Interface Set<E>

Type Parameters:
E - the type of elements maintained by this set
All Superinterfaces:
Collection<E>, Iterable<E>
All Known Subinterfaces:
EventSet, NavigableSet<E>, SequencedSet<E>, SortedSet<E>
All Known Implementing Classes:
AbstractSet, ConcurrentHashMap.KeySetView, ConcurrentSkipListSet, CopyOnWriteArraySet, EnumSet, HashSet, JobStateReasons, LinkedHashSet, TreeSet
public interface Set<E> extends Collection<E>
A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

The Set interface places additional stipulations, beyond those inherited from the Collection interface, on the contracts of all constructors and on the contracts of the add, equals and hashCode methods. Declarations for other inherited methods are also included here for convenience. (The specifications accompanying these declarations have been tailored to the Set interface, but they do not contain any additional stipulations.)

The additional stipulation on constructors is, not surprisingly, that all constructors must create a set that contains no duplicate elements (as defined above).

Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.

Some set implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the set may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

Unmodifiable Sets

The Set.of and Set.copyOf static factory methods provide a convenient way to create unmodifiable sets. The Set instances created by these methods have the following characteristics:

  • They are unmodifiable. Elements cannot be added or removed. Calling any mutator method on the Set will always cause UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the Set to behave inconsistently or its contents to appear to change.
  • They disallow null elements. Attempts to create them with null elements result in NullPointerException.
  • Unless otherwise specified, they are serializable if all elements are serializable.
  • They reject duplicate elements at creation time. Duplicate elements passed to a static factory method result in IllegalArgumentException.
  • The iteration order of set elements is unspecified and is subject to change.
  • They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
  • They are serialized as specified on the Serialized Form page.

This interface is a member of the Java Collections Framework.

Since:
1.2
See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    add(E e)
    Adds the specified element to this set if it is not already present (optional operation).
    boolean
    addAll(Collection<? extends E> c)
    Adds all of the elements in the specified collection to this set if they're not already present (optional operation).
    void
    clear()
    Removes all of the elements from this set (optional operation).
    boolean
    contains(Object o)
    Returns true if this set contains the specified element.
    boolean
    containsAll(Collection<?> c)
    Returns true if this set contains all of the elements of the specified collection.
    static <E> Set<E>
    copyOf(Collection<? extends E> coll)
    Returns an unmodifiable Set containing the elements of the given Collection.
    boolean
    equals(Object o)
    Compares the specified object with this set for equality.
    int
    hashCode()
    Returns the hash code value for this set.
    boolean
    isEmpty()
    Returns true if this set contains no elements.
    Iterator<E>
    iterator()
    Returns an iterator over the elements in this set.
    static <E> Set<E>
    of()
    Returns an unmodifiable set containing zero elements.
    static <E> Set<E>
    of(E e1)
    Returns an unmodifiable set containing one element.
    static <E> Set<E>
    of(E... elements)
    Returns an unmodifiable set containing an arbitrary number of elements.
    static <E> Set<E>
    of(E e1, E e2)
    Returns an unmodifiable set containing two elements.
    static <E> Set<E>
    of(E e1, E e2, E e3)
    Returns an unmodifiable set containing three elements.
    static <E> Set<E>
    of(E e1, E e2, E e3, E e4)
    Returns an unmodifiable set containing four elements.
    static <E> Set<E>
    of(E e1, E e2, E e3, E e4, E e5)
    Returns an unmodifiable set containing five elements.
    static <E> Set<E>
    of(E e1, E e2, E e3, E e4, E e5, E e6)
    Returns an unmodifiable set containing six elements.
    static <E> Set<E>
    of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)
    Returns an unmodifiable set containing seven elements.
    static <E> Set<E>
    of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)
    Returns an unmodifiable set containing eight elements.
    static <E> Set<E>
    of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)
    Returns an unmodifiable set containing nine elements.
    static <E> Set<E>
    of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)
    Returns an unmodifiable set containing ten elements.
    static <E> Set<E>
    ofLazy(Set<? extends E> elementCandidates, Predicate<? super E> computingFunction)
    Preview.
    Returns a new lazily computed set whose logical membership for each distinct element candidate in the set of elementCandidates is computed via the provided computingFunction on demand.
    boolean
    remove(Object o)
    Removes the specified element from this set if it is present (optional operation).
    boolean
    removeAll(Collection<?> c)
    Removes from this set all of its elements that are contained in the specified collection (optional operation).
    boolean
    retainAll(Collection<?> c)
    Retains only the elements in this set that are contained in the specified collection (optional operation).
    int
    size()
    Returns the number of elements in this set (its cardinality).
    default Spliterator<E>
    spliterator()
    Creates a Spliterator over the elements in this set.
    Object[]
    toArray()
    Returns an array containing all of the elements in this set.
    <T> T[]
    toArray(T[] a)
    Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

    Methods declared in interface Collection

    parallelStream, removeIf, stream, toArray
    Modifier and Type
    Method
    Description
    default Stream<E>
    parallelStream()
    Returns a possibly parallel Stream with this collection as its source.
    default boolean
    removeIf(Predicate<? super E> filter)
    Removes all of the elements of this collection that satisfy the given predicate (optional operation).
    default Stream<E>
    stream()
    Returns a sequential Stream with this collection as its source.
    default <T> T[]
    toArray(IntFunction<T[]> generator)
    Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.

    Methods declared in interface Iterable

    forEach
    Modifier and Type
    Method
    Description
    default void
    forEach(Consumer<? super E> action)
    Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.

  • Method Details

    • size

      int size()
      Returns the number of elements in this set (its cardinality). If this set contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
      Specified by:
      size in interface Collection<E>
      Returns:
      the number of elements in this set (its cardinality)

    • isEmpty

      boolean isEmpty()
      Returns true if this set contains no elements.
      Specified by:
      isEmpty in interface Collection<E>
      Returns:
      true if this set contains no elements

    • contains

      boolean contains(Object o)
      Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that Objects.equals(o, e).
      Specified by:
      contains in interface Collection<E>
      Parameters:
      o - element whose presence in this set is to be tested
      Returns:
      true if this set contains the specified element
      Throws:
      ClassCastException - if the type of the specified element is incompatible with this set (optional)
      NullPointerException - if the specified element is null and this set does not permit null elements (optional)

    • iterator

      Iterator<E> iterator()
      Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).
      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Returns:
      an iterator over the elements in this set

    • toArray

      Object[] toArray()
      Returns an array containing all of the elements in this set. If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

      The returned array will be "safe" in that no references to it are maintained by this set. (In other words, this method must allocate a new array even if this set is backed by an array). The caller is thus free to modify the returned array.

      This method acts as bridge between array-based and collection-based APIs.

      Specified by:
      toArray in interface Collection<E>
      Returns:
      an array containing all the elements in this set

    • toArray

      <T> T[] toArray(T[] a)
      Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. If the set fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this set.

      If this set fits in the specified array with room to spare (i.e., the array has more elements than this set), the element in the array immediately following the end of the set is set to null. (This is useful in determining the length of this set only if the caller knows that this set does not contain any null elements.)

      If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

      Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

      Suppose x is a set known to contain only strings. The following code can be used to dump the set into a newly allocated array of String: 

          String[] y = x.toArray(new String[0]);
      Note that toArray(new Object[0]) is identical in function to toArray().
      Specified by:
      toArray in interface Collection<E>
      Type Parameters:
      T - the component type of the array to contain the collection
      Parameters:
      a - the array into which the elements of this set are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
      Returns:
      an array containing all the elements in this set
      Throws:
      ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this set
      NullPointerException - if the specified array is null

    • add

      boolean add(E e)
      Adds the specified element to this set if it is not already present (optional operation). More formally, adds the specified element e to this set if the set contains no element e2 such that Objects.equals(e, e2). If this set already contains the element, the call leaves the set unchanged and returns false. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements.

      The stipulation above does not imply that sets must accept all elements; sets may refuse to add any particular element, including null, and throw an exception, as described in the specification for Collection.add. Individual set implementations should clearly document any restrictions on the elements that they may contain.

      Specified by:
      add in interface Collection<E>
      Parameters:
      e - element to be added to this set
      Returns:
      true if this set did not already contain the specified element
      Throws:
      UnsupportedOperationException - if the add operation is not supported by this set
      ClassCastException - if the class of the specified element prevents it from being added to this set
      NullPointerException - if the specified element is null and this set does not permit null elements
      IllegalArgumentException - if some property of the specified element prevents it from being added to this set

    • remove

      boolean remove(Object o)
      Removes the specified element from this set if it is present (optional operation). More formally, removes an element e such that Objects.equals(o, e), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)
      Specified by:
      remove in interface Collection<E>
      Parameters:
      o - object to be removed from this set, if present
      Returns:
      true if this set contained the specified element
      Throws:
      ClassCastException - if the type of the specified element is incompatible with this set (optional)
      NullPointerException - if the specified element is null and this set does not permit null elements (optional)
      UnsupportedOperationException - if the remove operation is not supported by this set

    • containsAll

      boolean containsAll(Collection<?> c)
      Returns true if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns true if it is a subset of this set.
      Specified by:
      containsAll in interface Collection<E>
      Parameters:
      c - collection to be checked for containment in this set
      Returns:
      true if this set contains all of the elements of the specified collection
      Throws:
      ClassCastException - if the types of one or more elements in the specified collection are incompatible with this set (optional)
      NullPointerException - if the specified collection contains one or more null elements and this set does not permit null elements (optional), or if the specified collection is null
      See Also:

    • addAll

      boolean addAll(Collection<? extends E> c)
      Adds all of the elements in the specified collection to this set if they're not already present (optional operation). If the specified collection is also a set, the addAll operation effectively modifies this set so that its value is the union of the two sets. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
      Specified by:
      addAll in interface Collection<E>
      Parameters:
      c - collection containing elements to be added to this set
      Returns:
      true if this set changed as a result of the call
      Throws:
      UnsupportedOperationException - if the addAll operation is not supported by this set
      ClassCastException - if the class of an element of the specified collection prevents it from being added to this set
      NullPointerException - if the specified collection contains one or more null elements and this set does not permit null elements, or if the specified collection is null
      IllegalArgumentException - if some property of an element of the specified collection prevents it from being added to this set
      See Also:

    • retainAll

      boolean retainAll(Collection<?> c)
      Retains only the elements in this set that are contained in the specified collection (optional operation). In other words, removes from this set all of its elements that are not contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the intersection of the two sets.
      Specified by:
      retainAll in interface Collection<E>
      Parameters:
      c - collection containing elements to be retained in this set
      Returns:
      true if this set changed as a result of the call
      Throws:
      UnsupportedOperationException - if the retainAll operation is not supported by this set
      ClassCastException - if the class of an element of this set is incompatible with the specified collection (optional)
      NullPointerException - if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:

    • removeAll

      boolean removeAll(Collection<?> c)
      Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.
      Specified by:
      removeAll in interface Collection<E>
      Parameters:
      c - collection containing elements to be removed from this set
      Returns:
      true if this set changed as a result of the call
      Throws:
      UnsupportedOperationException - if the removeAll operation is not supported by this set
      ClassCastException - if the class of an element of this set is incompatible with the specified collection (optional)
      NullPointerException - if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:

    • clear

      void clear()
      Removes all of the elements from this set (optional operation). The set will be empty after this call returns.
      Specified by:
      clear in interface Collection<E>
      Throws:
      UnsupportedOperationException - if the clear method is not supported by this set

    • equals

      boolean equals(Object o)
      Compares the specified object with this set for equality. Returns true if the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set (or equivalently, every member of this set is contained in the specified set). This definition ensures that the equals method works properly across different implementations of the set interface.
      Specified by:
      equals in interface Collection<E>
      Overrides:
      equals in class Object
      Parameters:
      o - object to be compared for equality with this set
      Returns:
      true if the specified object is equal to this set
      See Also:

    • hashCode

      int hashCode()
      Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a null element is defined to be zero. This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of Object.hashCode().
      Specified by:
      hashCode in interface Collection<E>
      Overrides:
      hashCode in class Object
      Returns:
      the hash code value for this set
      See Also:

    • spliterator

      default Spliterator<E> spliterator()
      Creates a Spliterator over the elements in this set.

      The Spliterator reports Spliterator.DISTINCT. Implementations should document the reporting of additional characteristic values.

      Specified by:
      spliterator in interface Collection<E>
      Specified by:
      spliterator in interface Iterable<E>
      Implementation Requirements:
      The default implementation creates a late-binding spliterator from the set's Iterator. The spliterator inherits the fail-fast properties of the set's iterator.

      The created Spliterator additionally reports Spliterator.SIZED.

      Implementation Note:
      The created Spliterator additionally reports Spliterator.SUBSIZED.
      Returns:
      a Spliterator over the elements in this set
      Since:
      1.8

    • of

      static <E> Set<E> of()
      Returns an unmodifiable set containing zero elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Returns:
      an empty Set
      Since:
      9

    • of

      static <E> Set<E> of(E e1)
      Returns an unmodifiable set containing one element. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the single element
      Returns:
      a Set containing the specified element
      Throws:
      NullPointerException - if the element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2)
      Returns an unmodifiable set containing two elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if the elements are duplicates
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3)
      Returns an unmodifiable set containing three elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3, E e4)
      Returns an unmodifiable set containing four elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      e4 - the fourth element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5)
      Returns an unmodifiable set containing five elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      e4 - the fourth element
      e5 - the fifth element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6)
      Returns an unmodifiable set containing six elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      e4 - the fourth element
      e5 - the fifth element
      e6 - the sixth element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)
      Returns an unmodifiable set containing seven elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      e4 - the fourth element
      e5 - the fifth element
      e6 - the sixth element
      e7 - the seventh element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)
      Returns an unmodifiable set containing eight elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      e4 - the fourth element
      e5 - the fifth element
      e6 - the sixth element
      e7 - the seventh element
      e8 - the eighth element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)
      Returns an unmodifiable set containing nine elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      e4 - the fourth element
      e5 - the fifth element
      e6 - the sixth element
      e7 - the seventh element
      e8 - the eighth element
      e9 - the ninth element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)
      Returns an unmodifiable set containing ten elements. See Unmodifiable Sets for details.
      Type Parameters:
      E - the Set's element type
      Parameters:
      e1 - the first element
      e2 - the second element
      e3 - the third element
      e4 - the fourth element
      e5 - the fifth element
      e6 - the sixth element
      e7 - the seventh element
      e8 - the eighth element
      e9 - the ninth element
      e10 - the tenth element
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null
      Since:
      9

    • of

      @SafeVarargs static <E> Set<E> of(E... elements)
      Returns an unmodifiable set containing an arbitrary number of elements. See Unmodifiable Sets for details.
      API Note:
      This method also accepts a single array as an argument. The element type of the resulting set will be the component type of the array, and the size of the set will be equal to the length of the array. To create a set with a single element that is an array, do the following: 
          String[] array = ... ;
    Set<String[]> list = Set.<String[]>of(array);
      This will cause the Set.of(E) method to be invoked instead.
      Type Parameters:
      E - the Set's element type
      Parameters:
      elements - the elements to be contained in the set
      Returns:
      a Set containing the specified elements
      Throws:
      IllegalArgumentException - if there are any duplicate elements
      NullPointerException - if an element is null or if the array is null
      Since:
      9

    • copyOf

      static <E> Set<E> copyOf(Collection<? extends E> coll)
      Returns an unmodifiable Set containing the elements of the given Collection. The given Collection must not be null, and it must not contain any null elements. If the given Collection contains duplicate elements, an arbitrary element of the duplicates is preserved. If the given Collection is subsequently modified, the returned Set will not reflect such modifications.
      Implementation Note:
      If the given Collection is an unmodifiable Set, calling copyOf will generally not create a copy.
      Type Parameters:
      E - the Set's element type
      Parameters:
      coll - a Collection from which elements are drawn, must be non-null
      Returns:
      a Set containing the elements of the given Collection
      Throws:
      NullPointerException - if coll is null, or if it contains any nulls
      Since:
      10

    • ofLazy

      static <E> Set<E> ofLazy(Set<? extends E> elementCandidates, Predicate<? super E> computingFunction)
      ofLazy is a preview API of the Java platform.
      Programs can only use ofLazy when preview features are enabled.
      Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.
      Returns a new lazily computed set whose logical membership for each distinct element candidate in the set of elementCandidates is computed via the provided computingFunction on demand.

      In the following, the term membership status is used to indicate whether an element belongs to the returned set or not. That is, if the membership status for element E is true, then E is a member of the returned set. Conversely, if the membership status for element E is false, then E is not a member of the returned set.

      The returned set is an unmodifiable set. The elements in the returned set are derived from the element candidates given at construction in combination with evaluating each element's membership status. The set's element membership statuses are lazily computed via the provided computingFunction when first accessed (e.g., via Set::contains). Once the membership status has been successfully computed for an element candidate, the associated membership status is initialized (i.e., either as a logical member or as a logical non-member).

      The provided computing function is guaranteed to be invoked at most once per element candidate, even in a multi-threaded environment. Competing threads accessing an element candidate already under membership status computation will block until the membership status of the element candidate is computed or the computing function completes abnormally.

      If evaluation of the provided computing function throws an unchecked exception (for an element candidate), the lazy membership status is not initialized but instead transitions to an error state whereafter a NoSuchElementException is thrown with the unchecked exception as a cause. Subsequent Set::contains calls for the same membership candidate throw NoSuchElementException (without ever invoking the computing function again) with no cause and with a message that includes the name of the original unchecked exception's class.

      All failures are handled in this way. There is a special case that causes unchecked exceptions to be thrown:

      If the computing function recursively invokes itself (for the same membership candidate) via the returned lazy set, a NoSuchElementException (with an IllegalStateException as a cause) will be thrown.

      The returned set's Object methods; equals(), hashCode(), and toString() methods may trigger initialization of one or more lazy elements. If initialization fails for at least one element, the Object methods may throw NoSuchElementException.

      The returned lazy set strongly references its underlying computing function used to compute membership status at least as long as there are uncomputed element candidates.

      The returned Set is not Serializable.

      If the provided Set of elementCandidates is subsequently modified, the returned Set will not reflect such modifications.

      The Set of elementCandidates must use equals() as its equivalence relation, or its comparison method must be consistent with equals(), otherwise the behavior is unspecified.

      Here is an example involving an application that manages various configurable options -- commonly referred to as "switches" -- that control its behavior. The state of these switches can be determined through the command line, a configuration file, or even a database connection. By using a lazy set, we ensure that the states of these switches are evaluated only once. Once computed, the results are eligible for constant folding by the JVM: 

      class Application {

    enum Option {VERBOSE, DRY_RUN, STRICT}

    // Lazily initialized Set of Options
    static final Set<Option> OPTIONS =
            Set.ofLazy(EnumSet.allOf(Option.class), Application::isEnabled);

    // Return true when the given Option is enabled
    private static boolean isEnabled(Option option) {
        // Parse command line, read configuration file, load database
        ...
    }

    public static void process() {
        // The if condition (and subsequent eliminated branch) is
        // eligible for constant folding (and code elimination).
        if (OPTIONS.contains(Option.DRY_RUN)) {
            // Skip processing in DRY_RUN mode
            return;
        }
        // Actual processing logic
    }

}

      The returned Set<E> can be thought of as a set backed by a Map<E, LazyConstant<Boolean>> field and where the contains(Object) operation is equivalent to: 

       class LazySet<E> extends AbstractCollection<E> implements Set<E> {

     private final Map<E, LazyConstant<Boolean>> backingMap;

     public LazySet(Set<E> elementCandidates, Predicate<E> computingFunction) {
         this.backingMap = elementCandidates.stream()
                 .collect(Collectors.toUnmodifiableMap(
                         Function.identity(),
                         k -> LazyConstant.of(() -> computingFunction.test(k))));
     }

     @Override
     public boolean contains(Object o) {
         var lazyConstant = backingMap.get(o);
         return lazyConstant == null
                 ? false
                 : lazyConstant.get();
     }
 }
      Except, performance and storage efficiency might be better.

      Elements in the returned set are eligible for certain performance optimizations such as constant folding as described in LazyConstant.

      Implementation Note:
      after all element membership statuses have been initialized successfully or transitioned to an error state, the computing function is no longer strongly referenced and becomes eligible for garbage collection.
      Type Parameters:
      E - the type of elements maintained by the returned set
      Parameters:
      elementCandidates - the (non-null) element candidates to be evaluated
      computingFunction - to invoke whenever the membership status of an element candidate is first computed
      Returns:
      a new lazily computed set whose logical membership for each distinct element candidate in the set of elementCandidates is computed via the provided computingFunction on demand
      Throws:
      NullPointerException - if the provided set of elementCandidates is null, if the set of elementCandidates contains a null element, or if the provided computingFunction is null
      Since:
      27
      See Also: