org.apache.commons.collections4

Class SetUtils

java.lang.Object

org.apache.commons.collections4.SetUtils

public class SetUtils
extends java.lang.Object

Provides utility methods and decorators for Set and SortedSet instances.

Since:

2.1

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	SetUtils.SetView<E> An unmodifiable view of a set that may be backed by other sets.

Field Summary

Fields

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

Method Summary

Modifier and Type	Method and Description
static <E> SetUtils.SetView<E>	difference(java.util.Set<? extends E> a, java.util.Set<? extends E> b) Returns a unmodifiable view containing the difference of the given Sets, denoted by a \ b (or a - b).
static <E> SetUtils.SetView<E>	disjunction(java.util.Set<? extends E> a, java.util.Set<? extends E> b) Returns a unmodifiable view of the symmetric difference of the given Sets.
static <T> java.util.Set<T>	emptyIfNull(java.util.Set<T> set) Returns an immutable empty set if the argument is null, or the argument itself otherwise.
static <E> java.util.Set<E>	emptySet() Get a typed empty unmodifiable Set.
static <E> java.util.SortedSet<E>	emptySortedSet() Get a typed empty unmodifiable sorted set.
static <T> int	hashCodeForSet(java.util.Collection<T> set) Generates a hash code using the algorithm specified in Set.hashCode().
static <E> SetUtils.SetView<E>	intersection(java.util.Set<? extends E> a, java.util.Set<? extends E> b) Returns a unmodifiable view of the intersection of the given Sets.
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 <E> java.util.Set<E>	newIdentityHashSet() Returns a new hash set that matches elements based on == not equals().
static <E> java.util.Set<E>	orderedSet(java.util.Set<E> set) Returns a set that maintains the order of elements that are added backed by the given set.
static <E> java.util.SortedSet<E>	predicatedNavigableSet(java.util.NavigableSet<E> set, Predicate<? super E> predicate) Returns a predicated (validating) navigable set backed by the given navigable set.
static <E> java.util.Set<E>	predicatedSet(java.util.Set<E> set, Predicate<? super E> predicate) Returns a predicated (validating) set backed by the given set.
static <E> java.util.SortedSet<E>	predicatedSortedSet(java.util.SortedSet<E> set, Predicate<? super E> predicate) Returns a predicated (validating) sorted set backed by the given sorted set.
static <E> java.util.Set<E>	synchronizedSet(java.util.Set<E> set) Returns a synchronized set backed by the given set.
static <E> java.util.SortedSet<E>	synchronizedSortedSet(java.util.SortedSet<E> set) Returns a synchronized sorted set backed by the given sorted set.
static <E> java.util.SortedSet<E>	transformedNavigableSet(java.util.NavigableSet<E> set, Transformer<? super E,? extends E> transformer) Returns a transformed navigable set backed by the given navigable set.
static <E> java.util.Set<E>	transformedSet(java.util.Set<E> set, Transformer<? super E,? extends E> transformer) Returns a transformed set backed by the given set.
static <E> java.util.SortedSet<E>	transformedSortedSet(java.util.SortedSet<E> set, Transformer<? super E,? extends E> transformer) Returns a transformed sorted set backed by the given set.
static <E> SetUtils.SetView<E>	union(java.util.Set<? extends E> a, java.util.Set<? extends E> b) Returns a unmodifiable view of the union of the given Sets.
static <E> java.util.SortedSet<E>	unmodifiableNavigableSet(java.util.NavigableSet<E> set) Returns an unmodifiable navigable set backed by the given navigable set.
static <E> java.util.Set<E>	unmodifiableSet(java.util.Set<? extends E> set) Returns an unmodifiable set backed by the given set.
static <E> java.util.SortedSet<E>	unmodifiableSortedSet(java.util.SortedSet<E> 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_SORTED_SET

public static final java.util.SortedSet EMPTY_SORTED_SET

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

Method Detail

emptySet

public static <E> java.util.Set<E> emptySet()

Get a typed empty unmodifiable Set.

Type Parameters:

E - the element type

Returns:

an empty Set

emptySortedSet

public static <E> java.util.SortedSet<E> emptySortedSet()

Get a typed empty unmodifiable sorted set.

Type Parameters:

E - the element type

Returns:

an empty sorted Set

emptyIfNull

public static <T> java.util.Set<T> emptyIfNull(java.util.Set<T> set)

Returns an immutable empty set if the argument is null, or the argument itself otherwise.

Type Parameters:

T - the element type

Parameters:

set - the set, possibly null

Returns:

an empty set if the argument is null

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 <T> int hashCodeForSet(java.util.Collection<T> 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.

Type Parameters:

T - the element type

Parameters:

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

Returns:

the hash code

See Also:

Set.hashCode()

newIdentityHashSet

public static <E> java.util.Set<E> newIdentityHashSet()

Returns a new hash set that matches elements based on == not equals().

This set will violate the detail of various Set contracts. As a general rule, don't compare this set to other sets. In particular, you can't use decorators like ListOrderedSet on it, which silently assume that these contracts are fulfilled.

Note that the returned set is not synchronized and is not thread-safe. If you wish to use this set from multiple threads concurrently, you must use appropriate synchronization. The simplest approach is to wrap this map using Collections.synchronizedSet(Set). This class may throw exceptions when accessed by concurrent threads without synchronization.

Type Parameters:

E - the element type

Returns:

a new identity hash set

Since:

4.1

synchronizedSet

public static <E> java.util.Set<E> synchronizedSet(java.util.Set<E> set)

Returns a synchronized set backed by the given set.

You must manually synchronize on the returned set'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 is just a wrapper for Collections.synchronizedSet(Set).

Type Parameters:

E - the element type

Parameters:

set - the set to synchronize, must not be null

Returns:

a synchronized set backed by the given set

Throws:

java.lang.NullPointerException - if the set is null

unmodifiableSet

public static <E> java.util.Set<E> unmodifiableSet(java.util.Set<? extends E> set)

Returns an unmodifiable set backed by the given set.

This method uses the implementation in the decorators subpackage.

Type Parameters:

E - the element type

Parameters:

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

Returns:

an unmodifiable set backed by the given set

Throws:

java.lang.NullPointerException - if the set is null

predicatedSet

public static <E> java.util.Set<E> predicatedSet(java.util.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.

Type Parameters:

E - the element type

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.NullPointerException - if the set or predicate is null

transformedSet

public static <E> java.util.Set<E> transformedSet(java.util.Set<E> set,
                                                  Transformer<? super E,? extends E> 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.

Existing entries in the specified set will not be transformed. If you want that behaviour, see TransformedSet.transformedSet(java.util.Set<E>, org.apache.commons.collections4.Transformer<? super E, ? extends E>).

Type Parameters:

E - the element type

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.NullPointerException - if the set or transformer is null

orderedSet

public static <E> java.util.Set<E> orderedSet(java.util.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.

Type Parameters:

E - the element type

Parameters:

set - the set to order, must not be null

Returns:

an ordered set backed by the given set

Throws:

java.lang.NullPointerException - if the set is null

synchronizedSortedSet

public static <E> java.util.SortedSet<E> synchronizedSortedSet(java.util.SortedSet<E> set)

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

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

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

This method is just a wrapper for Collections.synchronizedSortedSet(SortedSet).

Type Parameters:

E - the element type

Parameters:

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

Returns:

a synchronized set backed by the given set

Throws:

java.lang.NullPointerException - if the set is null

unmodifiableSortedSet

public static <E> java.util.SortedSet<E> unmodifiableSortedSet(java.util.SortedSet<E> set)

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

This method uses the implementation in the decorators subpackage.

Type Parameters:

E - the element type

Parameters:

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

Returns:

an unmodifiable set backed by the given set

Throws:

java.lang.NullPointerException - if the set is null

predicatedSortedSet

public static <E> java.util.SortedSet<E> predicatedSortedSet(java.util.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.

Type Parameters:

E - the element type

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.NullPointerException - if the set or predicate is null

transformedSortedSet

public static <E> java.util.SortedSet<E> transformedSortedSet(java.util.SortedSet<E> set,
                                                              Transformer<? super E,? extends E> 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.

Existing entries in the specified set will not be transformed. If you want that behaviour, see TransformedSortedSet.transformedSortedSet(java.util.SortedSet<E>, org.apache.commons.collections4.Transformer<? super E, ? extends E>).

Type Parameters:

E - the element type

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.NullPointerException - if the set or transformer is null

unmodifiableNavigableSet

public static <E> java.util.SortedSet<E> unmodifiableNavigableSet(java.util.NavigableSet<E> set)

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

This method uses the implementation in the decorators subpackage.

Type Parameters:

E - the element type

Parameters:

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

Returns:

an unmodifiable set backed by the given set

Throws:

java.lang.NullPointerException - if the set is null

Since:

4.1

predicatedNavigableSet

public static <E> java.util.SortedSet<E> predicatedNavigableSet(java.util.NavigableSet<E> set,
                                                                Predicate<? super E> predicate)

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

Type Parameters:

E - the element type

Parameters:

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

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

Returns:

a predicated navigable set backed by the given navigable set

Throws:

java.lang.NullPointerException - if the set or predicate is null

Since:

4.1

transformedNavigableSet

public static <E> java.util.SortedSet<E> transformedNavigableSet(java.util.NavigableSet<E> set,
                                                                 Transformer<? super E,? extends E> transformer)

Returns a transformed navigable set backed by the given navigable 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.

Existing entries in the specified set will not be transformed. If you want that behaviour, see TransformedNavigableSet.transformedNavigableSet(java.util.NavigableSet<E>, org.apache.commons.collections4.Transformer<? super E, ? extends E>).

Type Parameters:

E - the element type

Parameters:

set - the navigable 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.NullPointerException - if the set or transformer is null

Since:

4.1

union

public static <E> SetUtils.SetView<E> union(java.util.Set<? extends E> a,
                                            java.util.Set<? extends E> b)

Returns a unmodifiable view of the union of the given Sets.

The returned view contains all elements of a and b.

Type Parameters:

E - the generic type that is able to represent the types contained in both input sets.

Parameters:

a - the first set, must not be null

b - the second set, must not be null

Returns:

a view of the union of the two set

Throws:

java.lang.NullPointerException - if either input set is null

Since:

4.1

difference

public static <E> SetUtils.SetView<E> difference(java.util.Set<? extends E> a,
                                                 java.util.Set<? extends E> b)

Returns a unmodifiable view containing the difference of the given Sets, denoted by a \ b (or a - b).

The returned view contains all elements of a that are not a member of b.

Type Parameters:

E - the generic type that is able to represent the types contained in both input sets.

Parameters:

a - the set to subtract from, must not be null

b - the set to subtract, must not be null

Returns:

a view of the relative complement of of the two sets

Since:

4.1

intersection

public static <E> SetUtils.SetView<E> intersection(java.util.Set<? extends E> a,
                                                   java.util.Set<? extends E> b)

Returns a unmodifiable view of the intersection of the given Sets.

The returned view contains all elements that are members of both input sets (a and b).

Type Parameters:

E - the generic type that is able to represent the types contained in both input sets.

Parameters:

a - the first set, must not be null

b - the second set, must not be null

Returns:

a view of the intersection of the two sets

Since:

4.1

disjunction

public static <E> SetUtils.SetView<E> disjunction(java.util.Set<? extends E> a,
                                                  java.util.Set<? extends E> b)

Returns a unmodifiable view of the symmetric difference of the given Sets.

The returned view contains all elements of a and b that are not a member of the other set.

This is equivalent to union(difference(a, b), difference(b, a)).

Type Parameters:

E - the generic type that is able to represent the types contained in both input sets.

Parameters:

a - the first set, must not be null

b - the second set, must not be null

Returns:

a view of the symmetric difference of the two sets

Since:

4.1