org.apache.commons.collections

Class SetUtils


public class SetUtils
extends Object

Provides utility methods and decorators for Set and SortedSet instances.

Version:
$Revision: 1.26 $ $Date: 2004/04/01 20:12:00 $

Authors:
Paul Jack
Stephen Colebourne
Neil O'Toole
Matthew Hawthorne

Since:
Commons Collections 2.1

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 java.util.Set.hashCode().
static boolean
isEqualSet(Collection set1, Collection set2)
Tests two sets for equality as per the equals() contract in java.util.Set.equals(java.lang.Object).
static Set
orderedSet(Set set)
Returns a set that maintains the order of elements that are added backed by the given set.
static Set
predicatedSet(Set set, Predicate predicate)
Returns a predicated (validating) set backed by the given set.
static SortedSet
predicatedSortedSet(SortedSet set, Predicate predicate)
Returns a predicated (validating) sorted set backed by the given sorted set.
static Set
synchronizedSet(Set set)
Returns a synchronized set backed by the given set.
static SortedSet
synchronizedSortedSet(SortedSet set)
Returns a synchronized sorted set backed by the given sorted set.
static Set
transformedSet(Set set, Transformer transformer)
Returns a transformed set backed by the given set.
static SortedSet
transformedSortedSet(SortedSet set, Transformer transformer)
Returns a transformed sorted set backed by the given set.
static Set
typedSet(Set set, Class type)
Returns a typed set backed by the given set.
static SortedSet
typedSortedSet(SortedSet set, Class type)
Returns a typed sorted set backed by the given set.
static Set
unmodifiableSet(Set set)
Returns an unmodifiable set backed by the given set.
static SortedSet
unmodifiableSortedSet(SortedSet set)
Returns an unmodifiable sorted set backed by the given sorted set.

Field Details

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 Details

SetUtils

public SetUtils()
SetUtils should not normally be instantiated.

Method Details

hashCodeForSet

public static int hashCodeForSet(Collection set)
Generates a hash code using the algorithm specified in java.util.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:
java.util.Set.hashCode()


isEqualSet

public static boolean isEqualSet(Collection set1,
                                 Collection set2)
Tests two sets for equality as per the equals() contract in java.util.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:
java.util.Set


orderedSet

public static Set orderedSet(Set 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


predicatedSet

public static Set predicatedSet(Set set,
                                Predicate 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


predicatedSortedSet

public static SortedSet predicatedSortedSet(SortedSet set,
                                            Predicate 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


synchronizedSet

public static Set synchronizedSet(Set 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


synchronizedSortedSet

public static SortedSet synchronizedSortedSet(SortedSet 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


transformedSet

public static Set transformedSet(Set set,
                                 Transformer 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


transformedSortedSet

public static SortedSet transformedSortedSet(SortedSet set,
                                             Transformer 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


typedSet

public static Set typedSet(Set set,
                           Class type)
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


typedSortedSet

public static SortedSet typedSortedSet(SortedSet set,
                                       Class type)
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


unmodifiableSet

public static Set unmodifiableSet(Set 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


unmodifiableSortedSet

public static SortedSet unmodifiableSortedSet(SortedSet 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


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