Section 17.2. Collection Factories | Java Generics and Collections

17.2. Collection Factories

The Collections class provides convenient ways of creating some kinds of collections containing zero or more references to the same object. The simplest possible such collections are empty:

 <T> List<T> emptyList()    // return the empty list (immutable)    <K,V> Map<K,V> emptyMap()  // return the empty map (immutable) <T> Set<T> emptySet()      // return the empty set (immutable)

Empty collections can be useful in implementing methods to return collections of values, where they can be used to signify that there were no values to return. Each method returns a reference to an instance of a singleton inner class of Collections. Because these instances are immutable, they can safely be shared, so calling one of these factory methods does not result in object creation. The Collections fields EMPTY_SET, EMPTY_LIST, and EMPTY_MAP were commonly used for the same purpose in Java before generics, but are less useful now because their raw types generate unchecked warnings whenever they are used.

The Collections class also provides you with ways of creating collection objects containing only a single member:

 <T> Set<T> singleton(T o)         // return an immutable set containing only the specified object <T> List<T> singletonList(T o)         // return an immutable list containing only the specified object <K,V> Map<K,V> singletonMap(K key, V value)         // return an immutable map, mapping only the key K to the value V

Again, these can be useful in providing a single input value to a method that is written to accept a Collection of values.

Finally, it is possible to create a list containing a number of copies of a given object.

 <T> List<T> nCopies(int n, T o)         // return an immutable list containing n references to the object o

Because the list produced by nCopies is immutable, it need contain only a single physical element to provide a list view of the required length. Such lists are often used as the basis for building further collectionsfor example, as the argument to a constructor or an addAll method.