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

- - ### 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 SortedSet` | `EMPTY_SORTED_SET` An empty unmodifiable sorted set.

  - ### Method Summary

    All Methods Static Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `static <E> SetUtils.SetView<E>` | `difference(Set<? extends E> a, Set<? extends E> b)` Returns a 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> a, Set<? extends E> b)` Returns a 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()` Get a typed empty unmodifiable Set.
    `static <E> SortedSet<E>` | `emptySortedSet()` Get 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> SetUtils.SetView<E>` | `intersection(Set<? extends E> a, Set<? extends E> b)` Returns a 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(java.lang.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> a, Set<? extends E> b)` Returns a 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(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 Detail

    - #### EMPTY_SORTED_SET

          public static final SortedSet EMPTY_SORTED_SET

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

  - ### Method Detail

    - #### emptySet

          public static <E> Set<E> emptySet()

      Get a typed empty unmodifiable Set.
      Type Parameters:
        `E` - the element type
      Returns:
        an empty Set

    - #### emptySortedSet

          public static <E> 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> 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`

    - #### isEqualSet

          public static boolean isEqualSet(Collection<?> set1,
                                           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(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> 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> 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:

           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:
        `NullPointerException` - if the set is null

    - #### 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

    - #### 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

    - #### 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 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:
        `NullPointerException` - if the set or transformer is null

    - #### 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

    - #### 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

    - #### 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

    - #### 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

    - #### 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 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:
        `NullPointerException` - if the set or transformer is null

    - #### 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

    - #### 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

    - #### 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 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:
        `NullPointerException` - if the set or transformer is null
      Since:
        4.1

    - #### union

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

      Returns a 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:
        `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:
        `NullPointerException` - if either input set is null
      Since:
        4.1

    - #### difference

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

      Returns a 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:
        `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(Set<? extends E> a,
                                                             Set<? extends E> b)

      Returns a 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:
        `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(Set<? extends E> a,
                                                            Set<? extends E> b)

      Returns a 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:
        `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

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