Package io.vavr.collection

Class List.Nil<T>

java.lang.Object
io.vavr.collection.List.Nil<T>
Type Parameters:
T - Component type of the List.
All Implemented Interfaces:
Foldable<T>, LinearSeq<T>, List<T>, Seq<T>, Traversable<T>, Function1<Integer,T>, PartialFunction<Integer,T>, Value<T>, Serializable, Iterable<T>, Function<Integer,T>
Enclosing interface:
List<T>
public static final class List.Nil<T> extends Object implements List<T>, Serializable
Representation of the singleton empty List.
See Also:

  • Method Details

    • instance

      public static <T> List.Nil<T> instance()
      Returns the singleton instance of the linked list.
      Type Parameters:
      T - Component type of the List
      Returns:
      the singleton instance of the linked list.

    • head

      public T head()
      Description copied from interface: Traversable
      Returns the first element of a non-empty Traversable.
      Specified by:
      head in interface Traversable<T>
      Returns:
      The first element of this Traversable.

    • length

      public int length()
      Description copied from interface: Traversable
      Computes the number of elements of this Traversable.

      Same as Traversable.size().

      Specified by:
      length in interface List<T>
      Specified by:
      length in interface Traversable<T>
      Returns:
      the number of elements

    • tail

      public List<T> tail()
      Description copied from interface: Traversable
      Drops the first element of a non-empty Traversable.
      Specified by:
      tail in interface LinearSeq<T>
      Specified by:
      tail in interface List<T>
      Specified by:
      tail in interface Seq<T>
      Specified by:
      tail in interface Traversable<T>
      Returns:
      A new instance of Traversable containing all elements except the first.

    • isEmpty

      public boolean isEmpty()
      Description copied from interface: Traversable
      Checks if this Traversable is empty.
      Specified by:
      isEmpty in interface List<T>
      Specified by:
      isEmpty in interface Traversable<T>
      Specified by:
      isEmpty in interface Value<T>
      Returns:
      true, if this Traversable contains no elements, false otherwise.

    • equals

      public boolean equals(Object o)
      Description copied from interface: Traversable
      In Vavr there are four basic classes of collections:
      • Seq (sequential elements)
      • Set (distinct elements)
      • Map (indexed elements)
      • Multimap (indexed collections)
      Two collection instances of these classes are equal if and only if both collections
      • belong to the same basic collection class (Seq, Set, Map or Multimap)
      • contain the same elements
      • have the same element order, if the collections are of type Seq
      Two Map/Multimap elements, resp. entries, (key1, value1) and (key2, value2) are equal, if the keys are equal and the values are equal.

      Notes:

      • No collection instance equals null, e.g. Queue(1) not equals null.
      • Nulls are allowed and handled as expected, e.g. List(null, 1) equals Stream(null, 1) and HashMap((null, 1)) equals LinkedHashMap((null, 1)).
      • The element order is taken into account for Seq only. E.g. List(null, 1) not equals Stream(1, null) and HashMap((null, 1), ("a", null)) equals LinkedHashMap(("a", null), (null, 1)). The reason is, that we do not know which implementations we compare when having two instances of type Map, Multimap or Set (see Liskov Substitution Principle).
      • Other collection classes are equal if their types are equal and their elements are equal (in iteration order).
      • Iterator equality is defined to be object reference equality.
      Specified by:
      equals in interface Traversable<T>
      Specified by:
      equals in interface Value<T>
      Overrides:
      equals in class Object
      Parameters:
      o - an object, may be null
      Returns:
      true, if this collection equals the given object according to the rules described above, false otherwise.

    • hashCode

      public int hashCode()
      Description copied from interface: Traversable
      Returns the hash code of this collection.
      We distinguish between two types of hashes, those for collections with predictable iteration order (like Seq) and those with arbitrary iteration order (like Set, Map and Multimap).
      In all cases the hash of an empty collection is defined to be 1.
      Collections with predictable iteration order are hashed as follows: 
      
 int hash = 1;
 for (T t : this) { hash = hash * 31 + Objects.hashCode(t); }
      Collections with arbitrary iteration order are hashed in a way such that the hash of a fixed number of elements is independent of their iteration order. 
      
 int hash = 1;
 for (T t : this) { hash += Objects.hashCode(t); }
      Please note that the particular hashing algorithms may change in a future version of Vavr.
      Generally, hash codes of collections aren't cached in Vavr (opposed to the size/length). Storing hash codes in order to reduce the time complexity would increase the memory footprint. Persistent collections are built upon tree structures, it allows us to implement efficient memory sharing. A drawback of tree structures is that they make it necessary to store collection attributes at each tree node (read: element).
      The computation of the hash code is linear in time, i.e. O(n). If the hash code of a collection is re-calculated often, e.g. when using a List as HashMap key, we might want to cache the hash code. This can be achieved by simply using a wrapper class, which is not included in Vavr but could be implemented like this: 
      public final class Hashed<K> {

     private final K key;
     private final Lazy<Integer> hashCode;

     public Hashed(K key) {
         this.key = key;
         this.hashCode = Lazy.of(() -> Objects.hashCode(key));
     }

     public K key() {
         return key;
     }

     &#64;Override
     public boolean equals(Object o) {
         if (o == key) {
             return true;
         } else if (key != null && o instanceof Hashed) {
             final Hashed that = (Hashed) o;
             return key.equals(that.key);
         } else {
             return false;
         }
     }

     &#64;Override
     public int hashCode() {
         return hashCode.get();
     }

     &#64;Override
     public String toString() {
         return "Hashed(" + (key == null ? "null" : key.toString()) + ")";
     }
 }
      Specified by:
      hashCode in interface Traversable<T>
      Specified by:
      hashCode in interface Value<T>
      Overrides:
      hashCode in class Object
      Returns:
      The hash code of this collection

    • toString

      public String toString()
      Description copied from interface: Value
      Clarifies that values have a proper toString() method implemented.

      See Object.toString().

      Specified by:
      toString in interface Value<T>
      Overrides:
      toString in class Object
      Returns:
      A String representation of this object