Object model in Java: elements and application

1999, Information and Software Technology

Abstract

The Java programming language is a new object-oriented programming language that is gaining widespread popularity in the computer software industry because of its ease of learning, simplicity, generality, portability and networking capabilities. In this article, we discuss the semantic implementation in Java of each of the elements of the object model, identified by Booch [G. Booch, Object-Oriented Analysis and Design with Applications, 2nd Edition, Addison-Wesley, 1994], in addition to the mobility element. We show that Java, unlike other objectoriented languages, covers and simplifies all elements of the object model. An example illustrating those elements in Java will be drawn from a telecom software application.

Figures (9)

Fig. 1. Elements of the object model.

Fig. 2. Containment or composition relationship between objects. Another encapsulation and abstraction mechanism is provided by a Java interface which contains public abstract A hierarchy allows the organisation of various abstrac- tions mainly for the sake of reusability. Java supports two hierarchical classification structures: a class hierarchy and an interface hierarchy. The root of the class hierarchy consists of the most general class, i.e., the Object class, and the leaves consist of the most specialised classes. The recursive class/subclass hierarchy is governed by the inheri- tance rules which support code or class reusability. A subclass inherits all of the non-private methods and attri- butes of its superclass in the hierarchy. However, the inter- face hierarchy does not have a single root, and interfaces are not limited to a single superinterface. Therefore, an inter- face can inherit abstract methods and class variables from more than one superinterface. The mixture of class and

Encapsulation of attributes and methods in Java Table 1

Fig. 3. Modularization in a Java application. interface hierarchy can eliminate the complexities and ambiguities of multiple class inheritance existing in C++. 2.2.1. Typing

Fig. 4. Encapsulation of an object. Typing in an object-oriented programming language relates to the degree of enforcement of type checking when dealing with object assignment, parameter passing and manipulation. For the sake of writing robust programs, it is often recommended to use strongly typed languages. In Java, type checking is strong and performed at compile time so that the programmer is aware of a type conformance violation prior to run-time. Moreover, Java implements the dynamic binding mechanism for method calls in which the types of variables are only known at run-time. However, for the sake of efficiency, final methods (i.e., methods that cannot be overridden) are statically bound at compile time. Also, Java supports polymorphism meaning that some names can have different meaning or behaviour (i.e., method names). In fact, two aspects of polymorphism, method overriding and overloading, are enforced using dynamic binding.

Implementation of the object model elements in Java

Fig. 5. A class hierarchy and interface implementations.

Comparison between Java, C++, Smalltalk and Ada95 object models they can be retrieved properly. A partial solution is to dedi- cate one file for each class.

Subscribers can access this telecom service on the inter- net and engage in a communication session. Operators can monitor the state of the service at any time and over the internet. Records of subscribers (i.e., callers and callees) are

Key takeaways

In this article, we study Java from the object model point of view.
Java supports abstraction by allowing the definition of classes with their attributes and methods and the creation of objects form such classes.
In the following we discuss the four minor elements of the object model and their implementation in Java.
The Java programming language provides the syntax and semantic for supporting all the eight elements of the object model.
The power of the Java object model is exploited and illustrated in the following application example.