Class Declarations

A class is a stateful type declaration that:

  • may hold references to other objects,
  • may define initialization logic, and
  • except in the case of an abstract or formal class, may be instantiated.

A class may inherit another class, but classes are restricted to a single inheritance model. That is, a class inherits exactly one other class. Since single inheritance is quite often too restrictive, a class may also satisfy an arbitrary number of interfaces.


A trivial class declaration looks like this:

class Trivial() {
    /* declarations of class members */

A more general form of a class declaration with initializer parameters looks like this:

class Example
        satisfies SUPER-INTERFACES


An alternative to declaring a class with initializer parameters is to provide one or more constructors.

class Example
        /* no parameter list */
        extends SUPER-CLASS
        satisfies SUPER-INTERFACES


  • SUPER-CLASS is the type expression for the superclass (not an invocation)
  • INITIALIZER-STATEMENTS-AND-CONSTRUCTORS consists of statements in the intializer and constructor declarations
  • DECLARATIONS constitute the declaration section of the class.


Type parameters

A class declaration may have a list of type parameters enclosed in angle brackets (< and >) after the class name:

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

A class with type parameters is sometimes called a generic class.

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

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

Initializer parameters

Class declarations usually have a parameter list, the initializer parameters,

class WithParameters(Integer param1, String param2) {
    // ...

The initializer parameters are visible to statements in the class initializer.

If a class lacks a parameter list it must have one or more constructor declarations.

Callable type

A class may be viewed as a function that produces new instances of the class. The callable type of a class expresses, in terms of the interface Callable, the type of this function.

For example the callable type of

class CallableExample(Integer int, Boolean bool) => "";

is CallableExample(Integer, Boolean), because the class initializer takes Integer and Boolean parameters and invoking the class results in a CallableExample instance being returned to the caller.

(Regular functions also have a callable type.)

Extending classes

The extends clause is used to:

  • specify that the class being declared is a subtype of the given class type and,
  • invoke that class's initializer.

In the case of a class with initializer parameters the superclass constructor is invoked directly in the class's extends clause:

class S() extends C() {
    /* declarations of class members */

In the case of a class with constructors each constructor declaration has its own extends clause for invoking the appropriate superclass constructor:

class T extends C {
    shared new () extends C() {}
    new nonShared() extends C.nonShared() {}

If a class is declared without using the extends keywords, it is a subclass of Basic.

Satisfying interfaces

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

class C() satisfies I1 & I2 {
    /* declarations of class members */

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

If a class is declared without using the satisfies keyword, it does not directly inherit any interfaces. However, it may indirectly inherit interfaces via its superclass.

Enumerated classes

The subclasses of an abstract class can be constrained to a list of named class types (including toplevel anonymous classes) using the of clause. If the class C is permitted only two direct subclasses, S1 and S2, its declaration would look like this:

abstract class C() of S1 | S2 {
    /* declarations of class members */

The subclasses have to extend C:

class S1() extends C() {
class S"() extends C() {

Then S1 and S2 are called the cases of C.

If a class has enumerated subclasses we can use the subclasses as is cases in a switch statement.


The class initializer executes when instances of the class are created (also known as class instantiation). The parameters to the initializer (if any) are specified in parenthesis after the name of the class in the class declaration.

The body of a class must definitely initialize every member of the class. The following code will be rejected by the compiler because if bool is false greeting does not get initialized:

class C(Boolean bool) {
    shared String greeting;
    if (bool) {
        greeting = "hello";

The typechecker figures out for itself the point in the class at which all class members have been initialized. Everything before this point is in the initializer section of the class, and everything after this point is in the declaration section. In the initializer section you can't use a declaration before it's been declared.

Note that abstract classes cannot be invoked directly, but they are still invoked in the extends clause of their subclasses.

Constructor Declarations

If a class lacks initializer parameters it must have one or more constructor declarations.

Note: Constructors allow sophistocated initialization logic, but at a cost in terms of complexity and verbosity compared with classes with initializer parameters. It is recommended to prefer classes with initializer parameters where possible. Most classes can be written with initializer parameters.

The job of the constructor is to particpate in initialzing a class instance. A constructor is either be a value constructor or a callable constructor.

Value Constructors

A value constructor is simply a named instance of a class that it accessible by name, without requiring a parameter list:

class ValueConstructor {
    new example {
        // initialization logic

The instance is created lazily on first access (i.e. when ValueConstructor.example is first evaluated) and is initialized according to the body of the value constructor example. Thereafter further references to example refer to the same instance. This can be viewed as a form of the singleton pattern, rather like an object declaration. Unlike an object declaration, however, a value constructor is not declaring a new class, simply a named instance of a class with some initialization logic.

It is possible to constrain the class to have a finite set of named instances using the of clause:

class EnumeratedInstances 
        of one|two|more {
    new one {
    new two {
    new more {

Callable Constructors

A callable constructor is function that participates in initializing an instance of the class.

class CallableConstructor {
    new example() {
        // initialization logic

A callable constructor always initializes a fresh instance of the class, and multiple invocations of a callable constructor result in multiple instances of the class.

Default Constructors

The callable constructor with no name is the default constructor

class DefaultConstructor {
    shared new () {

This is the constructor used when the class is instantiated without an explicit constructor, for example via the invocation DefaultConstructor().

This has an important consequence with respect to refactoring: A class with a parameter list can be refactored into a class with a default constructor without affecting the sites where the class was instantiated.

Classes do not need to have a default constructor, but if it exists it must be shared.

Constructor Delegation

When a class has constructors the class's extends clause is the expression for the superclass type, and not an invocation expression. Delegation to the superclass for initialization is instead achieved via an extends clause on the constructor declaration:

class DelegationToDefault extends DefaultConstructor {
    shared new () extends DefaultConstructor() {
class DelegationToNamed extends CallableConstructor {
    shared new () extends CallableConstructor.example() {

Alternatively the constructor can delegate to another constructor of the same class:

class SelfDelegation {
    shared greet(String greeting, String subject) {
    shared new hello(String subject) extends greet("hello", subject) {
    shared new helloWorld extends hello("world") {

The above example shows a callable constructor hello delegating to a callable constructor greet, and a value constructor helloWorld delegating to hello.

abstract Constructors

A callable constructor usually has to initialize all the members of the class which are not initialized by the rest of the initializer section of the class.

An abstract constructor is allowed to leave members unitialized. An abstract constructor can only be used for other constructors to delegate to (otherwise you could obtain a partially initialized instance, which would defeat the whole point of having constructors). It follows that abstract constructors may not be shared.

Statements between constructors

Constructors occur in the initializer section of the class, but they can be preceeded by, separated by and followed by other statements, which are executed in order during initialization:

class StatementsAndConstructors {
    abstract foo() {
    shared new () extends foo() {

An invocation such as StatementsAndConstructors() would have the side-effect of printing


to standard output.

Declaration section


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

Member class refinement

An inner class of a class or interface can be subject to member class refinement, which means its instantiation will be polymorphic.

Here's an example where a Reader class declares that concrete subclasses must (because we used formal) provide an actual Buffer inner class.

shared abstract class Reader() {
    shared formal class Buffer(Character* chars)
            satisfies Sequence<Character> {}
    // ...

shared class FileReader(File file) 
        extends Reader() {
    shared actual class Buffer(Character* chars)
            extends super.Buffer(*chars) {
        // ...
    // ...

Within Reader (and elsewhere) we can instantiate the relevant kind of Buffer with a normal instantiation, Buffer(chars). This allows each subclass of Reader to implement an appropriate kind of Buffer.

Member class refinement is a lot like the 'abstract factory' pattern in other object-oriented languages, but it's a lot less verbose.

Only formal and default member classes are subject to member class refinement. A formal member class must be refined by concrete subtypes of the type declaring the member class—just like a formal method or attribute. A default member class may be refined—just like a default method or attribute.

In a subtype of the type declaring the member class, the member class (i.e. in FileReader.Buffer from the example above) must:

  • be declared actual,
  • have the same name as the member class in the declaring type (Buffer in the example),
  • have a parameter list with a compatible signature and,
  • extend the member class (you'll need to use super in the extends clause).

Refined member types are similar to, but not the same as, virtual types, which Ceylon does not support.

Different kinds of class

Concrete classes

A class that can be instantiated is concrete. It follows that abstract or formal classes are not concrete.

Abstract classes

An abstract class is a class that may not be instantiated. Abstract classes may declare formal members. An abstract class declaration must be annotated abstract:

abstract class C() {
    /* declarations of class members */

Naturally, abstract classes compete with interfaces, since both an abstract class and an interface may contain a mix of concrete and formal members. The crucial difference is:

  • an abstract class may contain or inherit state and initialization logic, whereas
  • interfaces support a full multiple inheritance model.

Nevertheless, it is often unclear whether a certain situation calls for an interface or an abstract class. Our advice is to incline in favor of using an interface, where reasonable.

shared classes

A toplevel class declaration, or a class 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 class is visible wherever the package that contains it is visible.
  • A shared class nested inside a class or interface is visible wherever the containing class or interface is visible.

formal classes

A class declaration nested inside the body of a containing class or interface may be annotated formal. A formal class must also be annotated shared.

Like abstract classes, formal classes may have formal members. Unlike abstract classes, formal classes may be instantiated.

A formal class must be refined by concrete subclasses of the containing class or interface.

default classes

A class declaration nested inside the body of a containing class or interface may be annotated default. A default class must also be annotated shared.

A default class may be refined by types which inherit the containing class or interface.

sealed classes

A class declaration annotated sealed cannot be instantiated (either in an invocation expression or in an extends clause) outside the module in which it is defined. This provides a way to share a class's type with other modules while retaining control over subclassing and instance creation.


A class alias is a kind of alias.


Class declarations can be manipulated at runtime via their representation as ClassDeclaration instances. An applied class (i.e. with all type parameters specified) corresponds to either a Class or MemberClass model instance.

See also