need help: I'm just not getting how this whole "private" / "public" class definition thing works.

[Holden Hodgdon]

We are using absolute Java 4th edition for this class, and chapter 4.1 prattles on for 32 pages of run on sentences. I've read 4.1 three times now, and all of chapter 4 twice, but I'm still not getting this ... I have this funny feeling that this book is kinda written as a guide to Java for people who already know how to program, I don't so instead of frolicking outside in the summer sun I'm pulling my hair out in front of a monitor.

additionally the teacher isn't "accessible" ...
(*slaps self* stop bitching!)

ok, so what I'm understanding is that you establish either public or private class "entities" outside of where the actual code is run. from there you can then use those established "classes" to dynamically generate instanced variables based on the root classes, that way you don't have to hard code in multiple copies of the same thing.

I get that (I think), but how the hell do I actually go about implementing that? the book has got a bunch of examples of different ways to go about this, but every single one of them only highlights the "no " parts of things while completely ignoring the "WTF is that doing" side of things.

I'm totally lost on how to establish the various elements of these self defined classes, that each instanced variable inherits from the parent class.

for example:

in the assignment that I'm working on (and is subsequently late at this point), we have to create a student record that calculates a final grade based on inputed supplied test scores.

the whole list of data we have to deal with is naming the student record, 3 quizes, 2 tests, the number of points the student got for each one of those, and the final grade.

I sorted that down to what I think I need:

the class it's self is the student's ID.

attached to that is the total number of grade points they got for the tests, and the final grade.

I've got the actual code to calculate the total points and then generate the final grade built, but it's currently just hard coded to run through one student at a time, and I'm lost on how to build the mechanic for generating instanced classes.

(this is project 6 of chapter 4 [P.248] if anyone wants to look up the un-bastardized details)

could someone provide me a with a walk though on how to set up these classes, and then instance them inside the actual code? preferably with some examples that start at just the basic formating of "this goes here, then that goes there" and then ratchet up more complicated implementations?

I'm kinda bright, and can pick things up quickly provided I've got something to get traction on ... currently this book is like Glass to me.

(P.S. I know beggars can't be choosers, but I sort of have to get this in by midnight tonight or I won't get any credit for it, so I'm gonna be hovering over this thread like a hawk and so even if you can't exactly answer this for me I am willing to take any discussion people throw out.)

[fred rosenberger]

on a very high level, (and forgetting about public and private), when you first approach a problem like this, you kind of talk through it. Tell yourself in words what needs to happen. Then, look for the nouns and verbs. Nouns become objects and variables. Verbs become methods (again, this is VERY broadly speaking).

some of the nouns you talk about are:
Student record
Final grade
test scores
quizzes
tests
points

Some of the verbs are
input (scores/points)
create (student records)
Calculate (final grade)

So now you start looking at this and say "What goes together? What can be eliminated? What else might I need?"

Personally, I would start by saying that test and quizzes are more or less the same thing - each has a 'max possible score' and a 'what the student scored'.

This implies i need a class, which I will call 'ExamResult'. It will have two member variables to hold those two items:

public class ExamResult
{
     int possibleScore;
     int studentScore;    //question for later - should this be a float?  Can a student get half a point???
}

So now I need to think about what i need to be able to DO with an ExamResult. I probably need a way to create one, to input the scores, possible change the scores (if i discover I made a mistake on one), and maybe get the percentage. So, I'll revise my class

public class ExamResult
{
     int possibleScore;
     int studentScore;    //question for later - should this be a float?  Can a student get half a point???
 
     //constructor to create a specific exam result
     public ExamResult(int total, int score)
     {
            possibleScore = total;
            studentScore = actual;
     }

    public void setStudScore(int score)
   {
        studentScore = score;
   }

   public float getPercentage()
   {
       if (possibleScore == 0)
       {
             return 0;
       }
       else
       {
            return (float)studentScore / possibleScore;
       }
   }
}

Now I have a usable examResult class (Note: i have done ZERO compiling/testing on any of this, so don't count on it being actual, usable code). Next I would develop my Student class that can (among other things) hold many ExamResult objects.

[Jesper de Jong]

Ok, let's see if I can help you so that we both can go frolicking outside in the sun... ;) although I don't understand exactly what you mean, this sounds a bit complicated:

Holden Hodgdon wrote:ok, so what I'm understanding is that you establish either public or private class "entities" outside of where the actual code is run. from there you can then use those established "classes" to dynamically generate instanced variables based on the root classes, that way you don't have to hard code in multiple copies of the same thing.

And I don't have the book that you're using.

When you design classes, you'd normally first start thinking about the real-world things that you want to model in your program, and what you can do with these things. You could create a class for each thing, and add a method to the class for each action that you can do with the thing. Things also have properties. Those will map to member variables of the class.

The keywords public, protected and private are used to control access to the member variables and methods of the class. When designing classes, it's important to carefully think about what the class exposes to the outside world. Normally you want a class to expose as little as possible - make as few things as possible public, because the smaller the number of public things in a class, the easier the class will be to use by other parts of the program, and the more it will be decoupled from other parts of the program. Loose coupling is important, because it makes components of the program more independent of each other, which will make the whole program simpler.

So, it's a good idea to make member variables and methods private, unless there is some good reason to make the member variable or method protected or public.

Can you explain in a bit more detail what exactly the requirements of the assignment are, what you had in mind and exactly where you're hitting the wall?

[Holden Hodgdon]

thats pretty much what I did.

I've got a int for the total score.
ints for each test sore that are then tallied up and applied as the value for the total score.
and then a string for the the final grade (A-F) that is established off of the total score.

there is also a string floating around for the student's ID.

my flow is:

get the student's ID.

get the grades for each test.

add up the grades for each test and apply the final result to a variable.

compare the variable to an "if" list of pre-calculated percentages and then return with the associated letter grade.

assign that letter grade to a variable.

print out a summary that lists the student ID, final result of points, and their letter grade.

I want / need to take those last three variables (student ID, point score, and final grade) and break them out into their own private clases so that this code could be reused to generate multiple entries into a database of point scores and final grades listed under student ID.

in order to start building a class like that would I do something like this:

public class student_record
{
	public String student_ID;
	public int total_points;
	public String final_grade;
}

or like this

public class student_ID
{
	public int total_points;
	public String final_grade;
}

edit:

Jesper de Jong wrote: ...

here is what I got so far:

import java.util.*;

public class student_record
{


    public static void main(String[] args)
    {

	 			Scanner keyboard = new Scanner(System.in);

	 			int total_points = 0; // elements used to calculate the point total.
	 			int test_midterm = 0;
	 			int test_final = 0;
	 			int quiz1 = 0;
	 			int quiz2 = 0;
	 			int quiz3 = 0;

// this is how I control my loops that regulate acceptable input and confirmation of correct entries.
	 			boolean gateA = false;
	 			boolean gateB = false;
	 			boolean gateC = false;
	 			boolean gateD = false;
	 			boolean gateE = false;
	 			boolean gateF = false;
	 			String gateA1 = "n";
	 			String gateA2 = "n";
	 			String gateB1 = "n";
	 			String gateC1 = "n";
	 			String gateD1 = "n";
	 			String gateE1 = "n";
 				
 				String student_ID; // these are self explanatory.
				String final_grade;





				System.out.print("Enter student's name. ");
				student_ID = keyboard.next();


// these are used to collect the grades for each test while regulating that the inputs are correct and within the limits of the points possible.
				while ((gateA != true))
					{
						System.out.print("\n" + "Enter a grade of 0-10 for Quiz #1: ");
						quiz1 = keyboard.nextInt();

							if (quiz1 <= 10)
							{
								System.out.println("\n" + "You have entered " + quiz1 + " as the grade for Quiz #1.");
								System.out.print("If this is correct enter (Y): ");
								gateA1 = keyboard.next();

									if (gateA1.equalsIgnoreCase("y"))
									{
										gateA = true;
									}
							}
					}


				while ((gateB != true))
					{
						System.out.print("\n" + "Enter a grade of 0-10 for Quiz #2: ");
						quiz2 = keyboard.nextInt();

							if (quiz2 <= 10)
							{
								System.out.println("\n" + "You have entered " + quiz2 + " as the grade for Quiz #2.");
								System.out.print("If this is correct enter (Y): ");
								gateB1 = keyboard.next();

									if (gateB1.equalsIgnoreCase("y"))
									{
										gateB = true;
									}
							}
					}

				while ((gateC != true))
					{
						System.out.print("\n" + "Enter a grade of 0-10 for Quiz #3: ");
						quiz3 = keyboard.nextInt();

							if (quiz3 <= 10)
							{
								System.out.println("\n" + "You have entered " + quiz3 + " as the grade for Quiz #3.");
								System.out.print("If this is correct enter (Y): ");
								gateC1 = keyboard.next();

									if (gateC1.equalsIgnoreCase("y"))
									{
										gateC = true;
									}
							}
					}

				while ((gateD != true))
					{
						System.out.print("\n" + "Enter a grade of 0-100 for the midterm test: ");
						test_midterm = keyboard.nextInt();

							if (test_midterm <= 100)
							{
								System.out.println("\n" + "You have entered " + test_midterm + " as the grade for the midterm test.");
								System.out.print("If this is correct enter (Y): ");
								gateD1 = keyboard.next();

									if (gateD1.equalsIgnoreCase("y"))
									{
										gateD = true;
									}
							}
					}

				while ((gateE != true))
					{
						System.out.print("\n" + "Enter a grade of 0-100 for the final test: ");
						test_final = keyboard.nextInt();

							if (test_final <= 100)
							{
								System.out.println("\n" + "You have entered " + test_final + " as the grade for the final test.");
								System.out.print("If this is correct enter (Y): ");
								gateE1 = keyboard.next();

									if (gateE1.equalsIgnoreCase("y"))
									{
										gateE = true;
									}
							}
					}
					
// this is to tally up the point total and compare it to grade boundaries based on the required percentages.
				total_points = (+ test_midterm + test_final + quiz1 + quiz2 + quiz3);

						if (total_points >= 207)
							{
								final_grade = "A";
							}

						else if ((total_points < 207) && (total_points >= 184))
							{
								final_grade = "B";
							}

						else if ((total_points < 184) && (total_points >= 161))
							{
								final_grade = "C";
							}

						else if ((total_points < 161) && (total_points >= 138))
							{
								final_grade = "D";
							}
						else if (total_points < 138)
							{
								final_grade = "F";
							}
				// summary of the end results.
				System.out.println(student_ID +
                                      " has earned a final grade of " + final_grade +
                                      " due to scoring a total of " + total_points + " out of a possible 230.");

    }
}

I don't think I'm going to be able to use that wholesale after setting up the classes that the project wants me to, but thats the flow I'm aiming for.

the project requires that I

define and use a class for the student record. the class should have instance variables for the quizzes, mid term, final, over all numeric score for the course, and final letter grade. the over all numeric score is a number in the range of 0 to 100 which represents the weighted average of the student's work. this class should have methods to compute the over all numeric grade that set the appropriate instance variables. your class should have a reasonable set of accessor and mutator methods, an equals method, and a toString method, whether or not your program uses them. you may add other methods if you wish.

[fred rosenberger]

define and use a class for the student record. the class should have instance variables for the quizzes, mid term, final, over all numeric score for the course, and final letter grade. the over all numeric score is a number in the range of 0 to 100 which represents the weighted average of the student's work. this class should have methods to compute the over all numeric grade that set the appropriate instance variables. your class should have a reasonable set of accessor and mutator methods, an equals method, and a toString method, whether or not your program uses them. you may add other methods if you wish.

I didn't look at your code at all, but this seems pretty clear...

define and use a class for the student record.

ok. so that tells me to do this:

public class StudentRecord
{
}

the class should have instance variables for the quizzes, mid term, final, over all numeric score for the course, and final letter grade.

so now do that:

public class StudentRecord
{
   ArrayList ??? quizzes; //what does this type need to be?  
   int midTerm;
   int final;
   int overall score;
   String letterGrade;
}

the over all numeric score is a number in the range of 0 to 100 which represents the weighted average of the student's work.

That tells me I may need a method that computes this score, and that it gets called whenever a new test score is added...

etc.

[Holden Hodgdon]

k:

(public/private/etc) class (name of the class)
{
     (stuff the applies to the class goes here)
}

... that helps, now how do I start instancing that class? the book hops around all over the place on this part so I don't have a consistent example to get a grip on what does what.

[fred rosenberger]

you call it's constructor - which is a method you may have to write yourself

public class MyClass
{
     //assuming i have some constructors defined...

    public static void main(String [] args)
    {
        MyClass refToAnInstance = new MyClass();
    } 
}

So what you probably need is to have in your main some code that loops to get user input. somthing like

ask if user wants to create a(nother) student
if no - print data on all students??? (not sure what you should do)
if yes - 
     create new student object
     have another loop that asks for input on all test scores for a given student.
     save that student object to a collection (arralist, for example

[Campbell Ritchie]

Getting better, but there are still things which can be improved.

You can simplify your if else blocks rather like this

if (score > 70)
{
   grade = "A";
}
else if (score > 60)
{
   grade = "B";
}
else if (score > 50)
{
   grade = "C";
}
else if (score > 40)
{
   grade = "D";
}
else
{
   grade = "Z";
}

Don't use tabs for indenting and don't write long lines in your code; they are difficult to read. Use spaces for indents; you can probably get your text editor to convert tabs to spaces automatically.
Never never write == true or == false. If you want something to happen when b for "boolean" is false, write while (!b) { . . . }
Your gateA boolean variables are poorly named; you can't tell by reading them what they mean. Maybe something likes finishedEntry3 would be better. When you have the String for finishing the loop, you can get a neater assignment like this: finishedEntry3 = "y".equalsIgnoreCase(gateC1); There is a reason for writing the equality test backwards; the bit on the left can never be null.

You ought to get all those values out of the main method and into a class. Your StudentRecord class should look something like this

public class StudentRecord
{
   private String name;
   private String id;
   private int score1;
   private int score2;
   private int score3;

   public StudentRecord(String name, String id)
   {
       this.name = name;
       this.id = id;
       score1 = 0;   // Better style to initialise all fields, even though we
       score2 = 0;   // already know their value is 0.
       score3 = 0;
   }

   public void setScore1(int score1)
   {
       this.score1 = score1;
   }

   public int getScore1()
   {
       return score1;
   }
/* Other methods required */
   public String getOverallGrade()
   {
       string grade;
       int totalScore = score1 + score2 + score3;
        . . .
       return grade;
   }
}

public class StudentDemo
{
   public static void main(String[] args)
   {
       StudentRecord s = new StudentRecord("Campbell" "ABC1234");
       s.setScore1(999);
       s.setScore2(3);
       s.setScore3(21);
       System.out.println("My student get a grade " + s.getOverallGrade());
   }
}

Also I think you are making things difficult for yourself by trying to do too much all at once.

Get that sort of thing working before you try any of the loops.