A model is a simplified representation of the real world and, as such, includes only those aspects of the real-world system relevant to the problem at hand. For example, a model airplane, used in wind tunnel tests, models only the external shape of the airplane. The power of the airplane engines, the number of seats and its cargo capacity do not affect the plane’s aerodynamic properties. Models are widely used in engineering since they can be used to focus on a particular aspect of a real-world system such as the aerodynamic properties of an airplane or the strength of a bridge. The reduction in scale and complexity achieved by modeling allows engineers to analyze properties such as the stress and strain on the structural components of a bridge. The earliest models used in engineering, such as airplane models for wind tunnels and ship models for drag tanks, were physical. Modern models tend to be mathematical in nature and as such can be analyzed using computers.

This article takes a modeling approach to the design of concurrent programs. Our models represent the behavior of real concurrent programs written in Java. The models abstract much of the detail of real programs concerned with data representation, resource allocation and user interaction. They let us focus on concurrency. We can animate these models to investigate the concurrent behavior of the intended program. More importantly, we can mechanically verify that a model satisfies particular safety and progress properties, which are required of the program when it is implemented. This mechanical or algorithmic verification is made possible by a model-checking tool LTSA (Labeled Transition System Analyzer). Exhaustive model checking using LTSA allows us to check for both desirable and undesirable properties for all possible sequences of events and actions. LTSA is available from the World Wide Web (http://www.wileyeurope.com/college/magee). As it has been implemented in Java, it runs on a wide range of platforms, either as an applet or as an application program.

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Between June 1985 and January 1987, a computerized radiation therapy machine called Therac-25 caused six known accidents involving massive overdoses with resultant deaths and serious injuries. Although most accidents are systemic involving complex interactions between various components and activities, and Therac-25 is not an exception in this respect, concurrent programming errors played an important part in these six accidents. Race conditions between different concurrent activities in the control program resulted in occasional erroneous control outputs. Furthermore, the sporadic nature of the errors caused by faulty concurrent programs contributed to the delay in recognizing that there was a problem. The designers of the Therac-25 software seemed largely unaware of the principles and practice of concurrent programming.

The wide acceptance of Java with its in-built concurrency constructs means that concurrent programming is no longer restricted to the minority of programmers involved in operating systems and embedded real-time applications. Concurrency is useful in a wide range of applications where responsiveness and throughput are issues. While most programmers are not engaged in the implementation of safety critical systems such as Therac-25, increasing numbers are using concurrent programming constructs in less esoteric applications. Errors in these applications and systems may not be directly life-threatening but they adversely affect our quality of life and may have severe financial implications. An understanding of the principles of concurrent programming and an appreciation of how it is practiced are an essential part of the education of computing science undergraduates and of the background of software engineering professionals. The pervasive nature of computing and the Internet makes it also an important topic for those whose primary activity may not be computing but who write programs none the less.

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There are two fundamental mechanisms for building new classes from existing ones: inheritance and aggregation. It makes sense to inherit from an existing class Vehicle to define a class Car, since a car is a vehicle. The class Vehicle has several parts; therefore, it makes sense to define a composite object of class Vehicle that has constituent objects of such classes as Motor, Axle, and GearBox, which make up a vehicle.

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Each object created will have its own copies of the fields defined in its class. The fields of an object are called instance variables. The values of the instance variables in an object comprise its state. Two distinct objects can have the same state, if their instance variables have the same values. The methods of an object define its behavior. These methods are called instance methods. It is important to note that these methods pertain to each object of the class. This should not be confused with the implementation of the methods, which is shared by all instances of the class. Instance variables and instance methods, which belong to objects, are collectively called instance members, to distinguish them from static members, which only belong to the class.

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Class Instantiation

The process of creating objects from a class is called instantiation. An object is an instance of a class. The object is constructed using the class as a blueprint and is a concrete instance of the abstraction that the class represents. An object must be created before it can be used in a program. In Java, objects are manipulated through object references (also called reference values or simply references). The process of creating objects usually involves the following steps:

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Introduction

Before embarking on the road to Java programmer certification, it is important to understand the basic terminology and concepts in object-oriented programming (OOP). In this chapter, the emphasis is on providing an introduction rather than an exhaustive coverage. In-depth coverage of the concepts follows in due course in subsequent chapters of the book.

Java supports the writing of many different kinds of executables: applications, applets, and servlets. The basic elements of a Java application are introduced in this chapter. The old adage that practice makes perfect is certainly true when learning a programming language. To encourage programming on the computer, the mechanics of compiling and running a Java application are outlined.

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