linkedList getting null element

[Tony Fabeets]

Debugger show that stuObj was constructed correctly, but when added to DeansList, the DeansList gets an Object NULL.

 public DLClass() {
        DeansList = new LinkedList();
    }//end DLClass()

    public boolean addStudent(String nameVal, int ssnVal, String gradesVal, String crHrVal)
    {
       Student stuObj = new Student(nameVal, ssnVal, gradesVal, crHrVal);
       
       if(stuObj.gpa >= 3.5)
       {
           DeansList.add(stuObj);
           return true;
       }
       else 
       {
           System.out.println(stuObj.name + "not added to list.  GPA " + stuObj.gpa + " < 3.5");
       }
       return false;
    }//end addStudent(String, int, String, String)

[David Peterson]

You've obviously cut down the code to try and show the relevant bit, but to be able to work out what your bug is, we need to see how you are accessing the linked-list when you get the "null" reference.
As an aside (but it is important): There are standard naming-conventions in the Java world. Classes should always begin with a capital letter, and objects should always begin with lower-case letters. It makes things very confusing to other people if you don't follow this convention. E.g. Your object "DeansList" should be named "deansList". Also, try and choose descriptive names for your variables (e.g. crHrVal is hard to understand).
Anyway, what you've written looks correct. Post a bit more code and we'll see if we can work out what the problem is.
Regards,
David Peterson

[Tony Fabeets]

[ February 16, 2004: Message edited by: Tony Fabeets ]

[Tony Fabeets]

I figured out why null was being added. Was a problem with the calling method, as you suggested.
As of now, i simplified this method...

String crHrVal;//holds credit hours in string
String gradesVal;//holds grades for stuObj
    public boolean addStudent(String nameVal, int ssnVal, String gradesVal, String crHrVal) {
        Student stuObj = new Student(nameVal, ssnVal, gradesVal, crHrVal);
        
        if(stuObj.gpa >= 3.5)
        {
            DeansList.add(stuObj);
            return true;
         }
         System.out.println("Student not added to DeansList: GPA " + stuObj.gpa + " too low\n");
        return false;
    }//end addStudent(nameVal, ssnVal, gradesVal, crHrVal)

what i'm trying to make it do is
*Creates Student obj and adds it to DeansList if student has gpa >= 3.5
*return true if add is successful else reutnrs false
*If stuObj is added, add in ascending order via ssn within name
*use overriding compareTo iin Student class
*/
when i try to implement an iterator via

  public boolean addStudent(String nameVal, int ssnVal, String gradesVal, String crHrVal) {        
        Student stuObj = new Student(nameVal, ssnVal, gradesVal, crHrVal);                
        if(stuObj.gpa >= 3.5){
            ListIterator i = DeansList.listIterator();
            while(i.hasNext())
            {
                if(stuObj.compareTo(i.next()) >=0)
                {                  
                    int loc = i.previousIndex();
                    DeansList.add(loc,stuObj);
                    return true;
                }
             }//end while         
          }//end if
          System.out.println("Student not added to DeansList: GPA " + stuObj.gpa + " too low\n");
          return false;    
       }//end addStudent(nameVal, ssnVal, gradesVal, crHrVal)

when i use that code, nothing is ever added to LinkedList DeansList. the condition if(stuObj.compareTo(listItr.next() >=0)
is never true.
my compareTo(Object) is as follows.

public int compareTo(Object o)
    {
        Student other = (Student)o;
        if(this.name < other.name) return -1;
        if(this.name == other.name)
        {
            if(this.ssn < other.ssn) return -1;
            if(this.ssn == other.ssn) return 0;
            if(this.ssn > other.ssn) return 1;
        }//end if
        return 1;
    }//end compareTo(Object)

thank you
[ February 16, 2004: Message edited by: Tony Fabeets ]
[ February 16, 2004: Message edited by: Tony Fabeets ]

[Joe Ess]

Originally posted by Tony Fabeets:

when i use that code, nothing is ever added to LinkedList DeansList.

There's a typo in that code so it will not compile. I hope you left something out when you transcribed it or you've been testing an old version of the class.

would someone be kind enough to draw up a compareTo(Object) that will compare stuObj.name properly?

Nope, we pretty much insist that students do their own work. You can feel free to post what you have and we will be glad to critique it.

[Tony Fabeets]

above post edited accordingly

[Joe Ess]

public int compareTo(Object o)
    {
        Student other = (Student)o;
        if(this.name < other.name) return -1;
        if(this.name == other.name)
        {
            if(this.ssn < other.ssn) return -1;
            if(this.ssn == other.ssn) return 0;
            if(this.ssn > other.ssn) return 1;
        }//end if
        return 1;
    }//end compareTo(Object)

Thanks for fixing that. Don't want to waste time on red herrings.
Comparing String objects using "==" actually evaluates if the String handles are pointing to the same instance of a String object. It does not evaluate the contents of the String instance. Consider the following code:

String str1 = "blah";
String str2 = new String("blah");
       
if (str1 == str2){ // will evaluate to false
   // this line will not be printed out
   System.out.println(" == evaluates to true");
}
       
if (str1.equals(str2)){ // will evaluate to true
   // this line will be printed out
   System.out.println(" .equals() evaluates to true");
}

You can see that even though str1 and str2 both contain the string "blah", they will not be "==" to each other. One should always use String.equals() when testing for equivalence.
IMPORTANT NOTE: if str2 were declared:

String str2 = "blah";

the test "==" would evaluate to true. This behavior fools a lot of people into thinking they can use "==" to evaluate String objects. The reason that "==" works in this case is because the compiler sets the literal references to "blah" to the same String instance in the literal pool.
Perhaps you should look at using String.compare() in your case. . .

[Tony Fabeets]

beginners mistake :-/
I now see how to compare if strings are equals, but would would i find whether to return -1 or 1?

 public int compareTo(Object o){
            Student other = (Student)o;
            if(this.name < other.name)) return -1;
            if(this.name.equals(other.name))
            {
                if(this.ssn < other.ssn) return -1;
                if(this.ssn == other.ssn) return 0;
            }//end if        
            return 1;    
        }//end compareTo(Object)

the above code will not compile and the code below never lets the condition to add student be true. With all the code provided, nothing is added to list, but when i strip the add method down, as in the first post, everything works fine. So, we've narrowed the problem down, but not seeing where.
[ February 16, 2004: Message edited by: Tony Fabeets ]

[Joe Ess]

I'm curious. If we have established that "==" evaluates the object handle and not the contents of the object, why would you assume that greater than or less than works differently?
HINT:
Perhaps you should look at using String.compare() in your case. . .
/HINT
[ February 16, 2004: Message edited by: Joe Ess ]

[Tony Fabeets]

Here's what i've come up with and still, nothing is true in the condition
if(stuObj.compare(i.next()) >=0)

public boolean addStudent(String nameVal, int ssnVal, String gradesVal, String crHrVal) {        
        Student stuObj = new Student(nameVal, ssnVal, gradesVal, crHrVal);                
        if(stuObj.gpa >= 3.5){
            ListIterator i = DeansList.listIterator();
            while(i.hasNext())
            {
                if(stuObj.compare(i.next()) >=0)
                {                  
                    int loc = i.previousIndex();
                    DeansList.add(loc,stuObj);
                    return true;
                }
             }//end while         
          }//end if
          System.out.println("Student not added to DeansList: GPA " + stuObj.gpa + " too low\n");
          return false;    
       }//end addStudent(nameVal, ssnVal, gradesVal, crHrVal)
//this method is in private class Student{
  public int compare(Object o){
            Student other = (Student)o;
            if((this.name).compareTo(other.name) < 0) return -1;
            if(this.name.equalsIgnoreCase(other.name))
            {
                if(this.ssn < other.ssn) return -1;
                if(this.ssn == other.ssn) return 0;
            }//end if        
            return 1;    
        }//end compareTo(Object)

[ February 16, 2004: Message edited by: Tony Fabeets ]

[Joe Ess]

Originally posted by Tony Fabeets:
Here's what i've come up with and still, nothing is true in the condition
if(stuObj.compare(i.next()) >=0)

What names are in your list and what are you adding?
[ February 16, 2004: Message edited by: Joe Ess ]

[Tony Fabeets]

not getting all the proper results, but have a good start. thanks for the help