Exam Question

Hey I'm new to Java and have been trying to teach myself using a book but I'm being particularly unsuccessful. I have an exam coming up and this is the sample question he gave us as an example.

QUESTION:

Write a Java class called ExamMarks, which specifies a module name (String), module code (int), year (int) and an array of 20 integer marks. The class should have an appropriate three argument constructor, a toString() method and methods displaying the highest mark and the average mark. It should have a separate method to obtain up to 20 marks from the user via the console.

Use appropriate visibility modifiers to ensure encapsulation. In the main(String[]) method create an ExamMarks object.

Marks will be given for good program design, code layout, and choice of identifiers as well as a satisfactory implementation of the requirements above. No marks will be given for comments or the inclusion of extra functionality not described above.

THIS IS WHAT I HAVE SO FAR:

class examMarks {
String name;
int studentNumber;
int marks;

public static void main (String [] args){
int year = 2013;
examMarks [] marks = new examMarks[20];
int z = 0;

while (z < 20) {
z = z + 1;
if (z == 1){
marks[z] = new examMarks();
marks[z].name = "Bob";
marks[z].studentNumber = 106708901;
marks[z].

System.out.println("Student name = " + marks[z].name);
System.out.println("Student number = " + marks[z].studentNumber);
System.out.println("Year = " + year);
}}}}

I know I haven't started encapsulation yet I was going to add that at the end. Can anyone point me in the right direction? I'm kinda stuck. Any thoughts appreciated thanks for your help.

Joe

A comment on coding style: Do NOT code more than one } on a line.
}s should be vertically inline below the start of the line with the pairing {

Please wrap code in code tags. See: http://www.java-forums.org/misc.php?do=bbcode#code


This is amazing:

Quote:

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No marks will be given for comments

---

What a poor way to teach programming.

I agree with Norm about the comments. On the formatting, start the class with a Capital letter. (And repost or edit the code so that it's readable.)

Go the problem as it was given to you and identify each of the things the class is is supposed to have. If you are unsure about what they are, say the things you have identified. If there is something in particular that you have identified but have no clue about how to implement it, then you will have something specific to ask.

Thanks for the tips on the formatting. Is this closer to the standard? I have added to the code and have a more specific question: Can you see why I'm getting a run-time error from this code?

Much appreciated.

Joe.

Code:

---




class ExamMarks {
  String name;
  int studentNumber;
  int grade;
 
  public static void main (String [] args){
    int year = 2013;
    ExamMarks [] marks = new ExamMarks[20];
    int z = -1;
   
    while (z < 20) {
     
      z++;
     
      if (z == 0) {
        marks[z] = new ExamMarks();
        marks[z].name = "Bob";
        marks[z].studentNumber = 106708901;
        marks[z].grade= 67;
      }
     
      if (z == 1) {
        marks[z] = new ExamMarks();
        marks[z].name = "Alan";
        marks[z].studentNumber = 106708902;
        marks[z].grade= 82; 
      }
     
    System.out.println("Student name = " + marks[z].name);
    System.out.println("Student number = " + marks[z].studentNumber);
    System.out.println("Year = " + year);
    System.out.println("Grade = " +marks[z].grade);
    System.out.println("************************");
   
    if (z > z + 1){
      z=z;
    }
   
    System.out.println("Highest grade = " +marks[z].grade);
   
    }
 
  }
}

---

The formatting looks much better now. There are still some statements that are not properly indented. What statement does the } on line 41 belong to? Looks like lines 29 to 39 be indented?

Quote:

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getting a run-time error

---

Please copy the full text of the error and post it.

} line 41 belongs to the while loop. 29-39 indent corrected: thank you.

ERROR CODE:

Welcome to DrJava. Working directory is C:\Users\jdp1\Downloads
> run ExamMarks
Student name = Bob
Student number = 106708901
Year = 2013
Grade = 67
************************
Highest grade = 67
Student name = Alan
Student number = 106708902
Year = 2013
Grade = 82
************************
Highest grade = 82
java.lang.NullPointerException
at ExamMarks.main(examMarks.java:29)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknow n Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Un known Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.ru nCommand(JavacCompiler.java:272)

Quote:

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java.lang.NullPointerException
at ExamMarks.main(examMarks.java:29)

---

There is a variable on line 29 with a null value when the code is executed. Look at line 29, find the variable with the null value and then backtrack to find out why it does not have a valid value.

This is an interesting statement:

Code:

---

  if (z > z + 1){

---

When will it be true?

Quote:

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Originally Posted by Norm

This is an interesting statement:

Code:

---

  if (z > z + 1){

---

When will it be true?

---

When z == Integer.MAX_VALUE :)

db

Thanks, I guess that happens when the value wraps: 7F to 80

Alright I'm getting closer. Thanks for all your help. After a lot of time tweaking I've come up with this. The array works but I haven't done it in the class. How would I go about doing that? Any other suggestions?

Cheers

Joe

Code:

---




import java.util.*;

class ExamMarks{
  String moduleName;
  int moduleCode;
  int year;
   
  public static void main (String [] args){
    ExamMarks grade = new ExamMarks();
    grade.moduleName="Object Oriented Programming";
    grade.moduleCode=5015;
    grade.year = 2013;
       
    Scanner percentages = new Scanner(System.in);
    int [] numbers = new int [20];
    System.out.println("Please enter 20 marks");
   
    for (int i = 0; i < numbers.length; i++){
      numbers[i] = percentages.nextInt();
      System.out.println("Mark acheived: " + numbers[i]);
      System.out.println("The title of this module is : " + grade.moduleName);
      System.out.println("The code number of this module is: " +grade.moduleCode);
      System.out.println("The year of the Exam is: " + grade.year);
    }
  }
}

---

Quote:

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I haven't done it in the class

---

Can you explain what you mean and what you are trying to do?

Quote:

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Originally Posted by joepeger

Write a Java class called ExamMarks, which specifies a module name (String), module code (int), year (int) and an array of 20 integer marks.

---

Please correct me if I am wrong. I have three specifications in the class: moduleName, moduleCode & year but I did the int [] in the 'main'. Is this OK with respect to the original specifications?

The array needs to be defined at the class level not inside of a method.
It can be given values in the method.

Slowly but surely wins the race. I feel like I'm learning!

How do I go about writing a method for the averageMarks and the highestMarks? There isn't a variable [i] yet defined so I can't yet write the code in the method. How would you suggest I go about doing this?
Any other advice appreciated.

Cheers, Joe.

Code:

---


import java.util.*;
 
public class ExamMarks{

  //fields
  private String moduleName;
  private int moduleCode;
  private int year;
  private int [] marks;
 
  //methods
  public void highestMark(){
    //code for highestMark
  }
 
  public void averageMark(){
    //code for averageMark
  }
 
  //Constructor
  public ExamMarks(String ExamMarksModuleName , int ExamMarksModuleCode, int ExamMarksYear){
    moduleName = ExamMarksModuleName;
    moduleCode = ExamMarksModuleCode;
    year = ExamMarksYear;
  }
 
  public static void main (String [] args){
    ExamMarks grade = new ExamMarks("Object Oriented Programming", 5015, 2013);
    Scanner percentages = new Scanner(System.in);
    int [] numbers = new int [20];
    System.out.println("Please enter 20 marks");
   
    for (int i = 0; i < numbers.length; i++){
      numbers[i] = percentages.nextInt();
      System.out.println("Mark acheived: " + numbers[i]);
      System.out.println("The title of this module is : " + grade.moduleName);
      System.out.println("The code number of this module is: " +grade.moduleCode);
      System.out.println("The year of the Exam is: " + grade.year);
    }
  }
}

---

Quote:

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How would you suggest I go about doing this?

---

What is the "this" you want to do?

Code:

---

//methods
  public void highestMark(){
    int highest = 0;
    int i=0;
    for (i = 0; i < numbers.length; i++){ 
      if (numbers[i]>numbers[i+1]){ 
        highest = numbers[i];
      }
    }
  }
  public void averageMark(){
    int sum = 0; 
    double average = 0.000;
    int i=0;
    for (i = 0; i < 19; i++){ 
      sum = numbers [i] + sum; 
      average = sum/numbers.length; 
    }
  }

---

I've had a go at creating the methods. This is what I have come up with. It's compiling but I'm not entirely sure how to access it in the object.

System.out.println(grade.averageMark()); doesn't work... Forgive me if it's a silly question: you may have guessed I'm just feeling my way through it at the moment. How do I check if it's working?

Neither of the methods are defined to return a value. They are both defined as returning void.

If the methods are to return anything they needed to be defined differently. See the tutorial:
Defining Methods (The Java™ Tutorials > Learning the Java Language > Classes and Objects)
Returning a Value from a Method (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

Will read those. thank you for the advice