DISCRETE COMPUTER SIMULATION WITH JAVA

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1 DISCRETE COMPUTER SIMULATION WITH JAVA Ricardo Augusto Cassel Department of Management Science - The Management School - Lancaster University Lancaster, LA1 4YX, United Kingdom Michael Pidd Department of Management Science - The Management School - Lancaster University Lancaster, LA1 4YX, United Kingdom Abstract: Simulation methods use a model of part of the real world as basis of exploration and experimentation. The three-phase approach provides a simple and robust way to develop discrete computer simulation programs. Object orientation is a save way of developing software, since it allows programs to be easily enhanced and enable important variables of the system to be protected and controlled. Java is one of the most popular object-oriented computer programming languages nowadays, specially because of its use in the World Wide Web and its capacity to create applets. This paper shows the use of the three-phase and the object-oriented approaches in discrete computer simulation, and describes the first stages of the simulation library written in Java. Keywords: JAVA language, discrete simulation, object orientation. 1. INTRODUCTION Since time immemorial, man has attempted to find ways to consider the problems that he might face in the future, and simulation is one way to do this. Simulation is the use of model as a basis for exploration and experimentation [Pidd, 1996]. Like all modelling approaches, simulation is used because it may be cheaper, safer, quicker and more secure than using the real system itself [Pidd, 1996]. Cassel and M ller (1996) created a diagram showing the difference between the flow of control of a real system and of a system which is simulated (Figure 1). As it can be seen, the experimentation, in a feedback approach, is realised on the system itself, while using simulation, the experimentation can be realised on the model and the results passed to the system, usually saving money and time. By the 1960's, a new approach for simulation has been started: the computer simulation. This approach has been enabled by the incredible development of computers, which have become quicker, cheaper, smaller and easier to use. Computer simulation methods have developed along with the computers, and may well be the most commonly used of all the analytical tools of management science[pidd, 1992]. In this paper, the word simulation will be used meaning discrete computer simulation.

2 FEEDBACK Inputs + System Outputs FEEDFORWARD FAST Inputs Model + System Outputs CORRECT FIGURE 1: Flow of Control of Real Systems The next sections of the paper will roughly describe the three-phase approach in discrete simulation, give an overview in object oriented programming, introduce the Java language, explain the difficulties that are being faced in learning the language, and the future steps to be followed in writing a library in Java for computer simulation. 2. THREE-PHASE APPROACH IN DISCRETE SIMULATION The three-phase approach was first described by Tocher (1963), and a further detailed description was given by Pidd (1992). This approach admits that most systems have two different kinds of activity: those whose starting time is known, allowing it to be scheduled; and those which depend on some conditions to be started. The activities that are bound to happen or whose starting time can be booked in advance are known as the B activities. Those activities that lie on some conditions to happen are known as the C activities. While in a process oriented model there is an attempt to form a chronological sequence of the activities, making the entities move as far through its process as possible at each time beat, in the three-phase approach the events and activities are treated as if they are independent of one another. Firstly, all the B activities due at the current time of the simulation are executed, and then all the C activities are tried. As an example, it can be seen that, in the process of the figure 2, there are five B activities and four C activities: B Activities (Bound activities) arrive in the petrol station; end of move to available pump; end fill the tank; end buy snack; end pay and leave.

3 Arrive in Petrol Station Go to a Free Pump Fill the Tank Y Buy a Snack? N Buy a Snack Pay Leave FIGURE 2: Example of a Petrol Station C Activities (Conditional activities) move to pump when available; start to fill the tank when pump activated; start to buy a snack; start to pay when cashier free. Having realised how to define the B's and C's activities, the three-phase approach is quite easy to be understood. The three phases to be followed are: A Phase: finds time of the next event and moves clock to that time; B Phase: executes all B's that are due at this time, keeping it constant; C Phase: holds clock constant and attempts all C's until none succeed. 3. OBJECT ORIENTATION Object orientation is a buzzword these days, used to describe languages, styles of programming, user interfaces, and just about anything else, however appropriately or inappropriately [Anuff, 1996]. One of the main claims made for object orientation is that it is the natural way of thinking about things. The world consists of objects, making it straightforward to understand a problem by considering the objects that are part of the system of interest. One of the clearest introduction to the features that characterise object orientation is given by Wegner (1990). To be consider as object-oriented, it seems to be generally agreed that a computer program should have the following features: Class Mechanism In a pure object-oriented view, all program variables are represented in the form of objects which are instances of classes. These classes contain two parts, the member data and the methods. The member data is the variables contained in the class, while the methods are functions used to deal with the data. New classes can be built from other classes, which are either pre-defined or are defined within the program being developed. This ability to define a new class in terms of one earlier class is known as inheritance and helps to avoid the need of defining new classes from the scratch.

4 Encapsulation As said before, a class in an object-oriented program has two parts: the member data and the methods. The member data can be hidden from code in other objects or classes and be only changed by the member functions of its own class. This process is called encapsulation. Encapsulation, therefore, defines the ways in which the member variables are to be manipulated, making sure the data fields will be only altered in manners strictly defined and controlled. This brings two benefits: - there is no need to have global data which can be changed by accident from elsewhere in the program; - it permits programs to be extended and enhanced at later dates as, once again, the risk of accidental damage to data can be fully controlled [Pidd, 1995a]. Polymorphism In an object-oriented language, any computation demanded of an object must be made by giving the name of the object as well as the name of the member function. This is needed because of the feature called polymorphism, which allows different classes to have functions with the same name. Polymorphism also allows an object to respond to the set of messages defined in superclasses of the object's class with methods implemented in the object class. Therefore, to call any member function, the following form is needed: ObjectName.MemberFunctionName The linking of the function to the object may be done by two ways: static binding or dynamic binding. In the static binding, the link of the object type and the function is known before the program is compiled, meaning that it cannot be re-defined as the program runs. In the other hand, in the dynamic binding, there is no need for the programmer to specify the precise type of object before compilation. The use of dynamic binding helps, specially, the creation of queues in which the programmer does not know what type of object is the next to enter the queue. After the small explanation of the three-phase approach and object orientation, there is one question that emerges: is it worth to use the three-phase simulation to link object orientation and discrete simulation? According to Pidd (1995b), though it may seem that process orientation is the obvious way to link object orientation and discrete simulation, when considering systems with complex interactions, two problems occur with this approach: the first is that true synchronicity is lost because the approach pretends that each entity class can be moved on it turn - whereas, in reality, they all move at the same time; the second problem is linked to the first: if the entities' movements are considered in the wrong order, deadlock is possible. Therefore, the linking of object orientation and three-phase simulation is worth serious consideration, specially in large and complex programs. 4. JAVA Java is an object-oriented language that has many elements in common with C and C++, and that is widely used in the World Wide Web, specially because of the Java applets. An applet is a small program that is automatically downloaded from a Web site and run

5 within a Web browser in response to instructions to do so contained within the Web page being viewed [Anuff, 1996]. Java can be called an almost pure object-oriented language, since it deals with everything as objects, besides simple data types, such as integers, characters, and floating point numbers. The language is both compiled an interpreted. This means that, after writing a Java program in a text editor, the programmer has to compile this source file to produce a type of binary object file called a class file. This file is not executable because it contains instructions that are not understandable by the processor. Nevertheless, they are very similar to native machine codes, allowing them to be executed with performance that approaches that of native compiled languages. To run it, a Java interpreter is needed. This may be seen as a drawback of the language, because a program can never be executed in a computer that does not have the interpreter installed. However, Java programs will run unmodified on any platform to which the Java interpreter has been ported. Java has the following characteristics related to the three features that characterise an object-oriented program: related to the class mechanism and inheritance: the Java language does not allow multiple inheritance, i.e., a subclass cannot be created from more than one superclass. In the other hand, Java allows a subclass be created from more than one "special kind of class" called interface. An interface, like a class, has methods and variables, but all variables in an interface are constants and all the methods are abstract methods. Abstract methods are methods that are only declared, and not defined, on the superclass or in the interface. They have to be defined in the class that extends the superclass or that implements the interface. related to the encapsulation: in Java, there are three modifier keywords used to implement encapsulation. Public: methods and variables marked as public can be seen by any other method in the program. Private: a private method or variable cannot be used by any other class, including subclasses. Protected: a protected variable or method can be used only from inside their class or in subclasses of that class. If none of the modifiers is used, the variable or the method can be used by other classes in the same package, but not by classes in other packages. Packages are similar to class libraries, i.e., collections of classes, usually grouped according to functionality. related to polymorphism: in Java, all classes are subclasses of the Object class. This means that, when a dynamic binding is needed, all classes can be considered an Object. 5. LEARNING JAVA Pidd (1996) describes six principles that can be applied when developing a model that will be useful in management science. These principles can also be applied to learn a programming language:

6 Model simple, think complicated In the beginning, when someone is learning a new language, all the programs that are written are very simple. However, the person that is learning the language has to be aware of the future applications of what is being learned. Even, later on, when the programs start to grow, it is important to leave them as simple as possible, allowing anyone to understand what has been done. This leads to the second principle. Be parsimonious, start small and add This principle is sometimes known as KISS, standing for Keep It Simple, Stupid. It is worthwhile to developed small programs, learn with them, and later add and enhance them. Divide and conquer, avoid mega-models It is usual to try to understand new concepts as quick as possible. Therefore, it is usual to try to develop programs that combine all the new concepts. This can be quite dangerous due to the great probability of the program to fail in running and the time spent to find the mistakes. Keeping the programs small and trying to develop new concepts in separated programs can help avoiding time waste. Use metaphors, analogies and similarities This principle can be interpreted in two different ways, concerning the learning of a new programming language. A language can be more easily learned by finding some similarities and making some analogies to other languages already known. In the other hand, this can bring some confusion due to the differences between the languages. Do not fall in love with data In this case, data can be considered all the books and references about the language. Usually, the best way to learn a language is programming, and getting the data just when it is needed. Therefore, the references should be used to answer the questions that arise during the programming, and not to be read over and over. Model building (Language learning) may feel like muddling through Learning a new language is not an activity in which smooth progress is made. Programmers have to know that, sometimes, they are going to get stuck, and it may take a while until they can make any improvement again. Even following the principles, it can take a while to learn Java, mainly because of the its big library of functions, and the difference between the use of references to objects in Java and the usual pointers in other languages like C and C FURTHER STEPS The main purpose of this study is to develop a simulation library written in Java that can be used over the World Wide Web. This library will be platform independent and will be used by anyone who has access to the internet. Besides, Java permits the use of many different graphical interfaces, making the program easy to be run and understood by the user. This can make simulation even more popular and more accessible to everyone interested in this technique.

7 The first steps towards this objective have already been done. The Java language has been learned and some parts of the library have already been developed and written. However, much more has to be done: the remaining parts of the library have to be written; the whole library has to be re-evaluated, since new features of the language have been learned during the developing process; the library has to be run as one, to find out some bugs or some linkage errors between the different parts; the documentation about the library has to be written; the library has to be put in the WWW. ACKNOWLEDGEMENTS Ricardo Augusto Cassel is a Ph.D. student in the Lancaster University and is sponsored by the Brazilian Government through CAPES - Funda ±o Coordena ±o de Aperfei oamento de Pessoal de N vel Superior. REFERENCES ANUFF, E. The Java sourcebook. United States of America, John Wiley & Sons Inc., CASSEL, R. A. & M LLER, C. J. "A simula ±o e os processos de mudan a." Proceedings of the 2nd International Congress of Industrial Engineering, Piracicaba - SP, oct GARSIDE, R. & MARIANI, J. draft version of Java: first contact. Great Britain, International Thomson Publishers, PIDD, M. Computer simulation in management science. Great Britain, John Wiley & Sons Ltd., PIDD, M. "The construction of an object-oriented traffic simulator." Presented to the European Working Group on Transportation, Universitat Politecnica de Catalunya, Barcelona, Sept. 1995a. PIDD, M. "Object-orientation, discrete simulation and the three-phase approach." Journal of the Operational Research Society, 1995b, vol. 46, No. 3, pp PIDD, M. Tools for thinking. Great Britain, John Wiley & Sons Ltd., SCHILDT, H. C++: the complete reference. United States of America, Osborne McGraw-Hill, TOCHER, K. D. The art of simulation. Great Britain, English University Press, WEGNER, P. "Concepts and paradigms of object-oriented programming." OOPS Messenger 1, 7-87.

What are the characteristics of Object Oriented programming language?

What are the characteristics of Object Oriented programming language? What are the various elements of OOP? Following are the various elements of OOP:- Class:- A class is a collection of data and the various operations that can be performed on that data. Object- This is