Set

See also:

• Collection
• List
• SortedSet
• HashSet
• TreeSet
• AbstractSet
• Collections.singleton(java.lang.Object)
• Collections.EMPTY_SET

add

public abstract 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.

addAll

public abstract 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.

clear

public abstract void clear ()

Removes all of the elements from this set (optional operation). The set will be empty after this call returns.

contains

public abstract 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).

containsAll

public abstract 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.

copyOf

public static 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.

equals

public abstract 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.

hashCode

public abstract 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.

isEmpty

public abstract boolean isEmpty ()

Returns true if this set contains no elements.

iterator

public abstract 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).

of

public static Set<E> of (E e1, 
                E e2, 
                E e3)

Returns an unmodifiable set containing three elements. See Unmodifiable Sets for details.

of

public static 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.

of

public static 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.

of

public static 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.

of

public static 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.

of

public static Set<E> of (E... elements)

Returns an unmodifiable set containing an arbitrary number of elements. See Unmodifiable Sets for details.

of

public static Set<E> of ()

Returns an unmodifiable set containing zero elements. See Unmodifiable Sets for details.

of

public static 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.

of

public static Set<E> of (E e1, 
                E e2)

Returns an unmodifiable set containing two elements. See Unmodifiable Sets for details.

of

public static 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.

of

public static Set<E> of (E e1, 
                E e2, 
                E e3, 
                E e4)

Returns an unmodifiable set containing four elements. See Unmodifiable Sets for details.

of

public static Set<E> of (E e1)

Returns an unmodifiable set containing one element. See Unmodifiable Sets for details.

remove

public abstract 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.)

removeAll

public abstract 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.

retainAll

public abstract 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.

size

public abstract 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.

spliterator

public 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.

toArray

public abstract 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.

toArray

public abstract 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]);