Class CollectionUtils

Method Details

• union

  public static Collection union(Collection a, 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(java.util.Collection<? extends E>)

• intersection

  public static Collection intersection(Collection a, 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(java.util.Collection<?>)
  • containsAny(java.util.Collection, java.util.Collection)

• disjunction

  public static Collection disjunction(Collection a, Collection b)

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

  The cardinality of each element e in the returned Collection will be equal to max(cardinality(e,a),cardinality(e,b)) - min(cardinality(e,a),cardinality(e,b)).

  This is equivalent to subtract(union(a,b),intersection(a,b)) or union(subtract(a,b),subtract(b,a)).

  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

• subtract

  public static Collection subtract(Collection a, 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(java.util.Collection<?>)

• containsAny

  public static boolean containsAny(Collection coll1, Collection coll2)

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

  In other words, this method returns true iff the intersection(java.util.Collection, java.util.Collection) of coll1 and coll2 is not empty.

  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

  Since:

  2.1

  See Also:

  • intersection(java.util.Collection, java.util.Collection)

• getCardinalityMap

  public static Map getCardinalityMap(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

• isSubCollection

  public static boolean isSubCollection(Collection a, 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(java.util.Collection, java.util.Collection)
  • Collection.containsAll(java.util.Collection<?>)

• isProperSubCollection

  public static boolean isProperSubCollection(Collection a, 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() < Integer.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(java.util.Collection, java.util.Collection)
  • Collection.containsAll(java.util.Collection<?>)

• isEqualCollection

  public static boolean isEqualCollection(Collection a, 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.

• cardinality

  public static int cardinality(Object obj, Collection coll)

  Returns the number of occurrences of obj in coll.

  Parameters:

  obj - the object to find the cardinality of

  coll - the collection to search

  Returns:

  the the number of occurrences of obj in coll

• 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

• 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

• 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

• 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

• 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

• 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:

  NullPointerException - if the input collection is null

• 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

• 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:

  NullPointerException - 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

• 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:

  NullPointerException - if the input 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)

• collect

  public static Collection collect(Collection inputCollection, Transformer transformer, 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:

  NullPointerException - if the output collection is null

• collect

  public static Collection collect(Iterator inputIterator, Transformer transformer, 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:

  NullPointerException - if the output collection is null

• addIgnoreNull

  public static boolean addIgnoreNull(Collection collection, Object object)

  Adds an element to the collection unless the element is null.

  Parameters:

  collection - the collection to add to, must not be null

  object - the object to add, if null it will not be added

  Returns:

  true if the collection changed

  Throws:

  NullPointerException - if the collection is null

  Since:

  Commons Collections 3.2

• 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, must not be null

  iterator - the iterator of elements to add, must not be null

  Throws:

  NullPointerException - if the collection or iterator is null

• 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, must not be null

  enumeration - the enumeration of elements to add, must not be null

  Throws:

  NullPointerException - if the collection or enumeration 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, must not be null

  elements - the array of elements to add, must not be null

  Throws:

  NullPointerException - if the collection or array is null

• index

  public static Object index(Object obj, int idx)

  Deprecated.

  use get(Object, int) instead. Will be removed in v4.0

  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:

  IndexOutOfBoundsException

  ArrayIndexOutOfBoundsException

• index

  public static Object index(Object obj, Object index)

  Deprecated.

  use get(Object, int) instead. Will be removed in v4.0

  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:

  IndexOutOfBoundsException

  ArrayIndexOutOfBoundsException

• 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.
  • Array -- the index-th array entry is returned, if there is such an entry; otherwise an IndexOutOfBoundsException 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:

  IndexOutOfBoundsException - if the index is invalid

  IllegalArgumentException - if the object type is invalid

• size

  public static int size(Object object)

  Gets the size of the collection/iterator specified.

  This method can handles objects as follows

  • Collection - the collection size
  • Map - the map size
  • Array - the array size
  • Iterator - the number of elements remaining in the iterator
  • Enumeration - the number of elements remaining in the enumeration

  Parameters:

  object - the object to get the size of

  Returns:

  the size of the specified collection

  Throws:

  IllegalArgumentException - thrown if object is not recognised or null

  Since:

  Commons Collections 3.1

• sizeIsEmpty

  public static boolean sizeIsEmpty(Object object)

  Checks if the specified collection/array/iterator is empty.

  This method can handles objects as follows

  • Collection - via collection isEmpty
  • Map - via map isEmpty
  • Array - using array size
  • Iterator - via hasNext
  • Enumeration - via hasMoreElements

  Note: This method is named to avoid clashing with isEmpty(Collection).

  Parameters:

  object - the object to get the size of, not null

  Returns:

  true if empty

  Throws:

  IllegalArgumentException - thrown if object is not recognised or null

  Since:

  Commons Collections 3.2

• isEmpty

  public static boolean isEmpty(Collection coll)

  Null-safe check if the specified collection is empty.

  Null returns true.

  Parameters:

  coll - the collection to check, may be null

  Returns:

  true if empty or null

  Since:

  Commons Collections 3.2

• isNotEmpty

  public static boolean isNotEmpty(Collection coll)

  Null-safe check if the specified collection is not empty.

  Null returns false.

  Parameters:

  coll - the collection to check, may be null

  Returns:

  true if non-null and non-empty

  Since:

  Commons Collections 3.2

• reverseArray

  public static void reverseArray(Object[] array)

  Reverses the order of the given array.

  Parameters:

  array - the array to reverse

• 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:

  NullPointerException - if the collection is null

• 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:

  NullPointerException - if the collection is null

• retainAll

  public static Collection retainAll(Collection collection, Collection retain)

  Returns a collection containing all the elements in collection that are also in retain. The cardinality of an element e in the returned collection is the same as the cardinality of e in collection unless retain does not contain e, in which case the cardinality is zero. This method is useful if you do not wish to modify the collection c and thus cannot call c.retainAll(retain);.

  Parameters:

  collection - the collection whose contents are the target of the #retailAll operation

  retain - the collection containing the elements to be retained in the returned collection

  Returns:

  a Collection containing all the elements of collection that occur at least once in retain.

  Throws:

  NullPointerException - if either parameter is null

  Since:

  Commons Collections 3.2

• removeAll

  public static Collection removeAll(Collection collection, Collection remove)

  Removes the elements in remove from collection. That is, this method returns a collection containing all the elements in c that are not in remove. The cardinality of an element e in the returned collection is the same as the cardinality of e in collection unless remove contains e, in which case the cardinality is zero. This method is useful if you do not wish to modify the collection c and thus cannot call collection.removeAll(remove);.

  Parameters:

  collection - the collection from which items are removed (in the returned collection)

  remove - the items to be removed from the returned collection

  Returns:

  a Collection containing all the elements of collection except any elements that also occur in remove.

  Throws:

  NullPointerException - if either parameter is null

  Since:

  Commons Collections 3.2

• 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:

  IllegalArgumentException - if the collection is null

• 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:

  IllegalArgumentException - if the collection is null

• predicatedCollection

  public static Collection predicatedCollection(Collection collection, Predicate predicate)

  Returns a predicated (validating) collection backed by the given collection.

  Only objects that pass the test in the given predicate can be added to the collection. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original collection after invoking this method, as it is a backdoor for adding invalid 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:

  IllegalArgumentException - if the Collection 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

• 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:

  IllegalArgumentException - if the Collection or Transformer is null