Arrays: Calling input from other methods and classes

[Maureen Charlton]

I have a line of code within a method called storeStudent in a class called storage.

 
            students [hash] = Student.upperCase(name);       

The error message I am getting is:

C:\java>javac StorageTest.java
StorageTest.java:130: incompatible types
found : java.lang.String
required: Student
students [hash] = Student.upperCase(Student.studentName);
^
The line of code in the method called storeStudent in a class called storage is:

 
public static int storeStudent (String studentName, int hashIndex, String name)
        {
            Student students [ ] = new Student [num];
 

The variables being passed from the public Main Program is:

 
  Storage.storeStudent(Student.studentName, Student.hashIndex, name) ; 

Firstly, I don't understand what the above error message is saying. Can anyone elaborate on this and point me in the right direction please?

Secondly, Is it okay to have a main program with various methods then have a number of classes that calls the input from different classes? I have assumed it is. But don't understand why this is not working. I have another line of code where I have successfully managed to do this but not in an array:

 
hash =  Student.hashIndex(studentName, hashIndex );         
 

My aim is to store the name in the array in the hash position i.e. students [hash] = student name.

(All help and suggestions is appreciated)

[Dun Dagda]

Hi Maureen,

The error message is telling you that you are trying to assign the output of a method that returns a String object, i.e.

 
  students [hash] = Student.upperCase(name);

The

  Student.upperCase(name);

Returns a String, presumably the name of the student in uppercase. However, you are trying to assign this to an array, students, that expects an object of the Student type.

You need to fix this along the lines:

   int num = // number of students, whatever
   String name = "Student name";
   String course = "Student course";
   Student s = new Student(name, course);
      // assuming you have defined a Constructor for your
      // Student class that takes name and course Strings as arguments
   Student [] students = new Student[num];
   students [hash] = s;

Thus you put the whole Student object, names, courses and all, into your array.

Does this help?

DD

[Maureen Charlton]

Mmmmmm - I think you have hit a note here on something I am not understanding correctly.........

You mention defining a constructor for my student class that takes name and course strings. I have a method in my student class that takes the name and course strings. When I put them in a constructor I was advised not too as a new instance was not instanciated. I also thought that a constructor was used only to initialise something. Have I missed something here?

[Dun Dagda]

You have been stuck on this for days now, so here is some example code to help you along.
Please note that this is only an example. You will have to write the rest of it yourself. (Hope this displays OK).

 
class Student {
    private String name = "";
    private String course = "";
    
    Student(String studentName, String studentCourse) {
        name = studentName;
        course = studentCourse;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String studentName) {
        name = studentName;
    }
    
    public String getCourse() {
        return course;
    }
    
    public void setCourse(String studentCourse) {
        course = studentCourse;
    }
    
    public int hashCode(int numStudents) {
        int total = 0;
        int hash = 0;
        byte [] bytes = name.getBytes();
        for (int i = 0; i < bytes.length; i++) {
            total += bytes[i];
        }
        hash = total % numStudents;
        return hash;
    }
}
 
class Storage {
    Student [] students;
    int numberOfStudents = 0;
    
    Storage(int numStudents) {
        students = new Student[numStudents];
        numberOfStudents = numStudents;
    }
    
    public void storeStudent(Student studentObject) {
        int hash = studentObject.hashCode(numberOfStudents);
        students[hash] = studentObject;
    }
    
    public Student retrieveStudent(int index) {
        return students[index];
    }
}
 
public class TestStudent {
    
    /** Creates a new instance of TestStudent */
    public TestStudent() {
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        int number = 3; //whatever you like, use JOptionPane in real program
        Student s1 = new Student("Joe Bloggs", "Java");
        Student s2 = new Student("Fred Smyth", "Visual Basic");
        Student s3 = new Student("Sally Collins", "Visual C#");
        // note that this is just an example to demonstrate the idea,
        // in real life you would prompt for all the info using JOptionPane
        // before trying to store it.
        // you will also have to modify the storeStudent method to check
        // for Student names with the same hash code
        // I will let you figure this out yourself
        
        Storage store = new Storage(number);
        
        store.storeStudent(s1);
        store.storeStudent(s2);
        store.storeStudent(s3);
        
        for (int i = 0; i < number; i++) {
            Student output = store.retrieveStudent(i);
            System.out.println("Student name:   " + output.getName());
            System.out.println("Student course: " + output.getCourse() + "\n");
        }
    }
    
}
 

DD

[Maureen Charlton]

Dun Dagda, many thanks for your reply. I will digest this.

I have been on this problem for months (since the beginning of August). Getting to the point when I think maybe me and java don't get along. Had many a sleepless night. Bought loads, and loads of books. Just trying to take name and course from user, put this into a hash index and then print it out. Gone into dynamic array (as nothing stated how many inputs the user would give) then went into hashmaps - managed these two successfully in the end. Absolute nightmare! Just feel I'm going round, and round in circles.

Guess I will get there in the end. Thanks for your help. No doubt I will be asking more questions.

Thanks again!

[Dun Dagda]

Hi Maureen,

Glad I could be of some help. Don't despair, I used to think the same about Java, thought I would never get it. But once I cracked this problem things got much easier. Are you using the online NETg material much? I found it quite helpful. Also, talk to your tutors - they are friendly guys and willing to help.

Keep it up! You can do it!!

DD

[Maureen Charlton]

Hi - I'm back again with a few questions on your sample code above.

In the Student class and again in the Storage constructor I notice you have used the variable numStudents. Am I assuming correctly that this hasn't been initialised anywhere because it is sample code?

In the Storage class and the storeStudent method you are passing a parameter called Student studentObject. Could you elaborate on this for me? If studentObject is a variable this has also not been declared anywhere. Again, is this because it is sample code?

I also notice you have used the following:

/**
*@param args the command line arguments
*/

I am assuming this is comments only?

In the public class TestStudent at the end, the first line in the for statement you use:
Student output = store.retrieveStudent(i);

I understand that retrieveStudent is a method in the Storage class, was store meant to be Storage then?

Thanking you again

[Nigel Browne]

Hi - I'm back again with a few questions on your sample code above.
In the Student class and again in the Storage constructor I notice you have used the variable numStudents. Am I assuming correctly that this hasn't been initialised anywhere because it is sample code?

You will notice that numStudents is a class variable in Storage and is indeed initialised to the value 0. When the constructor of that class is called in TestStudent the value is changed to 3.
In class Student numStudents is used as the name for the variable passed into the hashCode method, as in this case it is a local variable.

In the Storage class and the storeStudent method you are passing a parameter called Student studentObject. Could you elaborate on this for me? If studentObject is a variable this has also not been declared anywhere. Again, is this because it is sample code?

The Student instances are initialised in the TestStudent class, it is in this class that these instances are then passed to the Storage class.

I also notice you have used the following:

/**
*@param args the command line arguments
*/

I am assuming this is comments only?

Yes this is just a comment.

In the public class TestStudent at the end, the first line in the for statement you use:
Student output = store.retrieveStudent(i);

I understand that retrieveStudent is a method in the Storage class, was store meant to be Storage then?

Thanking you again

store is an instance of the class Storage and is created in the TestStudent class.

HTH

[Dun Dagda]

Dear Nigel,

I had to leave the forum for a while to get on with some other work, so thank you very much for responding to Maureen's questions for me.

DD

[Dun Dagda]

By the way, Maureen, can you get the code I posted to compile and run OK on your system? It worked fine on my system, so it should be alright for you (I hope!).

DD

[Maureen Charlton]

Dun,

Good to read your posting. Thanks for staying with me on this.

I ran your code and Yes, no problems at all.

I'm still trying to get my head around everything though. Trying a few things out. Will no doubt get back to ask you some more questions when I've tried things myself a bit more.

[Maureen Charlton]

Hi, well I'm back asking more questions.

The program compiles ok. But I now get the following error message.

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
at Storage.storeStudent(StorageTest.java:127)
at StorageTest.main(StorageTest.java:182)

I only put one name and course in and get the above error message. My array size has been set to 10 (which concerns me as I have no idea what the array size should be as the user could enter 50 or a million, but I will face that one again later. I assume I will have to use a dynamic array? Am I thinking along the right path? As this is where I started three months ago )

My main program is like this:

 
public class StorageTest
{
    //This is the main program to get input using Java's JOptionPane class 
    public static void main(String [ ]args) 
    {
        String name;      //One of the student names input by user
        String course;    //One of the enrolled courses input by user
        int numCt = 0;    //Count for number of names & course 
        int totalNum = 0; //Total of names and course input by user
        int number        //Maximum number of student details 
 
        //GET DATA INPUT FROM USER
        for (int lp = 0; lp<=numCt; lp++)
            {
            name=JOptionPane.showInputDialog
              ("Please Enter Student Name: \nEnter 'Exit' to end program");
            
            //EXIT LOOP IF "Exit" HAS BEEN ENTERED
            if (name.compareToIgnoreCase("Exit")==0)
                {
                break;
                }
            else
                {
               course=JOptionPane.showInputDialog
                         ("Please Enter Student Enrolled Course:  ");
      
               //TEST THE STUDENT CLASS
               System.out.println("TESTING THE STUDENT CLASS");
 
               Student studentDetail = new Student(name, course);
     
               //Test the name is being converted to upper case
               System.out.println(Student.upperCase(name)  );
 
               //Test the Total of the ASCII values for Upper Case Name 
               System.out.println(Student.sumASCII(Student.studentName)  );
 
               //Test the HashIndex - int based on total % name length -1
               System.out.println
                (Student.hashIndex(Student.studentName, Student.total)  );
 
              
              //TEST THE STORAGE CLASS
              System.out.println("\nTESTING THE STORAGE CLASS");
 
              Storage store = new Storage (number);
              store.storeStudent(Student.studentName, Student.hashIndex, studentDetail );
 
               numCt++;
                }//end if else
             }//end for statement     
        totalNum = numCt;  
        System.out.println("The number of Student Details entered =:  "+totalNum);
        System.out.println("\nYou have finished.  Thank you!");     
        System.exit(0);//this is needed when using JOptionPane     
    }//end main program
}//end public class 
 

The Student class works fine however the Storage class is a different story.

In the Student class I have the following code:

 
//Array to hold data for object student
public static Student [ ] StudentDetail; 
 
    //Constructor section
        Student (String name, String course)
        {
            int num = 10;
            studentDetail = new Student [num ];
        }
 

In the Storage class I have the following:

 
class Storage
{
    //Member section
        //Private members
           private static Student [ ] students;
 
        //Public members
           public static int hash;           
 
    //Constructor section
        public Storage ( int number )
        {
            students = new Student [number];
        }//ends Constructor Storage
 
    //Method section
        public static int storeStudent (String studentName, int hashIndex, Student studentDetail)
        {
            //Call the hashIndex method of the Student class
            hash =  studentDetail.hashIndex(studentName, hashIndex );    
            System.out.println("\nThe value of hash =  "+hash);                                        
            //Put the Student object in the array
            students [hash] = studentDetail;       
            System.out.println("Testing method storeStudent:  "+students[hash]);            
            return hash;     
        }//ends Method storeStudent   
}//ends class Storage
 

I think where the error is occuring is on the line students [hash]=studentDetail. But I don't understand why?

Thanking you in advance.

[Layne Lund]

You don't have to guess where the error is occuring. The error message tells you exactly which line causes the problem:

Storage.storeStudent(StorageTest.java:127)

This means that the error occurs in the file StorageTest.java on line 127 in the storeStudent() method in the Storage class. So, I think you are right that the line causing the problem is:

students [hash] = studentDetail;

Part of the issue is that you are using the variable hash directly. Typically, a hash function cannot guarantee that the hash code is a valid index into the container that uses the hash code. A typical solution is to use the modulus operator (%) to ensure it is the correct range:

students[hash % students.length] = studentDetail;

However, this causes additional problems. Assuming your array size is 10, what happens if one Student object has a hash code of 22 and another has a hash code of 52? Notice that 52 % 10 and 22 % 10 are BOTH 2!

This is called a collission. There are various methods to deal with this, but I'll leave it to you to google for a solution. There is plenty of literature available on hash tables that will explain it more clearly than I can.

I hope this explains the cause of the problem and also helps you get along the way to find a solution. I'd like to note that Java already implements many containers that take care of these kinds of details for you. However, it's always a good educational experience to implement them for yourself. Good luck!

Layne

[Maureen Charlton]

Layne, many thanks for your response.

The offending line of code: students [hash] = studentDetail;
is calculated on another method HashIndex in another class called Student. i.e.

            //Call the hashIndex method of the Student class
            hash =  studentDetail.hashIndex(studentName, hashIndex );    

I don't understand why I should have a collision though, as I have only entered one student when I get the error message?

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
at Storage.storeStudent(StorageTest.java:127)
at StorageTest.main(StorageTest.java:182)

//Method section
public static int storeStudent (String studentName, int hashIndex, Student studentDetail)
{
    //Call the hashIndex method of the Student class
    hash =  studentDetail.hashIndex(studentName, hashIndex );                                          
    //Put the Student object in the array
    students [hash] = studentDetail;       
    System.out.println("Testing method storeStudent:  "+students[hash]);            
    return hash;     
}//ends Method storeStudent
 

 
// Returns the HashIndex which is an int based on the total % name length -1
static int hashIndex (String studentName, int total)
{    
    maxNum = total % studentName.length( );        
    hashIndex = maxNum   -1;
    System.out.println("MaxNum =  "+maxNum + "\tHashIndex =  "+hashIndex);
    System.out.println("\nTotal =  " +total +"\tName length =  "+studentName.length( ) + "\tHashIndex =  " +hashIndex);
    return hashIndex;
}//ends Method hashIndex      
 

[Maureen Charlton]

Hi,

Worked this out for myself. Was calling the HashIndex twice on the same name, hence the collision.

A special huge thank you to Dun Dagna for staying with me on this!

[Dun Dagda]

OK Maureen, so I will repost my program with some comments to explain
what is going on in each part of the code. This still compiles
and runs OK for me; please try it yourself.

/** This is the Student class */
class Student {
    // OK, so this is my Student class. I want to make two variables that will hold
    // the name of the student and the course they are taking, as follows:
    private String name = "";
    private String course = "";
    // so I define two String variables, name and course, that will hold the data.
    // I want ALL the other variables and methods (i.e. "data members") within the
    // Student class to be able to access and change these variables if necessary,
    // so I am defining them here at the top of my class, outside any method, 
    // constructor or other coding blocks so that everything in Student can see them.
    // However, I don't want any other class to be able to change these variables by
    // mistake, so I mark them <B>private</B> so that nothing outside Student can see
    // them directly. This "revealing information on a need-to-know basis" is an
    // example of the object-oriented programming technique <I>encapsulation</I>.
    // Although not necessary for the purposes of this demo, it is a good habit
    // to get into using routinely.
    
    // This is the constructor for the Student class. When another class calls
    // the <B>new</B> command to make a new Student object, this is the piece
    // of code that will be used to make that object. It takes two arguments
    // of type String, one called studentName and the other called studentCourse.
    // These argument variables are only <B<"in Scope"</B> within the confines of
    // the constructor block, i.e.
    Student(String studentName, String studentCourse) { //<-- between here
        name = studentName;
        course = studentCourse;
    }                                                   //<-- and here
    // so to store the String values I have passed into the Student constructor,
    // I need to assign the contents of studentName and studentCourse to the 
    // "class-wide" variables name and course that were defined earlier, so that they
    // can be "permanently" stored within the newly-created Student object.
    // When the constructor exits, i.e. the execution point reaches the closing
    // curly brace }, the argument variables studentName and studentCourse go 
    // "out of scope" and are lost to the program, but that is OK as their contents
    // have by now been stored in name and course. This principle of "scope"
    // applies to method blocks too, as you will see below.
    
    
    
    // This method returns a String with the contents of the variable <B>name</B>
    // to whatever object calls it
    public String getName() {
        return name;
    }
    
    // This method allows the contents of <B>name</B> to be changed to store
    // a different String value, should that be required.
    // Again, the <B>local variable</B> studentName, defined in the method
    // argument, is only "in scope" between:
    public void setName(String studentName) { // <-- here
        name = studentName;
    }                                         // <-- and here
    
    
    // Same principle as the getName() method to get the course String
    public String getCourse() {
        return course;
    }
    
    // Same principle as the setName() method to set the course String
    public void setCourse(String studentCourse) {
        course = studentCourse;
    }
    
    // This method calculates the hashCode from the Student name.
    // You need to pass it the total number of Student objects being stored, so
    // that it can carry out the modulus calculation at the end
    public int hashCode(int numStudents) {
        int totalASCII = 0;
        int hash = 0;
        byte [] bytes = name.getBytes(); // the hashCode method can "see" the name
                                         // variable, and since name refers to a String
                                         // it can call the getBytes() method of the 
                                         // String class, where each character in the 
                                         // String is converted to its Unicode value
                                         // (ASCII on a US-ENGLISH platform), this value
                                         // is converted to a byte and returned as an
                                         // element of a byte array
        for (int i = 0; i < bytes.length; i++) {
            totalASCII += bytes[i];  // now use a for loop to add up all the ASCII values
        }                            // in the byte array
        // now use a remainder division (modulus) of the total ASCII codes in the byte array
        // that came from the name String, by the total number of Student objects, 
        // to calculate the hash code
        hash = totalASCII % numStudents;
        return hash;
    }
}
 
/** This is the Storage class that stores the Student objects */
class Storage {
    // OK, so I want some way to store the Student objects. How about an array?
    // This is good enough for the purposes of this demonstration, so here goes:
    private Student [] students;
    // hang on a minute, I don't know what size to make the array because
    // I don't know how many Student objects the user wants me to store.
    // However, at least I can define a variable called <B>students</B>.
    // I'll tell the compiler that this variable will be used to refer to
    // a Student array type of object (by typing in Student [])
    // and I'll leave the variable at the top of my class for now so that
    // other methods within Storage can access it.
    // Oh, I'll also make it <B>private</B> so that other classes 
    // outside of Storage can't change it
    
    // I'll also follow the same principle for the number of Student objects
    // I don't know how many of these there are going to be, so I'll define
    // a variable like this:
    private int numberOfStudents = 0;
    // again, this is at the top of my class, outside any methods or blocks so
    // that everything within Storage can see it. I'll set it to zero for now,
    // but that doesn't really matter as I'll be changing it later.
    // I'll also make it private to the Storage class to help prevent accidents
    
    // Now I'll define my Constructor for the Storage class.
    // Tell you what, I'll get any method that constructs a new Storage object
    // to pass in the number of students that it wants me to store as an 
    // int primitive called numStudents. Again, this numStudents variable is
    // only valid between the two curly braces {} of my constructor block,
    // but while its there, I'll use it:
    Storage(int numStudents) {
        students = new Student[numStudents]; // <-- here to set the size
                                             // of the array that will hold
                                             // the Student objects
        
        numberOfStudents = numStudents; // <-- again here to set the 
                                        // numberOfStudents variable to its
                                        // working value
    } // <-- the local variable numStudents now goes out of scope here
      // but this is OK, it has done its job.
    
    
    // Now I will define a method that actually stores the student objects.
    // As you already found out, this method does not prevent <B>collisions</B>
    // You will have to write collision-avoiding code in here yourself.
    // OK, so I pass this method a variable of type Student, called studentObject,
    // that acts as a "pointer" to the location of the actual Student object in memory.
    public void storeStudent(Student studentObject) {
        // Now I'm inside the method with my studentObject pointer, I'll use it
        // to call the hashCode method of the Student object being pointed to, 
        // to calculate the hash code of that particular student object.
        // Remember from the hashCode method definition in the Student class above,
        // that I need to tell the method the total number of Student objects being stored,
        // so here's where my numberOfStudents class-wide variable comes in.
        int hash = studentObject.hashCode(numberOfStudents);
        
        // OK, so now I've got my hashCode stored in an int primitive called hash.
        // Now, assuming no collisions (i.e. I haven't been given a hash code that
        // is the same as that of an already-existing Student object - NOT a safe
        // assumption to make as you already found out), I can use <B>"hash"</B> as an index
        // in <B>"students"</B>, my array of Student objects, and store the passed-in
        // studentObject reference there.
        students[hash] = studentObject;
    }
    // so that's all that the storeStudent() method does for now, but for your own
    // assignment you will have to include collision-avoiding code in it.
    
    
    // I also need to define a method that retrieves the Student objects from
    // the students array. I'll need this later to print out the student data.
    public Student retrieveStudent(int index) {
        return students[index];
    }
}
 
/** Class to test the operation of the above classes */
public class TestStudent {
    
    // This constructor is put in automatically by the NetBeansIDE that
    // I use to write my code in, but you don't need it for this demo.
    /** Creates a new instance of TestStudent */
    public TestStudent() {
    }
    
    /** This is just a comment inserted automatically by NetBeans
     * @param args the command line arguments
     */
    
    /** OK, so now we need a main method to run our demo */
    public static void main(String[] args) {
        // I'll first define the number of Student objects I want.
        int number = 3; //whatever you like, use JOptionPane in final program
        
        // now to make some Student objects for storage
        Student s1 = new Student("Joe Bloggs", "Java");
        // So the above code tells the JVM:
        // 1) call the new command to make a new Student object:
        // 2) Pass two String objects containing the values "Joe Bloggs" and "Java"
        //    to the Constructor of the Student class that makes the Student object.
        // 3) Assign a reference variable (a pointer) that points to the location
        //    in memory of the newly-created Student object. Call this reference
        //    variable "s1", and make it be of type Student, so that anything that
        //    uses it to refer to the object knows to expect Student-type behavour
        //    from the object being referred to.
        
        
        Student s2 = new Student("Fred Smyth", "Visual Basic");
        // Likewise for the second Student object, but store the Strings
        // "Fred Smyth" and "Visual Basic" in it, and refer to it with
        // a variable called s2.
        Student s3 = new Student("Sally Collins", "Visual C#");
        // same for s3.
        // note that these names have been carefully chosen to avoid the
        // data-collision problem mentioned above.
        // It will NOT work properly if you change the name Strings,
        // as you already found out.
        // In your final program you could prompt for the info using JOptionPane.
        
        // Now, I want to make an instance of my Storage class (a Storage object)
        // to store my Student object references in.
        Storage store = new Storage(number);
        // OK, so I call the <B>new</B> command with the constructor for
        // my Storage class, as defined above. The Storage class constructor
        // takes an int primitive representing the number of Student objects
        // that I want to store, so I can pass this in using the variable called
        // <B>"number"</B> that I defined at the top of my main method.
        // I'll use a reference variable called "store", of type Storage, to refer
        // to this newly-created object so that I can call its methods.
        
        
        store.storeStudent(s1);
        // Now, using the variable "store" as a reference to the Storage object,
        // I use the dot notation to call the storeStudent method of the Storage-
        // class object. This method takes an argument of type Student, so I can
        // pass in a reference to the first Student object I want to store, "s1".
        
        
        store.storeStudent(s2);
        // Likewise for the second Student object, "s2".
        
        store.storeStudent(s3);
        // and finally for s3.
        
        
        // To demonstrate that the data has been stored correctly, I'll print it out
        // using a for loop to step through each Student object in the Storage object.
        // Array indices in Java start from zero, so I need to start the for loop
        // from zero also.
        for (int i = 0; i < number; i++) {
            // make a temporary variable called "output", of type Student,
            // this will be used to refer to the Student object retrieved
            // on each pass through the for loop.
            Student output = store.retrieveStudent(i);
            // call the retrieveStudent method of the Storage object
            // using the for loop counter "i" as an index
            
            // Display the student name:
            System.out.println("Student name:   " + output.getName());
            // by using the reference variable "output" to refer to the retreived
            // Student object, and call its getName method to return the "name" String.
            
            // Likewise for display of the "course" String.
            System.out.println("Student course: " + output.getCourse() + "\n");
        }
    }
    // end of the program
}
 

[Maureen Charlton]

Hello again Dun Dagna,

I agree, I managed to compile your code no problem however I had a run time error of the following;

C:\java>java StorageTest
Testing
Student name: Sally Collins Student course: Visual C#
Testing
Exception in thread "main" java.lang.NullPointerException
at StorageTest.main(StorageTest.java:40)

The offending line of code is:

         
    Student output = store.retrieveStudent (i);

I have tested the Student class and have no problems.
In my Storage class I have declared a variable:

//Variable for number of Student objects
private static  int numberOfStudents = 0; 

In my Storage class I have a constructor, the same as you as follows:

 
      Storage (int numStudents)
      {
          //set the size of the array that will hold the Student objects
          students = new Student [numStudents];
 
          //set the numberOfStudents variable to its working value
          numberOfStudents = numStudents;
          
      }//end Consturctor Storage    

I then have the method store:

      //Method that stores the student objects
      public static void storeStudent (Student studentObject)
      {
          int hash = studentObject.hashCode (numberOfStudents);
          students[hash] = studentObject;
      }//end Method storeStudent

AND the method retrieveStudent

      //Method to retrieve the Student objects from the students array -needed to print out the student data
      public static Student retrieveStudent (int index)
      {
          return students[index];
      }//end Method retrieveStudent

In my main program I declare my Student objects and pass them to the storeStudent method. But unfortunately I am getting an array when I try to output three Students. My array size declared in the main program is not the problem and the Student class tested ok so I'm guessing (after trying many things over the last few days)that once again it is to do with my storeStudent method in Storage class but I can't see why? Are you able to enlighten me once more? Or alternatively pass me in the right direction?

Thanking you again!

[Dun Dagda]

Hi Maureen,

This is due to the data collision problem I mentioned. The names "Joe Bloggs", "Fred Smyth" (with a y) and "Sally Collins" do not cause this problem, because they all evaluate to a different hashCode. However, pretty much any other set of names will give you trouble.

One of the main objectives of this assignment from Computeach is to get you to devise your own code for circumventing this problem, and therefore to make you appreciate how useful the java.util.Collections classes are, whenever you come to use those, since they take care of this problem for you (by and large).

To avoid data collision, you need to insert code in your storeStudent() method that will:

1) check if the array position specified by the hash code is available for use.
2) If it isn't available (e.g. already taken by a different Student object), then search for the next available position and store the object there.

Each time you call the storeStudent method with a Student object, it should carry out the above steps before attempting to store the object. That way you should avoid collisions.

See how you get on with that, post again if you are stuck.

DD

[Dun Dagda]

Hi Maureen,

Looking at your own code again, I see that you are making a lot of your methods and variables static. There should be no need for you to do this, as you should be calling these methods on instances of the various Student objects and the Storage object created by your program, so Java should be able to find them and use them without needing to make them static.

In fact, making variables static in this way can interfere with correct operation of your code, as a static variable only has one instance per class, so all objects made from that class will all be looking at the same variable. This is probably not a good idea in your case, as you want your Student objects to be separate from each other, storing different name and course information for each Student.

DD

[Maureen Charlton]

Dun Dagna, again many thanks for staying with me on this.

With regard to the collisions (which I understand I need to tackle but for now I am leaving that until one of the many things I need to do later), I have ensured that no collisions are apparent. I have tested this and get the following hashindex's:

C:\java>java StudentTest

The student Joe Bloggs =hash index 7

The student Fred Smyth =hash index 3

The student Sally Collins =hash index 0

I have also ensured that static is not apparent (- can't believe I left these in, after editing the code over the weekend to attempt to find the problem, when I actually answered a thread about static methods this morning).

The error message I get now, when I print out is:
C:\java>java StorageTest
Testing
Student name: FRED SMYTH Student course: Visual Basic
Testing
Exception in thread "main" java.lang.NullPointerException
at StorageTest.main(StorageTest.java:39)

The offending line of code is:
System.out.print("Student name: " +output.getName ( ) );

So I looked at the getName method of the Student class which just returns name.

I also looked at the setName method of the Student class which just says name is = studentName.

However, I notice that the setName method allows the contents of name to be changed to store a different String value, should that be required and since I change the name variable to upper case in my Student hashCode method:

 
//This method calculates the hashCode from the Upper case 
Student name, accepting the size of the Students array
        public int hashCode (int numStudents)
        {
             int hash = 0;
             int subtotal = 0;
             int maxNum = 0;
 
            //Convert name to upper case
            name = name.toUpperCase ( );
                       
           //Convert each character in the upper case name 
           //and return it as an element of AsciiCode
             byte [ ] bytes = name.getBytes( );
        
           //Use a for loop to add up all the ASCII values in the byte array
           for (int i = 0; i < bytes.length; i++)
           {
            //Ignore spaces in the name
            if (bytes [i] != 32)
                {
                 subtotal += bytes [i];
                }//end if statement
           }//end for statement
 
            // Calculate hash  which is an int based 
            //on the total % number of students -1
            maxNum = subtotal % numStudents;        
            hash = maxNum   -1;
            return hash;
        }//end method hashCode
 

I thought I would have a look again at my understanding of get and set. Unfortunately I am finding very little. My course notes just say get and set are straightforward. "The get methods return the value of a member they are associated with and the set method sets the member to a new value. Are you able to elaborate here or alternatively suggest where I can find out more?

[Dun Dagda]

Hi Maureen,

Just a couple of suggestions: why are you subtracting one from the hash index? In the case where a Student name's total ASCII codes add up to a multiple of the number of students to be stored, the expression
totalASCII % numStudents will give a value of zero. If you then subtract 1 from this, you will get -1, which is not a valid index to use in an array (you will get an ArrayIndexOutOfBoundsException and crash the program).
So that is one thing. The other is that you are converting the name to upper case before adding up its ASCII codes. This means that the string "Fred Smyth" will give a different hash code to "FRED SMYTH" as the ASCII codes for lower and upper case characters are different. This might cause your code to behave in unexpected ways.
See the ASCII table web page for a more complete explanation.

DD

[Maureen Charlton]

Dun Dagna - Anyone tell you you're great!

I have done it!
(Not the assignment - still got A few more methods and classes to write but I have achieved and understood this part. Thanks to you for helping. I would of given up on java altogether if it wasn't for you explaining things to me and giving me things to think about. I can't thank you enough.
I've even helped some other people on their threads; although I must admit I'm pleased when someone more senior has a read in case I haven't explained things very well but I do find I learn even more from helping others as it makes me think about it as well).

C:\java>java StorageTest
Student name: JOE DOHN Student course: Visual Basic
Student name: SALLY COLLINS Student course: History
Student name: JOE DOHN Student course: Visual C#
Student name: JOE BLOGGS Student course: Java
Student name: FRED SMYTH Student course: Visual Basic
Student name: JOHN DOE Student course: Java

What can a say......... Thank you doesn't express my gratitude enough to you for staying with me on this!!!

[Dun Dagda]

Hi Maureen,

That's fantastic news, well done! I see you have been helping others out as well, so good for you!

I found these initial OOP conceptual hurdles that you have run into to be the hardest part of learning Java that I have encountered so far. The rest seems to be just remembering what the various packages do and applying the appropriate class and/or method to the problem at hand.

Once you submit and pass your first assignment (concentrate on writing up a good Test Plan and documenting your code well, the tutors like that), you will be invited by Computeach to attend an in-centre 2-day Java workshop at their training centre in Dudley, near Wolverhampton. This is a good way of getting to know your tutors and of meeting a few other students (there are usually 3 or 4 other people there at the same time). I found the course great for getting answers to questions face-to-face from qualified Java professionals and for making me more confident in my coding.

So once again, congratulations and very well done!!!
Thanks also for your kind comments, I like to help people out if I can.
DD