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org.apache.commons.collections

Class CollectionUtils

java.lang.Object
|
+--org.apache.commons.collections.CollectionUtils


public class CollectionUtils
extends java.lang.Object

Provides utility methods and decorators for Collection instances.
Since:
Commons Collections 1.0
Authors:
Rodney Waldhoff
Paul Jack
Stephen Colebourne
Steve Downey
Herve Quiroz
Peter KoBek
Matthew Hawthorne
Janek Bogucki
Phil Steitz

Field Summary

static Collection

EMPTY_COLLECTION

An empty unmodifiable collection.

Constructor Summary

CollectionUtils()

CollectionUtils should not normally be instantiated.

Method Summary

static void

addAll(Collection collection, Iterator iterator)

Adds all elements in the iteration to the given collection.

static void

addAll(Collection collection, Enumeration enumeration)

Adds all elements in the enumeration to the given collection.

static void

addAll(Collection collection, Object[] elements)

Adds all elements in the array to the given collection.

static int

cardinality(Object obj, final Collection col)

Returns the number of occurrences of obj in col.

static Collection

collect(Collection inputCollection, Transformer transformer)

Returns a new Collection consisting of the elements of inputCollection transformed by the given transformer.

static Collection

collect(Iterator inputIterator, Transformer transformer)

Transforms all elements from the inputIterator with the given transformer and adds them to the outputCollection.

static Collection

collect(Collection inputCollection, final Transformer transformer, final Collection outputCollection)

Transforms all elements from inputCollection with the given transformer and adds them to the outputCollection.

static Collection

collect(Iterator inputIterator, final Transformer transformer, final Collection outputCollection)

Transforms all elements from the inputIterator with the given transformer and adds them to the outputCollection.

static boolean

containsAny(final Collection coll1, final Collection coll2)

Returns true iff at least one element is in both collections.

static int

countMatches(Collection inputCollection, Predicate predicate)

Counts the number of elements in the input collection that match the predicate.

static Collection

disjunction(final Collection a, final Collection b)

Returns a Collection containing the exclusive disjunction (symmetric difference) of the given Collections.

static boolean

exists(Collection collection, Predicate predicate)

Answers true if a predicate is true for at least one element of a collection.

static void

filter(Collection collection, Predicate predicate)

Filter the collection by applying a Predicate to each element.

static Object

find(Collection collection, Predicate predicate)

Finds the first element in the given collection which matches the given predicate.

static void

forAllDo(Collection collection, Closure closure)

Executes the given closure on each element in the collection.

static Object

get(Object object, int index)

Returns the index-th value in object, throwing IndexOutOfBoundsException if there is no such element or IllegalArgumentException if object is not an instance of one of the supported types.

static Map

getCardinalityMap(final Collection coll)

Returns a Map mapping each unique element in the given Collection to an Integer representing the number of occurrences of that element in the Collection.

static Object

index(Object obj, int idx)

Given an Object, and an index, returns the nth value in the object.

static Object

index(Object obj, Object index)

Given an Object, and a key (index), returns the value associated with that key in the Object.

static Collection

intersection(final Collection a, final Collection b)

Returns a Collection containing the intersection of the given Collections.

static boolean

isEqualCollection(final Collection a, final Collection b)

Returns true iff the given Collections contain exactly the same elements with exactly the same cardinalities.

static boolean

isFull(Collection coll)

Returns true if no more elements can be added to the Collection.

static boolean

isProperSubCollection(final Collection a, final Collection b)

Returns true iff a is a proper sub-collection of b, that is, iff the cardinality of e in a is less than or equal to the cardinality of e in b, for each element e in a, and there is at least one element f such that the cardinality of f in b is strictly greater than the cardinality of f in a.

static boolean

isSubCollection(final Collection a, final Collection b)

Returns true iff a is a sub-collection of b, that is, iff the cardinality of e in a is less than or equal to the cardinality of e in b, for each element e in a.

static int

maxSize(Collection coll)

Get the maximum number of elements that the Collection can contain.

static Collection

predicatedCollection(Collection collection, Predicate predicate)

Returns a predicated collection backed by the given collection.

static void

reverseArray(Object[] array)

Reverses the order of the given array.

static Collection

select(Collection inputCollection, Predicate predicate)

Selects all elements from input collection which match the given predicate into an output collection.

static void

select(Collection inputCollection, Predicate predicate, Collection outputCollection)

Selects all elements from input collection which match the given predicate and adds them to outputCollection.

static Collection

selectRejected(Collection inputCollection, Predicate predicate)

Selects all elements from inputCollection which don't match the given predicate into an output collection.

static void

selectRejected(Collection inputCollection, Predicate predicate, Collection outputCollection)

Selects all elements from inputCollection which don't match the given predicate and adds them to outputCollection.

static Collection

subtract(final Collection a, final Collection b)

Returns a new Collection containing a - b.

static Collection

synchronizedCollection(Collection collection)

Returns a synchronized collection backed by the given collection.

static void

transform(Collection collection, Transformer transformer)

Transform the collection by applying a Transformer to each element.

static Collection

transformedCollection(Collection collection, Transformer transformer)

Returns a transformed bag backed by the given collection.

static Collection

typedCollection(Collection collection, Class type)

Returns a typed collection backed by the given collection.

static Collection

union(final Collection a, final Collection b)

Returns a Collection containing the union of the given Collections.

static Collection

unmodifiableCollection(Collection collection)

Returns an unmodifiable collection backed by the given collection.

Field Details

EMPTY_COLLECTION

public static final Collection EMPTY_COLLECTION

An empty unmodifiable collection. The JDK provides empty Set and List implementations which could be used for this purpose. However they could be cast to Set or List which might be undesirable. This implementation only implements Collection.

Constructor Details

CollectionUtils

public CollectionUtils()

CollectionUtils should not normally be instantiated.

Method Details

addAll

public static void addAll(Collection collection, Enumeration enumeration)

Adds all elements in the enumeration to the given collection.

Parameters:
collection - the collection to add to
enumeration - the enumeration of elements to add, may not be null
Throws:
- if the collection or enumeration is null

addAll

public static void addAll(Collection collection, Iterator iterator)

Adds all elements in the iteration to the given collection.

Parameters:
collection - the collection to add to
iterator - the iterator of elements to add, may not be null
Throws:
- if the collection or iterator is null

addAll

public static void addAll(Collection collection, Object[] elements)

Adds all elements in the array to the given collection.

Parameters:
collection - the collection to add to, may not be null
elements - the array of elements to add, may not be null
Throws:
- if the collection or array is null

cardinality

public static int cardinality(Object obj, final Collection col)

Returns the number of occurrences of obj in col.

Parameters:
obj - the object to find the cardinality of
col - the collection to search
Returns:
the the number of occurrences of obj in col

collect

public static Collection collect(Collection inputCollection, final Transformer transformer, final Collection outputCollection)

Transforms all elements from inputCollection with the given transformer and adds them to the outputCollection.

If the input collection or transformer is null, there is no change to the output collection.

Parameters:
inputCollection - the collection to get the input from, may be null
transformer - the transformer to use, may be null
outputCollection - the collection to output into, may not be null
Returns:
the outputCollection with the transformed input added
Throws:
- if the output collection is null

collect

public static Collection collect(Collection inputCollection, Transformer transformer)

Returns a new Collection consisting of the elements of inputCollection transformed by the given transformer.

If the input transformer is null, the result is an empty list.

Parameters:
inputCollection - the collection to get the input from, may not be null
transformer - the transformer to use, may be null
Returns:
the transformed result (new list)
Throws:
- if the input collection is null

collect

public static Collection collect(Iterator inputIterator, final Transformer transformer, final Collection outputCollection)

Transforms all elements from the inputIterator with the given transformer and adds them to the outputCollection.

If the input iterator or transformer is null, there is no change to the output collection.

Parameters:
inputIterator - the iterator to get the input from, may be null
transformer - the transformer to use, may be null
outputCollection - the collection to output into, may not be null
Returns:
the outputCollection with the transformed input added
Throws:
- if the output collection is null

collect

public static Collection collect(Iterator inputIterator, Transformer transformer)

Transforms all elements from the inputIterator with the given transformer and adds them to the outputCollection.

If the input iterator or transformer is null, the result is an empty list.

Parameters:
inputIterator - the iterator to get the input from, may be null
transformer - the transformer to use, may be null
Returns:
the transformed result (new list)

containsAny

public static boolean containsAny(final Collection coll1, final Collection coll2)

Since:
2.1
Parameters:
coll1 - the first collection, must not be null
coll2 - the first collection, must not be null
Returns:
true iff the intersection of the collections is non-empty
See Also:
intersection

countMatches

public static int countMatches(Collection inputCollection, Predicate predicate)

Counts the number of elements in the input collection that match the predicate.

A null collection or predicate matches no elements.

Parameters:
inputCollection - the collection to get the input from, may be null
predicate - the predicate to use, may be null
Returns:
the number of matches for the predicate in the collection

disjunction

public static Collection disjunction(final Collection a, final Collection b)

Parameters:
a - the first collection, must not be null
b - the second collection, must not be null
Returns:
the symmetric difference of the two collections

exists

public static boolean exists(Collection collection, Predicate predicate)

Answers true if a predicate is true for at least one element of a collection.

A null collection or predicate returns false.

Parameters:
collection - the collection to get the input from, may be null
predicate - the predicate to use, may be null
Returns:
true if at least one element of the collection matches the predicate

filter

public static void filter(Collection collection, Predicate predicate)

Filter the collection by applying a Predicate to each element. If the predicate returns false, remove the element.

If the input collection or predicate is null, there is no change made.

Parameters:
collection - the collection to get the input from, may be null
predicate - the predicate to use as a filter, may be null

find

public static Object find(Collection collection, Predicate predicate)

Finds the first element in the given collection which matches the given predicate.

If the input collection or predicate is null, or no element of the collection matches the predicate, null is returned.

Parameters:
collection - the collection to search, may be null
predicate - the predicate to use, may be null
Returns:
the first element of the collection which matches the predicate or null if none could be found

forAllDo

public static void forAllDo(Collection collection, Closure closure)

Executes the given closure on each element in the collection.

If the input collection or closure is null, there is no change made.

Parameters:
collection - the collection to get the input from, may be null
closure - the closure to perform, may be null

get

public static Object get(Object object, int index)

Returns the index-th value in object, throwing IndexOutOfBoundsException if there is no such element or IllegalArgumentException if object is not an instance of one of the supported types.

The supported types, and associated semantics are:

  • Map -- the value returned is the Map.Entry in position index in the map's entrySet iterator, if there is such an entry.
  • List -- this method is equivalent to the list's get method.
  • Object Array -- the index-th array entry is returned, if there is such an entry; otherwise an ArrayIndexOutOfBoundsException is thrown.
  • Collection -- the value returned is the index-th object returned by the collection's default iterator, if there is such an element.
  • Iterator or Enumeration -- the value returned is the index-th object in the Iterator/Enumeration, if there is such an element. The Iterator/Enumeration is advanced to index (or to the end, if index exceeds the number of entries) as a side effect of this method.

Parameters:
object - the object to get a value from
index - the index to get
Returns:
the object at the specified index
Throws:
-
-

getCardinalityMap

public static Map getCardinalityMap(final Collection coll)

Returns a Map mapping each unique element in the given Collection to an Integer representing the number of occurrences of that element in the Collection.

Only those elements present in the collection will appear as keys in the map.

Parameters:
coll - the collection to get the cardinality map for, must not be null
Returns:
the populated cardinality map

index

public static Object index(Object obj, int idx)

Given an Object, and an index, returns the nth value in the object.
  • If obj is a Map, returns the nth value from the keySet iterator, unless the Map contains an Integer key with integer value = idx, in which case the corresponding map entry value is returned. If idx exceeds the number of entries in the map, an empty Iterator is returned.
  • If obj is a List or an array, returns the nth value, throwing IndexOutOfBoundsException, ArrayIndexOutOfBoundsException, resp. if the nth value does not exist.
  • If obj is an iterator, enumeration or Collection, returns the nth value from the iterator, returning an empty Iterator (resp. Enumeration) if the nth value does not exist.
  • Returns the original obj if it is null or not a Collection or Iterator.

Parameters:
obj - the object to get an index of, may be null
idx - the index to get
Throws:
-
-

index

public static Object index(Object obj, Object index)

Given an Object, and a key (index), returns the value associated with that key in the Object. The following checks are made:
  • If obj is a Map, use the index as a key to get a value. If no match continue.
  • Check key is an Integer. If not, return the object passed in.
  • If obj is a Map, get the nth value from the keySet iterator. If the Map has fewer than n entries, return an empty Iterator.
  • If obj is a List or an array, get the nth value, throwing IndexOutOfBoundsException, ArrayIndexOutOfBoundsException, resp. if the nth value does not exist.
  • If obj is an iterator, enumeration or Collection, get the nth value from the iterator, returning an empty Iterator (resp. Enumeration) if the nth value does not exist.
  • Return the original obj.

Parameters:
obj - the object to get an index of
index - the index to get
Returns:
the object at the specified index
Throws:
-
-

intersection

public static Collection intersection(final Collection a, final Collection b)

Returns a Collection containing the intersection of the given Collections.

The cardinality of each element in the returned Collection will be equal to the minimum of the cardinality of that element in the two given Collections.

Parameters:
a - the first collection, must not be null
b - the second collection, must not be null
Returns:
the intersection of the two collections
See Also:
Collection.retainAll
containsAny

isEqualCollection

public static boolean isEqualCollection(final Collection a, final Collection b)

Returns true iff the given Collections contain exactly the same elements with exactly the same cardinalities.

That is, iff the cardinality of e in a is equal to the cardinality of e in b, for each element e in a or b.

Parameters:
a - the first collection, must not be null
b - the second collection, must not be null
Returns:
true iff the collections contain the same elements with the same cardinalities.

isFull

public static boolean isFull(Collection coll)

Returns true if no more elements can be added to the Collection.

This method uses the BoundedCollection interface to determine the full status. If the collection does not implement this interface then false is returned.

The collection does not have to implement this interface directly. If the collection has been decorated using the decorators subpackage then these will be removed to access the BoundedCollection.

Parameters:
coll - the collection to check
Returns:
true if the BoundedCollection is full
Throws:
- if the collection is null

isProperSubCollection

public static boolean isProperSubCollection(final Collection a, final Collection b)

Returns true iff a is a proper sub-collection of b, that is, iff the cardinality of e in a is less than or equal to the cardinality of e in b, for each element e in a, and there is at least one element f such that the cardinality of f in b is strictly greater than the cardinality of f in a.

The implementation assumes

  • a.size() and b.size() represent the total cardinality of a and b, resp.
  • a.size() <32Integer.MAXVALUE

Parameters:
a - the first (sub?) collection, must not be null
b - the second (super?) collection, must not be null
Returns:
true iff a is a proper sub-collection of b
See Also:
isSubCollection
Collection.containsAll

isSubCollection

public static boolean isSubCollection(final Collection a, final Collection b)

Returns true iff a is a sub-collection of b, that is, iff the cardinality of e in a is less than or equal to the cardinality of e in b, for each element e in a.

Parameters:
a - the first (sub?) collection, must not be null
b - the second (super?) collection, must not be null
Returns:
true iff a is a sub-collection of b
See Also:
isProperSubCollection
Collection.containsAll

maxSize

public static int maxSize(Collection coll)

Get the maximum number of elements that the Collection can contain.

This method uses the BoundedCollection interface to determine the maximum size. If the collection does not implement this interface then -1 is returned.

The collection does not have to implement this interface directly. If the collection has been decorated using the decorators subpackage then these will be removed to access the BoundedCollection.

Parameters:
coll - the collection to check
Returns:
the maximum size of the BoundedCollection, -1 if no maximum size
Throws:
- if the collection is null

predicatedCollection

public static Collection predicatedCollection(Collection collection, Predicate predicate)

Returns a predicated collection backed by the given collection. Only objects that pass the test in the given predicate can be added to the collection. Throws an IllegalArgumentException on adding an element if it is invalid.

It is important not to use the original collection after invoking this method, as it is a backdoor for adding unvalidated objects.

Parameters:
collection - the collection to predicate, must not be null
predicate - the predicate for the collection, must not be null
Returns:
a predicated collection backed by the given collection
Throws:
- if the Collection is null

reverseArray

public static void reverseArray(Object[] array)

Reverses the order of the given array.

Parameters:
array - the array to reverse

select

public static void select(Collection inputCollection, Predicate predicate, Collection outputCollection)

Selects all elements from input collection which match the given predicate and adds them to outputCollection.

If the input collection or predicate is null, there is no change to the output collection.

Parameters:
inputCollection - the collection to get the input from, may be null
predicate - the predicate to use, may be null
outputCollection - the collection to output into, may not be null

select

public static Collection select(Collection inputCollection, Predicate predicate)

Selects all elements from input collection which match the given predicate into an output collection.

A null predicate matches no elements.

Parameters:
inputCollection - the collection to get the input from, may not be null
predicate - the predicate to use, may be null
Returns:
the elements matching the predicate (new list)
Throws:
- if the input collection is null

selectRejected

public static void selectRejected(Collection inputCollection, Predicate predicate, Collection outputCollection)

Selects all elements from inputCollection which don't match the given predicate and adds them to outputCollection.

If the input predicate is null, no elements are added to outputCollection.

Parameters:
inputCollection - the collection to get the input from, may be null
predicate - the predicate to use, may be null
outputCollection - the collection to output into, may not be null

selectRejected

public static Collection selectRejected(Collection inputCollection, Predicate predicate)

Selects all elements from inputCollection which don't match the given predicate into an output collection.

If the input predicate is null, the result is an empty list.

Parameters:
inputCollection - the collection to get the input from, may not be null
predicate - the predicate to use, may be null
Returns:
the elements not matching the predicate (new list)
Throws:
- if the input collection is null

subtract

public static Collection subtract(final Collection a, final Collection b)

Returns a new Collection containing a - b. The cardinality of each element e in the returned Collection will be the cardinality of e in a minus the cardinality of e in b, or zero, whichever is greater.

Parameters:
a - the collection to subtract from, must not be null
b - the collection to subtract, must not be null
Returns:
a new collection with the results
See Also:
Collection.removeAll

synchronizedCollection

public static Collection synchronizedCollection(Collection collection)

Returns a synchronized collection backed by the given collection.

You must manually synchronize on the returned buffer's iterator to avoid non-deterministic behavior:

 Collection c = CollectionUtils.synchronizedCollection(myCollection);
 synchronized (c) {
     Iterator i = c.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
This method uses the implementation in the decorators subpackage.

Parameters:
collection - the collection to synchronize, must not be null
Returns:
a synchronized collection backed by the given collection
Throws:
- if the collection is null

transform

public static void transform(Collection collection, Transformer transformer)

Transform the collection by applying a Transformer to each element.

If the input collection or transformer is null, there is no change made.

This routine is best for Lists, for which set() is used to do the transformations "in place." For other Collections, clear() and addAll() are used to replace elements.

If the input collection controls its input, such as a Set, and the Transformer creates duplicates (or are otherwise invalid), the collection may reduce in size due to calling this method.

Parameters:
collection - the collection to get the input from, may be null
transformer - the transformer to perform, may be null

transformedCollection

public static Collection transformedCollection(Collection collection, Transformer transformer)

Returns a transformed bag backed by the given collection.

Each object is passed through the transformer as it is added to the Collection. It is important not to use the original collection after invoking this method, as it is a backdoor for adding untransformed objects.

Parameters:
collection - the collection to predicate, must not be null
transformer - the transformer for the collection, must not be null
Returns:
a transformed collection backed by the given collection
Throws:
- if the Collection or Transformer is null

typedCollection

public static Collection typedCollection(Collection collection, Class type)

Returns a typed collection backed by the given collection.

Only objects of the specified type can be added to the collection.

Parameters:
collection - the collection to limit to a specific type, must not be null
type - the type of objects which may be added to the collection
Returns:
a typed collection backed by the specified collection

union

public static Collection union(final Collection a, final Collection b)

Returns a Collection containing the union of the given Collections.

The cardinality of each element in the returned Collection will be equal to the maximum of the cardinality of that element in the two given Collections.

Parameters:
a - the first collection, must not be null
b - the second collection, must not be null
Returns:
the union of the two collections
See Also:
Collection.addAll

unmodifiableCollection

public static Collection unmodifiableCollection(Collection collection)

Returns an unmodifiable collection backed by the given collection.

This method uses the implementation in the decorators subpackage.

Parameters:
collection - the collection to make unmodifiable, must not be null
Returns:
an unmodifiable collection backed by the given collection
Throws:
- if the collection is null