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An interface in Vala is a non-instantiable type. A class may implement any number of interfaces, thereby declaring that an instance of that class should also be considered an instance of those interfaces. Interfaces are part of the GType system, and so compact classes may not implement interfaces (see Classes/Types of class.)

The simplest interface declaration looks like this:

interface InterfaceName {

Unlike C# or Java, Vala's interfaces may include implemented methods, and so provide premade functionality to an implementing class, similar to mixins in other languages. All methods defined in a Vala interface are automatically considered to be virtual. Interfaces in Vala may also have prerequisites - classes or other interfaces that implementing classes must inherit from or implement. This is a more general form of the interface inheritence found in other languages. It should be noted that if you want to guarantee that all implementors of an interface are GObject type classes, you should give that class as a prerequisite for the interface.

Interfaces in Vala have a static scope, identified by the name of the interface. This is the only scope associated with them (i.e. there is no class or instance scope created for them at any time.) Non-instance members of the interface (static members and other declarations,) can be identified using this scope.

For an overview of object oriented programming, see Concepts/Object oriented programming.

Interface declaration

  • interface-declaration:

    • [ access-modifier ] interface qualified-interface-name [ inheritance-list ] { [ interface-members ] }


    • [ qualified-namespace-name . ] interface-name


    • identifier


    • : prerequisite-classes-and-interfaces


    • qualified-class-name [ , prerequisite-classes-and-interfaces ]
      qualified-interface-name [ , prerequisite-classes-and-interfaces ]


    • interface-member [ interface-members ]


    • interface-constant-declaration





    • interface-instance-method-declaration


    • interface-static-field-declaration

Interface fields

As an interface is not instantiable, it may not contain data on a per instance basis. It is though allowable to define static fields in an interface. These are equivalent to static fields in a class: they exist exactly once regardless of how many instances there are of classes that implement the interface.

The syntax for static interface fields is the same as the static class fields: See Classes/Class fields. For more explanation of static vs instance members, see Classes/Types of class members.

Interface methods

Interfaces can contain abstract and non abstract methods. A non-abstract class that implements the interface must provide implementations of all abstract methods in the interface. All methods defined in an interface are automatically virtual.

Vala interfaces may also define static methods. These are equivalent to static methods in classes.

  • interface-instance-method-declaration:

    • [ class-member-visibility-modifier ] return-type method-name ( [ params-list ] ) method-contracts [ throws exception-list ] { statement-list }


    • [ class-member-visibility-modifier ] abstract return-type method-name ( [ params-list ] ) method-contracts [ throws exception-list ] ;


    • [ class-member-visibility-modifier ] static return-type method-name ( [ params-list ] ) method-contracts [ throws exception-list ] { statement-list }

For discussion of methods in classes, see: Classes/Class methods. For information about methods in general, see Methods. Of particular note is that an abstract method of an interface defines a method that can always be called in an instance of an interface, because that instance is guaranteed to be of a non-abstract class that implements the interface's abstract methods.

Interface properties

Interfaces can contain properties in a similar way to classes. As interfaces can not contain per instance data, interface properties cannot be created automatically. This means that all properties must either be declared abstract (and implemented by implementing classes,) or have explicit get and set clauses as appropriate. Vala does not allow an abstract property to be partially implemented, instead it should just define which actions (get, set or both) should be implemented.

Interfaces are not constructed, and so there is not concept of a construction property in an interface. For more on properties in classes, see: Classes/Properties.

  • interface-instance-property-declaration:

    • [ class-member-visibility-modifier ] [ class-method-type-modifier ] qualified-type-name property-name { accessors [ default-value ] } ;
      [ class-member-visibility-modifier ] abstract qualified-type-name property-name { automatic-accessors } ;

Interface signals

Signals can be defined in interfaces. They have exactly the same semantics as when directly defined in the implementing class.

  • interface-instance-signal-declaration:

    • class-instance-signal-declaration

Other interface members

Constants, Enums, Delegates and Inner Classes all function the same as when they are declared in a class. See Classes. When declared in an interface, all these members can be accessed either using the name of the interface (that is, of the static interface scope), or through and instance of an implementing class.


Here is an example implementing (and overriding) an abstract interface method,

   This example gives you a simple interface, Speaker, with
   - one abstract method, speak

   It shows you three classes to demonstrate how these and overriding them behaves:
   - Fox, implementing Speaker
   - ArcticFox, extending Fox AND implementing Speaker
     (ArcticFox.speak () replaces superclasses' .speak())
   - RedFox, extending Fox BUT NOT implementing speaker
     (RedFox.speak () does not replace superclasses' .speak())

   Important notes:
   - generally an object uses the most specific class's implementation
   - ArcticFox extends Fox (which implements Speaker) and implements Speaker itself,
     - ArcticFox defines speak () with new, so even casting to Fox or Speaker still
       gives you ArcticFox.speak ()
   - RedFox extends from Fox, but DOES NOT implement Speaker
     - RedFox speak () gives you RedFox.speak ()
     - casting RedFox to Speaker or Fox gives you Fox.speak ()

/* Speaker: extends from GObject */
interface Speaker : Object {
  /* speak: abstract without a body */
  public abstract void speak ();

/* Fox: implements Speaker, implements speak () */
class Fox : Object, Speaker {
  public void speak () {
    stdout.printf ("  Fox says Ow-wow-wow-wow\n");

/* ArcticFox: extends Fox; must also implement Speaker to re-define
 *            inherited methods and use them as Speaker */
class ArcticFox : Fox, Speaker {
  /* speak: uses 'new' to replace speak () from Fox */
  public new void speak () {
    stdout.printf ("  ArcticFox says Hatee-hatee-hatee-ho!\n");

/* RedFox: extends Fox, does not implement Speaker */
class RedFox : Fox {
  public new void speak () {
    stdout.printf ("  RedFox says Wa-pa-pa-pa-pa-pa-pow!\n");

public static int main () {
  Speaker f = new Fox ();
  Speaker a = new ArcticFox ();
  Speaker r = new RedFox ();

  stdout.printf ("\n\n// Fox implements Speaker, speak ()\n");
  stdout.printf ("Fox as Speaker:\n");
  (f as Speaker).speak ();   /* Fox.speak () */
  stdout.printf ("\nFox as Fox:\n");
  (f as Fox).speak ();       /* Fox.speak () */

  stdout.printf ("\n\n// ArcticFox extends Fox, re-implements Speaker and " +
                 "replaces speak ()\n");
  stdout.printf ("ArcticFox as Speaker:\n");
  (a as Speaker).speak ();   /* ArcticFox.speak () */
  stdout.printf ("\nArcticFox as Fox:\n");
  (a as Fox).speak ();       /* ArcticFox.speak () */
  stdout.printf ("\nArcticFox as ArcticFox:\n");
  (a as ArcticFox).speak (); /* ArcticFox.speak () */

  stdout.printf ("\n\n// RedFox extends Fox, DOES NOT re-implement Speaker but" +
                 " does replace speak () for itself\n");
  stdout.printf ("RedFox as Speaker:\n");
  (r as Speaker).speak ();   /* Fox.speak () */
  stdout.printf ("\nRedFox as Fox:\n");
  (r as Fox).speak ();       /* Fox.speak () */
  stdout.printf ("\nRedFox as RedFox:\n");
  (r as RedFox).speak ();    /* RedFox.speak () */

  return 0;

Here is an example of implementing (and inheriting) a virtual interface method. Note that the same rules for subclasses re-implementing methods that apply to the abstract interface method above apply here.

   This example gives you a simple interface, Yelper, with
   - one virtual default method, yelp

   It shows you two classes to demonstrate how these and overriding them behaves:
   - Cat, implementing Yelper (inheriting yelp)
   - Fox, implementing Yelper (overriding yelp)

   Important notes:
   - generally an object uses the most specific class's implementation
   - Yelper provides a default yelp (), but Fox overrides it
     - Fox overriding yelp () means that even casting Fox to Yelper still gives
       you Fox.yelp ()
   - as with the Speaker/speak() example, if a subclass wants to override an
     implementation (e.g. Fox.yelp ()) of a virtual interface method
     (e.g. Yelper.yelp ()), it must use 'new'
   - 'override' is used when overriding regular class virtual methods,
     but not when implementing interface virtual methods.

interface Yelper : Object {
  /* yelp: virtual, if we want to be able to override it */
  public virtual void yelp () {
    stdout.printf ("  Yelper yelps Yelp!\n");

/* Cat: implements Yelper, inherits virtual yelp () */
class Cat : Object, Yelper {

/* Fox: implements Yelper, overrides virtual yelp () */
class Fox : Object, Yelper {
  public void yelp () {
    stdout.printf ("  Fox yelps Ring-ding-ding-ding-dingeringeding!\n");

public static int main () {
  Yelper f = new Fox ();
  Yelper c = new Cat ();

  stdout.printf ("// Cat implements Yelper, inherits yelp\n");
  stdout.printf ("Cat as Yelper:\n");
  (c as Yelper).yelp ();  /* Yelper.yelp () */
  stdout.printf ("\nCat as Cat:\n");
  (c as Cat).yelp ();     /* Yelper.yelp () */

  stdout.printf ("\n\n// Fox implements Yelper, overrides yelp ()\n");
  stdout.printf ("Fox as Yelper:\n");
  (f as Yelper).yelp ();  /* Fox.yelp () */
  stdout.printf ("\nFox as Fox:\n");
  (f as Fox).yelp ();     /* Fox.yelp () */

  return 0;

Projects/Vala/Manual/Interfaces (last edited 2017-02-13 19:44:14 by AlThomas)