org.apache.commons.collections
Class SetUtils

java.lang.Object
  org.apache.commons.collections.SetUtils

public class SetUtils
extends Object

Provides utility methods and decorators for Set and SortedSet instances.

Since:

Commons Collections 2.1

Version:

$Revision: 1.2 $ $Date: 2005/05/23 14:42:13 $

Author:

Paul Jack, Stephen Colebourne, Neil O'Toole, Matt Hall, John Watkinson, Matthew Hawthorne

Field Summary

static Set	EMPTY_SET An empty unmodifiable set.
static SortedSet	EMPTY_SORTED_SET An empty unmodifiable sorted set.

Constructor Summary

SetUtils()
SetUtils should not normally be instantiated.

Method Summary

static int	hashCodeForSet(Collection set) Generates a hash code using the algorithm specified in Set.hashCode().
static boolean	isEqualSet(Collection set1, Collection set2) Tests two sets for equality as per the equals() contract in Set.equals(java.lang.Object).
static <E> Set<E>	orderedSet(Set<E> set) Returns a set that maintains the order of elements that are added backed by the given set.
static <E> Set<E>	predicatedSet(Set<E> set, Predicate<? super E> predicate) Returns a predicated (validating) set backed by the given set.
static <E> SortedSet<E>	predicatedSortedSet(SortedSet<E> set, Predicate<? super E> predicate) Returns a predicated (validating) sorted set backed by the given sorted set.
static <E> Set<E>	synchronizedSet(Set<E> set) Returns a synchronized set backed by the given set.
static <E> SortedSet<E>	synchronizedSortedSet(SortedSet<E> set) Returns a synchronized sorted set backed by the given sorted set.
static <I,O> Set<O>	transformedSet(Set<I> set, Transformer<? super I,? extends O> transformer) Returns a transformed set backed by the given set.
static <I,O> SortedSet<O>	transformedSortedSet(SortedSet<I> set, Transformer<? super I,? extends O> transformer) Returns a transformed sorted set backed by the given set.
static <E> Set<E>	typedSet(Set<E> set, Class<E> type) Deprecated. Made obsolete by Java 1.5 generics.
static <E> SortedSet<E>	typedSortedSet(SortedSet<E> set, Class<E> type) Deprecated. made obsolete by Java 1.5 generics.
static <E> Set<E>	unmodifiableSet(Set<E> set) Returns an unmodifiable set backed by the given set.
static <E> SortedSet<E>	unmodifiableSortedSet(SortedSet<E> set) Returns an unmodifiable sorted set backed by the given sorted set.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

EMPTY_SET

public static final Set EMPTY_SET

An empty unmodifiable set. This uses the Collections implementation and is provided for completeness.

EMPTY_SORTED_SET

public static final SortedSet EMPTY_SORTED_SET

An empty unmodifiable sorted set. This is not provided in the JDK.

Constructor Detail

SetUtils

public SetUtils()

SetUtils should not normally be instantiated.

Method Detail

isEqualSet

public static boolean isEqualSet(Collection set1,
                                 Collection set2)

Tests two sets for equality as per the equals() contract in Set.equals(java.lang.Object).

This method is useful for implementing Set when you cannot extend AbstractSet. The method takes Collection instances to enable other collection types to use the Set implementation algorithm.

The relevant text (slightly paraphrased as this is a static method) is:

Two sets are considered equal if they have the same size, and every member of the first set is contained in the second. This ensures that the equals method works properly across different implementations of the Set interface.

This implementation first checks if the two sets are the same object: if so it returns true. Then, it checks if the two sets are identical in size; if not, it returns false. If so, it returns a.containsAll((Collection) b).

Parameters:

set1 - the first set, may be null

set2 - the second set, may be null

Returns:

whether the sets are equal by value comparison

See Also:

Set

hashCodeForSet

public static int hashCodeForSet(Collection set)

Generates a hash code using the algorithm specified in Set.hashCode().

This method is useful for implementing Set when you cannot extend AbstractSet. The method takes Collection instances to enable other collection types to use the Set implementation algorithm.

Parameters:

set - the set to calculate the hash code for, may be null

Returns:

the hash code

See Also:

Set.hashCode()

synchronizedSet

public static <E> Set<E> synchronizedSet(Set<E> set)

Returns a synchronized set backed by the given set.

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

 Set s = SetUtils.synchronizedSet(mySet);
 synchronized (s) {
     Iterator i = s.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
 

This method uses the implementation in the decorators subpackage.

Parameters:

set - the set to synchronize, must not be null

Returns:

a synchronized set backed by the given set

Throws:

IllegalArgumentException - if the set is null

unmodifiableSet

public static <E> Set<E> unmodifiableSet(Set<E> set)

Returns an unmodifiable set backed by the given set.

This method uses the implementation in the decorators subpackage.

Parameters:

set - the set to make unmodifiable, must not be null

Returns:

an unmodifiable set backed by the given set

Throws:

IllegalArgumentException - if the set is null

predicatedSet

public static <E> Set<E> predicatedSet(Set<E> set,
                                       Predicate<? super E> predicate)

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

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

Parameters:

set - the set to predicate, must not be null

predicate - the predicate for the set, must not be null

Returns:

a predicated set backed by the given set

Throws:

IllegalArgumentException - if the Set or Predicate is null

typedSet

public static <E> Set<E> typedSet(Set<E> set,
                                  Class<E> type)

Deprecated. Made obsolete by Java 1.5 generics.

Returns a typed set backed by the given set.

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

Parameters:

set - the set to limit to a specific type, must not be null

type - the type of objects which may be added to the set

Returns:

a typed set backed by the specified set

transformedSet

public static <I,O> Set<O> transformedSet(Set<I> set,
                                          Transformer<? super I,? extends O> transformer)

Returns a transformed set backed by the given set.

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

Parameters:

set - the set to transform, must not be null

transformer - the transformer for the set, must not be null

Returns:

a transformed set backed by the given set

Throws:

IllegalArgumentException - if the Set or Transformer is null

orderedSet

public static <E> Set<E> orderedSet(Set<E> set)

Returns a set that maintains the order of elements that are added backed by the given set.

If an element is added twice, the order is determined by the first add. The order is observed through the iterator or toArray.

Parameters:

set - the set to order, must not be null

Returns:

an ordered set backed by the given set

Throws:

IllegalArgumentException - if the Set is null

synchronizedSortedSet

public static <E> SortedSet<E> synchronizedSortedSet(SortedSet<E> set)

Returns a synchronized sorted set backed by the given sorted set.

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

 Set s = SetUtils.synchronizedSet(mySet);
 synchronized (s) {
     Iterator i = s.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
 

This method uses the implementation in the decorators subpackage.

Parameters:

set - the sorted set to synchronize, must not be null

Returns:

a synchronized set backed by the given set

Throws:

IllegalArgumentException - if the set is null

unmodifiableSortedSet

public static <E> SortedSet<E> unmodifiableSortedSet(SortedSet<E> set)

Returns an unmodifiable sorted set backed by the given sorted set.

This method uses the implementation in the decorators subpackage.

Parameters:

set - the sorted set to make unmodifiable, must not be null

Returns:

an unmodifiable set backed by the given set

Throws:

IllegalArgumentException - if the set is null

predicatedSortedSet

public static <E> SortedSet<E> predicatedSortedSet(SortedSet<E> set,
                                                   Predicate<? super E> predicate)

Returns a predicated (validating) sorted set backed by the given sorted set.

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

Parameters:

set - the sorted set to predicate, must not be null

predicate - the predicate for the sorted set, must not be null

Returns:

a predicated sorted set backed by the given sorted set

Throws:

IllegalArgumentException - if the Set or Predicate is null

typedSortedSet

public static <E> SortedSet<E> typedSortedSet(SortedSet<E> set,
                                              Class<E> type)

Deprecated. made obsolete by Java 1.5 generics.

Returns a typed sorted set backed by the given set.

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

Parameters:

set - the set to limit to a specific type, must not be null

type - the type of objects which may be added to the set

Returns:

a typed set backed by the specified set

transformedSortedSet

public static <I,O> SortedSet<O> transformedSortedSet(SortedSet<I> set,
                                                      Transformer<? super I,? extends O> transformer)

Returns a transformed sorted set backed by the given set.

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

Parameters:

set - the set to transform, must not be null

transformer - the transformer for the set, must not be null

Returns:

a transformed set backed by the given set

Throws:

IllegalArgumentException - if the Set or Transformer is null