# 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

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  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
  Description
  `static final SortedSet`
  `EMPTY_SORTED_SET`
  An empty unmodifiable sorted set.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static <E> SetUtils.SetView<E>`
  `difference(Set<? extends E> setA, Set<? extends E> setB)`
  Returns an unmodifiable view containing the difference of the given `Set`s, denoted by `a \ b` (or `a - b`).
  `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 `Set`s.
  `static <T> Set<T>`
  `emptyIfNull(Set<T> set)`
  Returns an immutable empty set if the argument is `null`, or the argument itself otherwise.
  `static <E> Set<E>`
  `emptySet()`
  Gets a typed empty unmodifiable Set.
  `static <E> SortedSet<E>`
  `emptySortedSet()`
  Gets a typed empty unmodifiable sorted set.
  `static <T> int`
  `hashCodeForSet(Collection<T> set)`
  Generates a hash code using the algorithm specified in `Set.hashCode()`.
  `static <E> HashSet<E>`
  `hashSet(E... items)`
  Creates a set from the given items.
  `static <E> SetUtils.SetView<E>`
  `intersection(Set<? extends E> setA, Set<? extends E> setB)`
  Returns an unmodifiable view of the intersection of the given `Set`s.
  `static boolean`
  `isEqualSet(Collection<?> set1, Collection<?> set2)`
  Tests two sets for equality as per the `equals()` contract in `Set.equals(Object)`.
  `static <E> Set<E>`
  `newIdentityHashSet()`
  Returns a new hash set that matches elements based on `==` not `equals()`.
  `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> SortedSet<E>`
  `predicatedNavigableSet(NavigableSet<E> set, Predicate<? super E> predicate)`
  Returns a predicated (validating) navigable set backed by the given navigable 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 <E> SortedSet<E>`
  `transformedNavigableSet(NavigableSet<E> set, Transformer<? super E,? extends E> transformer)`
  Returns a transformed navigable set backed by the given navigable set.
  `static <E> Set<E>`
  `transformedSet(Set<E> set, Transformer<? super E,? extends E> transformer)`
  Returns a transformed set backed by the given set.
  `static <E> SortedSet<E>`
  `transformedSortedSet(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(Set<? extends E> setA, Set<? extends E> setB)`
  Returns an unmodifiable view of the union of the given `Set`s.
  `static <E> SortedSet<E>`
  `unmodifiableNavigableSet(NavigableSet<E> set)`
  Returns an unmodifiable navigable set backed by the given navigable set.
  `static <E> Set<E>`
  `unmodifiableSet(E... items)`
  Creates an unmodifiable set from the given items.
  `static <E> Set<E>`
  `unmodifiableSet(Set<? extends 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 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 `Set`s, 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 `Set`s.

    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:
      - `Set.hashCode()`

  - ### 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 `Set`s.

    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:
      - `Set`

  - ### 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 `Set`s.

    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