org.apache.commons.collections

Class SetUtils

java.lang.Object

org.apache.commons.collections.SetUtils

public class SetUtils
extends java.lang.Object

Provides utility methods and decorators for Set and SortedSet instances.

Since:

Commons Collections 2.1

Field Summary

Fields

Modifier and Type	Field and Description
static java.util.Set	EMPTY_SET An empty unmodifiable set.
static java.util.SortedSet	EMPTY_SORTED_SET An empty unmodifiable sorted set.

Constructor Summary

Constructors

Constructor and Description
SetUtils() SetUtils should not normally be instantiated.

Method Summary

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

Methods inherited from class java.lang.Object

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

Field Detail

EMPTY_SET

public static final java.util.Set EMPTY_SET

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

EMPTY_SORTED_SET

public static final java.util.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(java.util.Collection set1,
                                 java.util.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(java.util.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 java.util.Set synchronizedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the set is null

unmodifiableSet

public static java.util.Set unmodifiableSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the set is null

predicatedSet

public static java.util.Set predicatedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the Set or Predicate is null

typedSet

public static java.util.Set typedSet(java.util.Set set,
                                     java.lang.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

transformedSet

public static java.util.Set transformedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the Set or Transformer is null

orderedSet

public static java.util.Set orderedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the Set is null

synchronizedSortedSet

public static java.util.SortedSet synchronizedSortedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the set is null

unmodifiableSortedSet

public static java.util.SortedSet unmodifiableSortedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the set is null

predicatedSortedSet

public static java.util.SortedSet predicatedSortedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the Set or Predicate is null

typedSortedSet

public static java.util.SortedSet typedSortedSet(java.util.SortedSet set,
                                                 java.lang.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

transformedSortedSet

public static java.util.SortedSet transformedSortedSet(java.util.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

Throws:

java.lang.IllegalArgumentException - if the Set or Transformer is null