterator over no elements.
static OrderedIterator
EMPTY_ORDERED_ITERATOR
An ordered iterator over no elements.
static OrderedMapIterator
EMPTY_ORDERED_MAP_ITERATOR
An ordered map iterator over no elements.

Constructor Summary

IteratorUtils()
IteratorUtils is not normally instantiated.

Method Summary

static ResettableIterator
arrayIterator(Object array)
Gets an iterator over an object or primitive array.
static ResettableIterator
arrayIterator(Object array, int start)
Gets an iterator over the end part of an object or primitive array.
static ResettableIterator
arrayIterator(Object array, int start, int end)
Gets an iterator over part of an object or primitive array.
static ResettableIterator
arrayIterator(Object[] array)
Gets an iterator over an object array.
static ResettableIterator
arrayIterator(Object[] array, int start)
Gets an iterator over the end part of an object array.
static ResettableIterator
arrayIterator(Object[] array, int start, int end)
Gets an iterator over part of an object array.
static ResettableListIterator
arrayListIterator(Object array)
Gets a list iterator over an object or primitive array.
static ResettableListIterator
arrayListIterator(Object array, int start)
Gets a list iterator over the end part of an object or primitive array.
static ResettableListIterator
arrayListIterator(Object array, int start, int end)
Gets a list iterator over part of an object or primitive array.
static ResettableListIterator
arrayListIterator(Object[] array)
Gets a list iterator over an object array.
static ResettableListIterator
arrayListIterator(Object[] array, int start)
Gets a list iterator over the end part of an object array.
static ResettableListIterator
arrayListIterator(Object[] array, int start, int end)
Gets a list iterator over part of an object array.
static Enumeration
asEnumeration(Iterator iterator)
Gets an enumeration that wraps an iterator.
static Iterator
asIterator(Enumeration enumeration)
Gets an iterator that provides an iterator view of the given enumeration.
static Iterator
asIterator(Enumeration enumeration, Collection removeCollection)
Gets an iterator that provides an iterator view of the given enumeration that will remove elements from the specified collection.
static Iterator
chainedIterator(Collection iterators)
Gets an iterator that iterates through a collections of Iterators one after another.
static Iterator
chainedIterator(Iterator iterator1, Iterator iterator2)
Gets an iterator that iterates through two Iterators one after another.
static Iterator
chainedIterator(Iterator[] iterators)
Gets an iterator that iterates through an array of Iterators one after another.
static Iterator
collatedIterator(Comparator comparator, Collection iterators)
Gets an iterator that provides an ordered iteration over the elements contained in a collection of Iterators.
static Iterator
collatedIterator(Comparator comparator, Iterator iterator1, Iterator iterator2)
Gets an iterator that provides an ordered iteration over the elements contained in a collection of ordered Iterators.
static Iterator
collatedIterator(Comparator comparator, Iterator[] iterators)
Gets an iterator that provides an ordered iteration over the elements contained in an array of Iterators.
static ResettableIterator
emptyIterator()
Gets an empty iterator.
static ResettableListIterator
emptyListIterator()
Gets an empty list iterator.
static MapIterator
emptyMapIterator()
Gets an empty map iterator.
static OrderedIterator
emptyOrderedIterator()
Gets an empty ordered iterator.
static OrderedMapIterator
emptyOrderedMapIterator()
Gets an empty ordered map iterator.
static Iterator
filteredIterator(Iterator iterator, Predicate predicate)
Gets an iterator that filters another iterator.
static ListIterator
filteredListIterator(ListIterator listIterator, Predicate predicate)
Gets a list iterator that filters another list iterator.
static Iterator
getIterator(Object obj)
Gets a suitable Iterator for the given object.
static ResettableIterator
loopingIterator(Collection coll)
Gets an iterator that loops continuously over the supplied collection.
static Iterator
objectGraphIterator(Object root, Transformer transformer)
Gets an iterator that operates over an object graph.
static ResettableIterator
singletonIterator(Object object)
Gets a singleton iterator.
static ListIterator
singletonListIterator(Object object)
Gets a singleton list iterator.
static Object[]
toArray(Iterator iterator)
Gets an array based on an iterator.
static Object[]
toArray(Iterator iterator, Class arrayClass)
Gets an array based on an iterator.
static List
toList(Iterator iterator)
Gets a list based on an iterator.
static List
toList(Iterator iterator, int estimatedSize)
Gets a list based on an iterator.
static ListIterator
toListIterator(Iterator iterator)
Gets a list iterator based on a simple iterator.
static Iterator
transformedIterator(Iterator iterator, Transformer transform)
Gets an iterator that transforms the elements of another iterator.
static Iterator
unmodifiableIterator(Iterator iterator)
Gets an immutable version of an Iterator.
static ListIterator
unmodifiableListIterator(ListIterator listIterator)
Gets an immutable version of a ListIterator.
static MapIterator
unmodifiableMapIterator(MapIterator mapIterator)
Gets an immutable version of a MapIterator.

Field Details

EMPTY_ITERATOR

public static final ResettableIterator EMPTY_ITERATOR
An iterator over no elements.

WARNING: This constant is binary incompatible with Commons Collections 2.1 and 2.1.1. Use EmptyIterator.INSTANCE for compatability with Commons Collections 2.1.1.


EMPTY_LIST_ITERATOR

public static final ResettableListIterator EMPTY_LIST_ITERATOR
A list iterator over no elements.

WARNING: This constant is binary incompatible with Commons Collections 2.1 and 2.1.1. Use EmptyListIterator.INSTANCE for compatability with Commons Collections 2.1.1.


EMPTY_MAP_ITERATOR

public static final MapIterator EMPTY_MAP_ITERATOR
A map iterator over no elements.

EMPTY_ORDERED_ITERATOR

public static final OrderedIterator EMPTY_ORDERED_ITERATOR
An ordered iterator over no elements.

EMPTY_ORDERED_MAP_ITERATOR

public static final OrderedMapIterator EMPTY_ORDERED_MAP_ITERATOR
An ordered map iterator over no elements.

Constructor Details

IteratorUtils

public IteratorUtils()
IteratorUtils is not normally instantiated.

Method Details

arrayIterator

public static ResettableIterator arrayIterator(Object array)
Gets an iterator over an object or primitive array.

This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.

Parameters:
array - the array over which to iterate
Returns:
an iterator over the array

arrayIterator

public static ResettableIterator arrayIterator(Object array,
                                               int start)
Gets an iterator over the end part of an object or primitive array.

This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.

Parameters:
array - the array over which to iterate
start - the index to start iterating at
Returns:
an iterator over part of the array

arrayIterator

public static ResettableIterator arrayIterator(Object array,
                                               int start,
                                               int end)
Gets an iterator over part of an object or primitive array.

This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.

Parameters:
array - the array over which to iterate
start - the index to start iterating at
end - the index to finish iterating at
Returns:
an iterator over part of the array

arrayIterator

public static ResettableIterator arrayIterator(Object[] array)
Gets an iterator over an object array.

WARNING: This method is binary incompatible with Commons Collections 2.1 and 2.1.1. Use new ArrayIterator(array) for compatability.

Parameters:
array - the array over which to iterate
Returns:
an iterator over the array

arrayIterator

public static ResettableIterator arrayIterator(Object[] array,
                                               int start)
Gets an iterator over the end part of an object array.

WARNING: This method is binary incompatible with Commons Collections 2.1 and 2.1.1. Use new ArrayIterator(array,start) for compatability.

Parameters:
array - the array over which to iterate
start - the index to start iterating at
Returns:
an iterator over part of the array

arrayIterator

public static ResettableIterator arrayIterator(Object[] array,
                                               int start,
                                               int end)
Gets an iterator over part of an object array.

WARNING: This method is binary incompatible with Commons Collections 2.1 and 2.1.1. Use new ArrayIterator(array,start,end) for compatability.

Parameters:
array - the array over which to iterate
start - the index to start iterating at
end - the index to finish iterating at
Returns:
an iterator over part of the array

arrayListIterator

public static ResettableListIterator arrayListIterator(Object array)
Gets a list iterator over an object or primitive array.

This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.

Parameters:
array - the array over which to iterate
Returns:
a list iterator over the array

arrayListIterator

public static ResettableListIterator arrayListIterator(Object array,
                                                       int start)
Gets a list iterator over the end part of an object or primitive array.

This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.

Parameters:
array - the array over which to iterate
start - the index to start iterating at
Returns:
a list iterator over part of the array

arrayListIterator

public static ResettableListIterator arrayListIterator(Object array,
                                                       int start,
                                                       int end)
Gets a list iterator over part of an object or primitive array.

This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.

Parameters:
array - the array over which to iterate
start - the index to start iterating at
end - the index to finish iterating at
Returns:
a list iterator over part of the array

arrayListIterator

public static ResettableListIterator arrayListIterator(Object[] array)
Gets a list iterator over an object array.
Parameters:
array - the array over which to iterate
Returns:
a list iterator over the array

arrayListIterator

public static ResettableListIterator arrayListIterator(Object[] array,
                                                       int start)
Gets a list iterator over the end part of an object array.
Parameters:
array - the array over which to iterate
start - the index to start iterating at
Returns:
a list iterator over part of the array

arrayListIterator

public static ResettableListIterator arrayListIterator(Object[] array,
                                                       int start,
                                                       int end)
Gets a list iterator over part of an object array.
Parameters:
array - the array over which to iterate
start - the index to start iterating at
end - the index to finish iterating at
Returns:
a list iterator over part of the array

asEnumeration

public static Enumeration asEnumeration(Iterator iterator)
Gets an enumeration that wraps an iterator.
Parameters:
iterator - the iterator to use, not null
Returns:
a new enumeration

asIterator

public static Iterator asIterator(Enumeration enumeration)
Gets an iterator that provides an iterator view of the given enumeration.
Parameters:
enumeration - the enumeration to use
Returns:
a new iterator

asIterator

public static Iterator asIterator(Enumeration enumeration,
                                  Collection removeCollection)
Gets an iterator that provides an iterator view of the given enumeration that will remove elements from the specified collection.
Parameters:
enumeration - the enumeration to use
removeCollection - the collection to remove elements from
Returns:
a new iterator

chainedIterator

public static Iterator chainedIterator(Collection iterators)
Gets an iterator that iterates through a collections of Iterators one after another.
Parameters:
iterators - the iterators to use, not null or empty or contain nulls
Returns:
a combination iterator over the iterators

chainedIterator

public static Iterator chainedIterator(Iterator iterator1,
                                       Iterator iterator2)
Gets an iterator that iterates through two Iterators one after another.
Parameters:
iterator1 - the first iterators to use, not null
iterator2 - the first iterators to use, not null
Returns:
a combination iterator over the iterators

chainedIterator

public static Iterator chainedIterator(Iterator[] iterators)
Gets an iterator that iterates through an array of Iterators one after another.
Parameters:
iterators - the iterators to use, not null or empty or contain nulls
Returns:
a combination iterator over the iterators

collatedIterator

public static Iterator collatedIterator(Comparator comparator,
                                        Collection iterators)
Gets an iterator that provides an ordered iteration over the elements contained in a collection of Iterators.

Given two ordered Iterators A and B, the Iterator.next() method will return the lesser of A.next() and B.next() and so on.

The comparator is optional. If null is specified then natural order is used.

Parameters:
comparator - the comparator to use, may be null for natural order
iterators - the iterators to use, not null or empty or contain nulls
Returns:
a combination iterator over the iterators

collatedIterator

public static Iterator collatedIterator(Comparator comparator,
                                        Iterator iterator1,
                                        Iterator iterator2)
Gets an iterator that provides an ordered iteration over the elements contained in a collection of ordered Iterators.

Given two ordered Iterators A and B, the Iterator.next() method will return the lesser of A.next() and B.next().

The comparator is optional. If null is specified then natural order is used.

Parameters:
comparator - the comparator to use, may be null for natural order
iterator1 - the first iterators to use, not null
iterator2 - the first iterators to use, not null
Returns:
a combination iterator over the iterators

collatedIterator

public static Iterator collatedIterator(Comparator comparator,
                                        Iterator[] iterators)
Gets an iterator that provides an ordered iteration over the elements contained in an array of Iterators.

Given two ordered Iterators A and B, the Iterator.next() method will return the lesser of A.next() and B.next() and so on.

The comparator is optional. If null is specified then natural order is used.

Parameters:
comparator - the comparator to use, may be null for natural order
iterators - the iterators to use, not null or empty or contain nulls
Returns:
a combination iterator over the iterators

emptyIterator

public static ResettableIterator emptyIterator()
Gets an empty iterator.

This iterator is a valid iterator object that will iterate over nothing.

WARNING: This method is binary incompatible with Commons Collections 2.1 and 2.1.1. Use EmptyIterator.INSTANCE for compatability with Commons Collections 2.1.1.

Returns:
an iterator over nothing

emptyListIterator

public static ResettableListIterator emptyListIterator()
Gets an empty list iterator.

This iterator is a valid list iterator object that will iterate over nothing.

WARNING: This method is binary incompatible with Commons Collections 2.1 and 2.1.1. Use EmptyListIterator.INSTANCE for compatability with Commons Collections 2.1.1.

Returns:
a list iterator over nothing

emptyMapIterator

public static MapIterator emptyMapIterator()
Gets an empty map iterator.

This iterator is a valid map iterator object that will iterate over nothing.

Returns:
a map iterator over nothing

emptyOrderedIterator

public static OrderedIterator emptyOrderedIterator()
Gets an empty ordered iterator.

This iterator is a valid iterator object that will iterate over nothing.

Returns:
an ordered iterator over nothing

emptyOrderedMapIterator

public static OrderedMapIterator emptyOrderedMapIterator()
Gets an empty ordered map iterator.

This iterator is a valid map iterator object that will iterate over nothing.

Returns:
a map iterator over nothing

filteredIterator

public static Iterator filteredIterator(Iterator iterator,
                                        Predicate predicate)
Gets an iterator that filters another iterator.

The returned iterator will only return objects that match the specified filtering predicate.

Parameters:
iterator - the iterator to use, not null
predicate - the predicate to use as a filter, not null
Returns:
a new filtered iterator

filteredListIterator

public static ListIterator filteredListIterator(ListIterator listIterator,
                                                Predicate predicate)
Gets a list iterator that filters another list iterator.

The returned iterator will only return objects that match the specified filtering predicate.

Parameters:
listIterator - the list iterator to use, not null
predicate - the predicate to use as a filter, not null
Returns:
a new filtered iterator

getIterator

public static Iterator getIterator(Object obj)
Gets a suitable Iterator for the given object.

This method can handles objects as follows

  • null - empty iterator
  • Iterator - returned directly
  • Enumeration - wrapped
  • Collection - iterator from collection returned
  • Map - values iterator returned
  • Dictionary - values (elements) enumeration returned as iterator
  • array - iterator over array returned
  • object with iterator() public method accessed by reflection
  • object - singleton iterator
Parameters:
obj - the object to convert to an iterator
Returns:
a suitable iterator, never null

loopingIterator

public static ResettableIterator loopingIterator(Collection coll)
Gets an iterator that loops continuously over the supplied collection.

The iterator will only stop looping if the remove method is called enough times to empty the collection, or if the collection is empty to start with.

Parameters:
coll - the collection to iterate over, not null
Returns:
a new looping iterator

objectGraphIterator

public static Iterator objectGraphIterator(Object root,
                                           Transformer transformer)
Gets an iterator that operates over an object graph.

This iterator can extract multiple objects from a complex tree-like object graph. The iteration starts from a single root object. It uses a Transformer to extract the iterators and elements. Its main benefit is that no intermediate List is created.

For example, consider an object graph:

                 |- Branch -- Leaf
                 |         \- Leaf
         |- Tree |         /- Leaf
         |       |- Branch -- Leaf
  Forest |                 \- Leaf
         |       |- Branch -- Leaf
         |       |         \- Leaf
         |- Tree |         /- Leaf
                 |- Branch -- Leaf
                 |- Branch -- Leaf
The following Transformer, used in this class, will extract all the Leaf objects without creating a combined intermediate list:
 public Object transform(Object input) {
   if (input instanceof Forest) {
     return ((Forest) input).treeIterator();
   }
   if (input instanceof Tree) {
     return ((Tree) input).branchIterator();
   }
   if (input instanceof Branch) {
     return ((Branch) input).leafIterator();
   }
   if (input instanceof Leaf) {
     return input;
   }
   throw new ClassCastException();
 }

Internally, iteration starts from the root object. When next is called, the transformer is called to examine the object. The transformer will return either an iterator or an object. If the object is an Iterator, the next element from that iterator is obtained and the process repeats. If the element is an object it is returned.

Under many circumstances, linking Iterators together in this manner is more efficient (and convenient) than using nested for loops to extract a list.

Parameters:
root - the root object to start iterating from, null results in an empty iterator
transformer - the transformer to use, see above, null uses no effect transformer
Returns:
a new object graph iterator
Since:
Commons Collections 3.1

singletonIterator

public static ResettableIterator singletonIterator(Object object)
Gets a singleton iterator.

This iterator is a valid iterator object that will iterate over the specified object.

WARNING: This method is binary incompatible with Commons Collections 2.1 and 2.1.1. Use new SingletonIterator(object) for compatability.

Parameters:
object - the single object over which to iterate
Returns:
a singleton iterator over the object

singletonListIterator

public static ListIterator singletonListIterator(Object object)
Gets a singleton list iterator.

This iterator is a valid list iterator object that will iterate over the specified object.

Parameters:
object - the single object over which to iterate
Returns:
a singleton list iterator over the object

toArray

public static Object[] toArray(Iterator iterator)
Gets an array based on an iterator.

As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, this is converted to an array.

Parameters:
iterator - the iterator to use, not null
Returns:
an array of the iterator contents

toArray

public static Object[] toArray(Iterator iterator,
                               Class arrayClass)
Gets an array based on an iterator.

As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, this is converted to an array.

Parameters:
iterator - the iterator to use, not null
arrayClass - the class of array to create
Returns:
an array of the iterator contents

toList

public static List toList(Iterator iterator)
Gets a list based on an iterator.

As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, the list is returned.

Parameters:
iterator - the iterator to use, not null
Returns:
a list of the iterator contents

toList

public static List toList(Iterator iterator,
                          int estimatedSize)
Gets a list based on an iterator.

As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, the list is returned.

Parameters:
iterator - the iterator to use, not null
estimatedSize - the initial size of the ArrayList
Returns:
a list of the iterator contents

toListIterator

public static ListIterator toListIterator(Iterator iterator)
Gets a list iterator based on a simple iterator.

As the wrapped Iterator is traversed, a LinkedList of its values is cached, permitting all required operations of ListIterator.

Parameters:
iterator - the iterator to use, not null
Returns:
a new iterator

transformedIterator

public static Iterator transformedIterator(Iterator iterator,
                                           Transformer transform)
Gets an iterator that transforms the elements of another iterator.

The transformation occurs during the next() method and the underlying iterator is unaffected by the transformation.

Parameters:
iterator - the iterator to use, not null
transform - the transform to use, not null
Returns:
a new transforming iterator

unmodifiableIterator

public static Iterator unmodifiableIterator(Iterator iterator)
Gets an immutable version of an Iterator. The returned object will always throw an UnsupportedOperationException for the Iterator.remove method.
Parameters:
iterator - the iterator to make immutable
Returns:
an immutable version of the iterator

unmodifiableListIterator

public static ListIterator unmodifiableListIterator(ListIterator listIterator)
Gets an immutable version of a ListIterator. The returned object will always throw an UnsupportedOperationException for the Iterator.remove, ListIterator.add and ListIterator.set methods.
Parameters:
listIterator - the iterator to make immutable
Returns:
an immutable version of the iterator

unmodifiableMapIterator

public static MapIterator unmodifiableMapIterator(MapIterator mapIterator)
Gets an immutable version of a MapIterator. The returned object will always throw an UnsupportedOperationException for the Iterator.remove, MapIterator.setValue(Object) methods.
Parameters:
mapIterator - the iterator to make immutable
Returns:
an immutable version of the iterator

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