Package org.apache.commons.collections4

Class SetUtils

java.lang.Object
org.apache.commons.collections4.SetUtils
public class SetUtils extends Object
Provides utility methods and decorators for Set and SortedSet instances.
Since:
2.1

  • Field Details

    • EMPTY_SORTED_SET

      public static final SortedSet EMPTY_SORTED_SET
      An empty unmodifiable sorted set. This is not provided in the JDK.

  • Method Details

    • difference

      public static <E> SetUtils.SetView<E> difference(Set<? extends E> setA, Set<? extends E> setB)
      Returns an 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:
      setA - the set to subtract from, must not be null
      setB - the set to subtract, must not be null
      Returns:
      a view of the relative complement of the two sets
      Since:
      4.1

    • disjunction

      public static <E> SetUtils.SetView<E> disjunction(Set<? extends E> setA, Set<? extends E> setB)
      Returns an 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:
      setA - the first set, must not be null
      setB - the second set, must not be null
      Returns:
      a view of the symmetric difference of the two sets
      Since:
      4.1

    • emptyIfNull

      public static <T> Set<T> emptyIfNull(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

    • emptySet

      public static <E> Set<E> emptySet()
      Gets a typed empty unmodifiable Set.
      Type Parameters:
      E - the element type
      Returns:
      an empty Set

    • emptySortedSet

      public static <E> SortedSet<E> emptySortedSet()
      Gets a typed empty unmodifiable sorted set.
      Type Parameters:
      E - the element type
      Returns:
      an empty sorted Set

    • hashCodeForSet

      public static <T> int hashCodeForSet(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:

    • hashSet

      public static <E> HashSet<E> hashSet(E... items)
      Creates a set from the given items. If the passed var-args argument is null, then the method returns null.
      Type Parameters:
      E - the element type
      Parameters:
      items - the elements that make up the new set
      Returns:
      a set
      Since:
      4.3

    • intersection

      public static <E> SetUtils.SetView<E> intersection(Set<? extends E> setA, Set<? extends E> setB)
      Returns an 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:
      setA - the first set, must not be null
      setB - the second set, must not be null
      Returns:
      a view of the intersection of the two sets
      Since:
      4.1

    • isEqualSet

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

    • newIdentityHashSet

      public static <E> 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

    • 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.

      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:
      NullPointerException - if the set is null

    • predicatedNavigableSet

      public static <E> SortedSet<E> predicatedNavigableSet(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:
      NullPointerException - if the set or predicate is null
      Since:
      4.1

    • 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.

      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:
      NullPointerException - if the set or predicate 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.

      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:
      NullPointerException - if the set or predicate is null

    • 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 set's iterator to avoid non-deterministic behavior: 

       Sets 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:
      NullPointerException - 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 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:
      NullPointerException - if the set is null

    • transformedNavigableSet

      public static <E> SortedSet<E> transformedNavigableSet(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 behavior, 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:
      NullPointerException - if the set or transformer is null
      Since:
      4.1

    • transformedSet

      public static <E> Set<E> transformedSet(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 behavior, 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:
      NullPointerException - if the set or transformer is null

    • transformedSortedSet

      public static <E> SortedSet<E> transformedSortedSet(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 behavior, 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:
      NullPointerException - if the set or transformer is null

    • union

      public static <E> SetUtils.SetView<E> union(Set<? extends E> setA, Set<? extends E> setB)
      Returns an 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:
      setA - the first set, must not be null
      setB - the second set, must not be null
      Returns:
      a view of the union of the two set
      Throws:
      NullPointerException - if either input set is null
      Since:
      4.1

    • unmodifiableNavigableSet

      public static <E> SortedSet<E> unmodifiableNavigableSet(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:
      NullPointerException - if the set is null
      Since:
      4.1

    • unmodifiableSet

      public static <E> Set<E> unmodifiableSet(E... items)
      Creates an unmodifiable set from the given items. If the passed var-args argument is null, then the method returns null.
      Type Parameters:
      E - the element type
      Parameters:
      items - the elements that make up the new set
      Returns:
      a set
      Since:
      4.3

    • unmodifiableSet

      public static <E> Set<E> unmodifiableSet(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:
      NullPointerException - 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.

      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:
      NullPointerException - if the set is null