Module java.base
Package java.util

Interface Set<E>

Type Parameters:
E - the type of elements maintained by this set
All Superinterfaces:
Collection<E>, Iterable<E>
All Known Subinterfaces:
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.
  • 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.

See Also: