An iterable collection of elements of finite Iterable.size, with a well-defined notion of value equality. Collection is the abstract supertype of List, Map, and Set.

A Collection forms a Category of its elements, and is Iterable. The elements of a collection are not necessarily distinct when compared using Object.equals().

A Collection may be cloned. If a collection is immutable, it is acceptable that clone() produce a reference to the collection itself. If a collection is mutable, clone() should produce a collection containing references to the same elements, with the same structure as the original collection—that is, it should produce a shallow copy of the collection.

All Collections are required to support a well-defined notion of value equality, but the definition of equality depends upon the kind of collection. Equality for Maps and Sets has a quite different definition to equality for Lists. Instances of two different kinds of collection are never equal—for example, a Map is never equal to a List.

See also List, Map, Set

no type hierarchy

emptySource Codeshared actual default Boolean empty

Determine if the collection is empty, that is, if it has no elements.

permutationsSource Codeshared {[Element+]*} permutations

The permutations of this collection, as a stream of nonempty sequences. That is, a stream producing every distinct ordering of the elements of this collection.

For example,


is the stream of strings { "ABC", "ACB", "BAC", "BCA", "CAB", "CBA" }.

If this collection is empty, the resulting stream is empty.

The permutations are enumerated lexicographically according to the order in which each distinct element of this collection is first produced by its iterator. No permutation is repeated.

Two elements are considered distinct if either:

  • they are both instances of Object, and are unequal, or
  • one element is an Object and the other is null.
stringSource Codeshared actual default String string

A string of form "{ x, y, z }" where x, y, and z are the string representations of the elements of this collection, as produced by the iterator of the collection, or the string "{}" if this collection is empty. If the collection iterator produces the value null, the string representation contains the string "<null>".

Refines Iterable.string ultimately refines Object.string
Inherited Attributes
Attributes inherited from: Object
Attributes inherited from: Iterable<Element,Absent>
cloneSource Codeshared formal Collection<Element> clone()

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.

containsSource Codeshared actual default Boolean contains(Object element)

Return true if the given object is an element of this collection. In this default implementation, and in most refining implementations, return false otherwise. An acceptable refining implementation may return true for objects which are not elements of the collection, but this is not recommended. (For example, the contains() method of String returns true for any substring of the string.)

Refines Iterable.contains ultimately refines Category.contains
Inherited Methods
Methods inherited from: Object
Methods inherited from: Category<Element>
Methods inherited from: Iterable<Element,Absent>