4.11 Writing Object-Oriented Programs

4.11 Writing Object-Oriented Programs

Object interaction was explained in Chapter 2. When one object sends a message to another object, the first object invokes or calls a function of the second object. In the class definition of the second object, this function must have been defined as public; otherwise, the function is not accessible to other objects.

Function main is special; execution of the entire application starts in this function, and terminates here. In a typical program (or application), function main carries out the following general sequence of tasks:

1. Declares constants, simple variables, and object variables, as necessary

2. Creates one or more objects of the previously defined classes

3. Invokes one or more methods of the various objects. This delegates the tasks and subtasks to the objects for carrying the complete solution of the application.

4.11.1 A Simple Object-Oriented Program

Consider a definition of class Ball, similar to the one presented in Chapter 3. The attributes of the class are:

• size of type real; represents the diameter of the object in inches

• color of type character (not string); the possible values are 'B' for blue, 'R' for red, 'Y' for yellow, 'W' for white, and 'O' for orange

• move_status of type character; represents the state of the ball object. 'M' is the value of this attribute if the ball is moving. 'S' is the value of this attribute if the ball object is not moving and/or has been stopped.

The methods (functions) of this class are:

• move, which starts movement of the ball object

• stop, which stops the movement of the ball object

• show_status, which displays the values of the attributes color and move_status

• get_color, which reads the value of attribute color from the console

• get_size, which reads the value of attribute size from the console

The program has two classes: Ball and Mball. The second class (Mball) is the class with function main. This function is defined to carry out the following sequence of activities:

1. Declare two object variables, obj_1 and obj_2, of class Ball.

2. Create the two objects.

3. Invoke functions get_color and get_size for each object.

4. Invoke function show_status for each object.

5. Invoke functions move and then show_status for each object.

6. Terminate execution of the entire program.

On the CD

The complete definition of class Ball in KJP follows and is stored in the file Ball.kpl on the CD-ROM that accompanies this book.

       description         This is a simple class. The attributes are color,         size, and move_status. Nov 2002, J Garrido    */       class Ball is         private         // attributes         variables             character color             character move_status             real size         public         // public methods         description           This method reads the value of attribute color           from the console  */         function get_color is         begin           display "type single-character color value "           read color         endfun get_color         description           This method reads the value of attribute color           from the console. */         function get_size is         begin           display "type value of size "           read size         endfun get_size         description             This function displays the color, status,             and size of the object. */         function show_status is           begin             display "Color of ball object: ", color             display "Size of ball object: ", size             display "Status of ball object: ", move_status         endfun show_status         description             This function changes the move_status of the             object.      */         function move is           begin             set move_status = 'M'         endfun move         description             This function changes the move_status of the             object      */         function stop is           begin             set move_status = 'S'         endfun stop       endclass Ball 

On the CD

The implementation in KJP of class Mball is stored in the file Mball.kpl. The two Java classes produced by the KJP translator are stored in the files Ball.java and Mball.java. The complete definition of class Mball follows.

       description          This program illustrates the general structure          of a KJP program. It creates two objects of          class Ball, and invokes some of their methods. */       class Mball is         public           description              This function controls the program.    */           function main is             objects                object obj_1 of class Ball                object obj_2 of class Ball             begin                display "Creating object 1"                create obj_1 of class Ball                display "Creating object 2"                create obj_2 of class Ball                display "Invoking methods of object 1"                call get_color of obj_1                call get_size of obj_1                display "Invoking methods of object 2"                call get_color of obj_2                call get_size of obj_2                // to starting moving the ball objects                call move of obj_1                call move of obj_2                call show_status of obj_1                call show_status of obj_2                // now stop moving the objects                call stop of obj_1                call stop of obj_2                call show_status of obj_1                call show_status of obj_2           endfun main       endclass Mball 

The details for some of the KJP statements used in this example are explained in Chapters 5 and 6. Appendix A contains detailed explanations on compiling and executing KJP and Java programs with jGRASP. Figure 4.5 shows the main jGRASP screen with class Mball.

Figure 4.5: jGRASP with class Mball on the main window.

Figures 4.6 and 4.7 show the console input and output produced during execution of the program.

Figure 4.6: Console input data.

Figure 4.7: Console output data.

4.11.2 A Single-Class Program

This section presents a complete and extremely simple KJP program with only one class and only one function. This is an extreme case of a simple program. No objects are necessary in this program because the entire task is carried out in function main.

Because this program does not involve objects, it is not a real object-oriented program. The program consists of a single-class called Salary1.

On the CD

The KJP code for class Salary1 follows and is stored in the file Salary1.kpl.

       description         This program computes the salary increase for         an employee. If his/her salary is greater than         $45,000 the salary increase is 4.5%; otherwise,         the salary increase is 5%.  */       class Salaryl is         public         description             This function computes the salary increase             and updates the salary of an employee.  */         function main is           constants             real percentl = 0.045  // percent increase             real percent2 = 0.05           variables             real salary             real increase           // body of function starts           begin             display "enter salary: "             read salary             if salary > 45000 then                set increase = salary * percentl             else                set increase = salary * percent2             endif             add increase to salary             print "increase: ", increase,                                  " salary: ", salary         endfun main       endclass Salaryl