Abstract Data Types
Abstract data types are descriptions and representations of a real-world construct. Many languages support aggregation of disparate intrinsic data types to represent some real world construct. For example in "C" the struct keyword denotes such an aggregation. However an abstract data type is more than just an aggregation of disparate data types. An abstract data type also defines behaviors represented by the abstraction. In many object-oriented languages the combination of data and its associated behaviors denotes a class. Languages such as C++, Java, and C# provide the class keyword to identify abstract data types.
Notice that the function keyword is overloaded and serves as both the constructor function for objects as well as identifying procedural functions:
The difference between MyClass being interpreted as a constructor or as a procedure is the new operator. The new operator instantiates an object of class MyClass calling the constructor function, while in the second call a procedural call is made to the function MyClass expecting a return result.
The problem however is that only the instance of MyClass referenced by myClassObj possesses the additional data properties. Subsequent instances will not have any properties. What is needed is a way to defined properties to all instances of MyClass. Using the this keyword in the constructor function data properties are now defined on all instances of MyClass:Figure 5. Defining data properties to all instances of the class
MyClass is still incomplete because there are no behaviors assigned to it. To add methods to MyClass, properties that reference functions are added to MyClass:
Using MyClass as defined above permits accessibility of its internal data representation as well as having its methods and variable names global in scope increasing the risk of name collisions. Encapsulation supports data hiding and the concept of viewing objects as self-contained entities providing services to consumers.
Figure 7. Dynamically assigning properties to all object instances
In the example below, the Shape prototype object defines three properties, GetArea, GetPerimeter, and Draw, that reference the functions, Shape_GetArea, Shape_GetParameter, and Shape_Draw. Every instance of Shape inherits the prototype allowing them to call the Shape functions through the properties.
Circle and Rectangle prototypes are assigned to the prototype of the Shape class through an object instance thereby "inheriting" the methods assigned to the prototype array of the Shape class. An instance of the Shape class is necessary to create a copy of the Shape prototype array. This permits overriding of the Shape methods yet preserving the original prototype array associated to all Shape objects.
Both the Circle and Shape classes extend the Shape class by overriding Draw method supplying its own respective implementation while inheriting the Shape implementation of the GetArea and GetParameter methods:
Polymorphism defines disparate behaviors and actions by objects to the same function invocation. The Draw property of Shape, Circle, and Rectangle object types is polymorphic. The Draw property invokes a different function depending upon its object type.