qooxdoo allows you to easily leverage many key concepts of object-oriented programming without bothering about limited native support in JavaScript.

The main actors of qooxdoo OO are:

• Classes
• Interfaces
• Mixins

When trying to get a grip of the framework code, you should probably understand all those three concepts. As a regular application developer you often get by just knowing how to use classes.

A “class” is a central concept in most object-oriented languages, and as a programmer you are certainly familiar with it. qooxdoo supports a “closed form” of class declaration, i.e. the entire declaration is provided within a qx.Class.define(name, config) statement, where name is the fully-qualified class name, and config is a configuration map with various keys (or “sections”).

There are several types of classes available, which are specified by the type key within the config map:

• regular class:
  May contain class variables/methods (in a statics section) and instance variables/methods (in a members section). An instance of the class can be created using the new keyword, so a constructor needs to be given in construct.
• static class:
  Only contains class variables and class methods. Often a helper or utility class. Use type : “static”.
• abstract class:
  Does not allow an instance to be created. Typically classes derive from it and provide concrete implementations. type is abstract.
• singleton:
  Not more than a single instance of the class may exists at any time. A static method getInstance() returns the instance. Use type : “singleton”.

qooxdoo’s interfaces are similar to the ones in Java. Similar to the declaration of class they are created by qx.Interface.define(name, config). They specify an “interface” (typically a set of empty methods), that classes must implement.

Mixins are a very practical concept that not all programming languages provide. Unlike interfaces, which require a class to provide concrete implementations to fulfill the interface contract, mixins do include code. This code needs to be generic, if it is “mixed into” different existing classes. Mixins usually cover only a single aspect of functionality and therefore tend to be small. They are declared by qx.Mixin.define(name, config).

Like most programming languages qooxdoo only supports single-inheritance for classes, not multiple-inheritance, i.e. a class can only derive directly from a single super class. This is easily modeled by the extend key in the class declaration map.

Since a class may implement/include one or many interfaces/mixins, which themselves can extend others, some advanced forms of multiple-inheritance could be realized.