The hash value of the value, which allows the value to be an element of a hash-based set or key of a hash-based map. Implementations must respect the constraint that:

• if x==y then x.hash==y.hash.

Therefore, a class which refines equals must also refine hash.

The number of elements returned by the iterator of this stream, if the iterator terminates. In the case of an infinite stream, this operation never terminates.

Removes every element

A shallow copy of this collection, that is, a collection with identical elements which does not change if this collection changes. If this collection is immutable, it is acceptable to return a reference to this collection. If this collection is mutable, a newly instantiated collection must be returned.

Returns true if the given value belongs to this Category, that is, if it is an element of this Category, or false otherwise.

For most Categorys the following relationship is satisfied by every pair of elements x and y:

• if x==y, then x in category == y in category

However, it is possible to form a useful Category consistent with some other equivalence relation, for example ===. Therefore implementations of contains() which do not satisfy this relationship are tolerated.

Produces the number of elements in this stream that satisfy the given predicate function (selecting). For an infinite stream, this method never terminates.

Determine if two values are equal. Implementations should respect the constraints that:

• if x===y then x==y (reflexivity),
• if x==y then y==x (symmetry),
• if x==y and y==z then x==z (transitivity).

Furthermore it is recommended that implementations ensure that if x==y then x and y have the same concrete class.

A class which explicitly refines equals() is said to support value equality, and the equality operator == is considered much more meaningful for such classes than for a class which simply inherits the default implementation of identity equality from Identifiable.

An iterator for the elements belonging to this stream.