Note: information on this page refers to Ceylon 1.1, not to the current release.

Interface Declarations

An interface is a stateless type declaration that, unlike a class declaration:

  • may not hold references to other objects,
  • does not define initialization logic, and
  • cannot be directly instantiated.

Interfaces support a more flexible inheritance model: multiple inheritance. "Diamond" inheritance is not a problem for interfaces, because interfaces don't have state or logic to initialize that state.


A trivial interface declaration looks like this:

interface Trivial {
    /* declarations of interface members */

The general form of an interface declaration looks like this:

interface Example
        satisfies SUPER-INTERFACES


Due to the stateless nature of interfaces, their declarations have:

  • no parameter list after the interface name (as there is in a class declaration).
  • no extends clause
  • no initializer section in the interface body: It contains only member declarations.


Type parameters

An interface may have a list of type parameters enclosed in angle brackets (< and >) after the interface name.

interface Generic<Foo, Bar> {
    /* declarations of interface members 
       type parameters Foo and Bar are treated as a types */

An interface with type parameters is sometimes called a generic interface.

An interface declaration with type parameters may also have a given clause for each declared type parameter to constrain the argument types:

interface Constrained<Foo, Bar>
        given Foo satisfies Baz1&Baz2
        given Bar of Gee1|Gee2 {
    /* declarations of interface members 
       type parameters Foo and Bar treated as a types */

Satisfying interfaces

The satisfies clause is used to specify that the interface being declared is a subtype of the given interface type.

interface I satisfies I1 & I2 {
    /* declarations of class members */

& is used as the separator between satisfied interface types because the interface (I) is being declared as a subtype of an intersection type (I1&I2).

If an interface is declared without using the satisfies keyword, it does not directly inherit any interfaces. However, all interfaces are considered to inherit the class Object.

Enumerated interfaces

The subtypes of an enumerated interface can be constrained to a list of named interfaces, classes, or toplevel anonymous classes using the of clause. If the interface I is permitted only two direct subtypes, the interface I1, and the class C1, its declaration would look like this:

interface I of I1 | C1 {
    /* declarations of interface members */

Then I1 and I2 are called the cases of I.

Declaration section


The permitted members of interfaces are classes, interfaces, methods, and attributes.

Note that an interface cannot have an object member.

Different kinds of interface

shared interfaces

A toplevel interface declaration, or an interface declaration nested inside the body of a containing class or interface, may be annotated shared:

shared class C() {
    /* declarations of class members */
  • A toplevel shared interface is visible wherever the package that contains it is visible.
  • A shared interface nested inside a class or interface is visible wherever the containing class or interface is visible.

sealed interfaces

An interface declaration can be annotated sealed which prevents the declaration of subclasses and subinterfaces outside the module in which the sealed interface is declared. This provides a way to share an interface's type with other modules.


An interface alias is a kind of alias.


Interface declarations can be manipulated at runtime via their representation as InterfaceDeclaration instances. An applied interface (i.e. with all type parameters specified) corresponds to either an Interface or MemberInterface model instance.

See also