Class_Member

Class Member Assignment Peter Komisar
rev. 27 / 4 / 2000

Note:
This source code contains two classes. Only one class can be the top level class, that is the class
that lends it's name to the file to which the source code will be saved. In this case, the top level
class will need to be the class that has the main method, (that's the Average_Engine class) This is
because the compiler looks for a main method in the file which is provided to it as an argument
at the command line. Another note, for reference, only this top level class can be marked public.

class Student{

/* fields describing the student ID and course marks */

private String ID="1234546";
        int   math = 85;
        int english = 55;
final int history = 65;
        int science = 92;
        int copyNo = 0;

/* The constructor I am using to get this action done everytime the
class instantiates. Notice the constructor has the same name as the
class and looks like a method except with no return type */

Student( ){
copyNo = copyNo+1;
}

/* a method returning the students marks */

public int getAverage(){
        int total = math + english + history + science;
        int average = total / 4;
        return average;
        }
/* a method returning the student's ID */

public String getID(){
return ID;
}

}
class Average_Engine{
public static void main(String [] argv) {
Student av = new Student();
Student av2 = new Student();
Student av3 = new Student();
Student av4 = new Student();
Student av5 = new Student();
int average = av.getAverage( );
System.out.println("Student " + av.getID() + " has an average of " + average);
System.out.println("The student's science mark is " + av.science);
av.science=23; // here's the work of the prankster
average = av.getAverage( );
System.out.println("The student's science mark is " + av.science);
System.out.println("Student " + av.getID() + " has an average of " + average);
System.out.println("av5 is copy number " + av5.copyNo + ".");
}
}

Questions

Students are presented their marks via new instances of classes that represent their records.
Assume you have access to the Average_Engine class but no direct access to the student class.
You can only reference the Student class instances in the manner the prankster below did.
(The 5 instances of the student class are there for use of Q6 and Q7)

1. Notice how a prankster has taken the luster off of this students average. Change the mark
back to what it is suppose to be from within the source code for the Average_Engine class.

2. This student actually should have received 99% in math. Change that mark too by the same
technique. This will give the student the correct impression of his mark until someone can
change it back in the class.

3. Try to change the student ID by the same technique. from within the Average_Engine class.
What happens? Why can't the ID be changed?

4. Try to change the history mark. What happens why?

5. What role does the method getID( ) play. What barrier does the method circumvent?

6. Break the rule and paste the following print line into the constructor immediately after the line,
copyNo = copyNo+1;

System.out.println("I'm in Student class constructor and copyNo is " + copyNo);

Recompile and run the code. The copy numbers are the same why?

7. Break the rule again and go into the Student class and mark the the copyNo variable static.
(Do this where copyNo is declared) Redo Step 6. What has changed and why?