toString() using queues with nodes

[Adrian Meneses]

Hello all,
I'm trying to print the contents of a queue using nodes but I get duplicates for some reason and I don't understand why. Here is my code:

public class Student {
       //Variables
       private String name;
       private int studentID;

       //Other methods such as setName and setID go here...

       public String toString() {
             return "Student name: " + this.name + " and the ID is: " + this.studentID;
       }
}

public class Node<T> {
      //Variables
      private T data;
      private Node<T> link;

      //Non-static methods
      public void setData(String newData) {
            name = newData;
      }

      public T getData() {
           return data;
      }

      public void setLink(Node<T> newNode) {
           link = newNode;
      }

      public Node<T> getLink() {
           return link;
      }
}

public class StudentNodeBasedQueue<T> {
      //Other methods such as enqueue() and dequeue() go here...

      public String toString() {
            Node<T> currentStudent = front; //To set front item to currentStudent
            String allStudents = "";

            while(currentStudent != null) {
                  allStudents = allStudents + "\n" + currentStudent.getData(); //Stores the data (student name and ID) 
                  currentStudent = currentStudent.getLink(); //To set currentStudent to next node to print
            }

             return allStudents;
      }
}

public class StudentQueueDemo {

      public static void main(String[] args) {
            UnboundedQueueInterface<Student> studentQueue = new StudentNodeBasedQueue<Student>();
            Student newStudent = new Student();
            Scanner keyboard = new Scanner(System.in);

            System.out.println("Enter the student's name:);
            String name = keyboard.nextLine();
            System.out.println("Enter the student's ID:);
            int id = keyboard.nextInt();

            newStudent.setName(name);
            newStudent.setID(id);

            studentQueue.enqueue(newStudent); //No need for a try-catch block because it's overridden in the unbounded interface plus is node based 

            System.out.println(studentQueue.toString());
      }
}

The unbounded interface just overrides a queue interface where the enqueue, dequeue, size, and toString methods are. The console prints the last item added to the queue. For example, if I add Mary with ID 10, Jon with ID 20, and Adrian with ID 30, the program will print:
Student name: Adrian and the ID is: 30
Student name: Adrian and the ID is: 30
Student name: Adrian and the ID is: 30

Please help me by pointing where the error/bug is, tell me in a conceptual way so I see what's wrong and to code it.  

[Knute Snortum]

Where is the loop in your demo class?  Can you make sure you posted the same classes that you tested?

[Junilu Lacar]

Based on what you described, it looks like you're only using the one instance of Student that you created on line 5 in main(). To fix the problem, you need to do a new Student() for each new student you create and add to your queue. Otherwise, you're going to have all your Nodes referencing the same Student object, which is why you see the same output for all nodes.

[Adrian Meneses]

Adrian Meneses wrote:Hello all,
I'm trying to print the contents of a queue using nodes but I get duplicates for some reason and I don't understand why. Here is my code:

public class Student {
       //Variables
       private String name;
       private int studentID;

       //Other methods such as setName and setID go here...

       public String toString() {
             return "Student name: " + this.name + " and the ID is: " + this.studentID;
       }
}

public class Node<T> {
      //Variables
      private T data;
      private Node<T> link;

      //Non-static methods
      public void setData(String newData) {
            name = newData;
      }

      public T getData() {
           return data;
      }

      public void setLink(Node<T> newNode) {
           link = newNode;
      }

      public Node<T> getLink() {
           return link;
      }
}

public class StudentNodeBasedQueue<T> {
      //Other methods such as enqueue() and dequeue() go here...

      public String toString() {
            Node<T> currentStudent = front; //To set front item to currentStudent
            String allStudents = "";

            while(currentStudent != null) {
                  allStudents = allStudents + "\n" + currentStudent.getData(); //Stores the data (student name and ID) 
                  currentStudent = currentStudent.getLink(); //To set currentStudent to next node to print
            }

             return allStudents;
      }
}

public class StudentQueueDemo {

      public static void main(String[] args) {
            UnboundedQueueInterface<Student> studentQueue = new StudentNodeBasedQueue<Student>();
            Student newStudent = new Student();
            Scanner keyboard = new Scanner(System.in);

            int option = 0;

            do {

                 System.out.println("Press 1 to add a student");
                 System.out.println("Press 2 to view all students");
                 System.out.println("Press 3 to end the program");
                 option = keyboard.nextInt();
                 keyboard.nextLine();

                 if(option == 1) {
                       System.out.println("Enter the student's name:);
                       String name = keyboard.nextLine();
                       System.out.println("Enter the student's ID:);
                       int id = keyboard.nextInt();

                       newStudent.setName(name);
                       newStudent.setID(id);

                       studentQueue.enqueue(newStudent); //No need for a try-catch block because it's overridden in the unbounded interface plus is node based 
                 }
                 else if(option == 2) {                
                       System.out.println(studentQueue.toString());
                 }
                 else if(option == 3) {
                       System.out.println("Good bye!");
                       System.exit();
                 }

            }while(option != 3);
      }
}

The unbounded interface just overrides a queue interface where the enqueue, dequeue, size, and toString methods are. The console prints the last item added to the queue. For example, if I add Mary with ID 10, Jon with ID 20, and Adrian with ID 30, the program will print:
Student name: Adrian and the ID is: 30
Student name: Adrian and the ID is: 30
Student name: Adrian and the ID is: 30

Please help me by pointing where the error/bug is, tell me in a conceptual way so I see what's wrong and to code it.  

I added the do-while loop in the demo class that I forgot to post. The program, as I see it, adds a new student to the queue but when it comes to print all the students in the queue it only prints the last student added multiple times.

[Junilu Lacar]

In your main() method:
Student newStudent = new Student();
This line is executed outside of your loop and that means it gets executed EXACTLY one time.  Now, since you only have one instance of a Student object, here's what basically happens:

Node1 ----->[Student]<------- Node2
                ^
                |
Node3 ----------+

You create Node1 and make it refer to the Student object as its data. However, you never instantiate another Student object. So, next time you use the newStudent reference, you're still referring to the Student object you created on line 5. Every time you set the Student's field values, you overwrite the previous values with the new ones. That's why in the end, after "adding" what you thought was three students (when in fact you just kept overwriting the same ONE student), you only see the last student values because all three Nodes refer to the same Student object!

You need to move line 5 inside your loop. This way, you'll get a different student object from what you created in the previous iteration.

[Adrian Meneses]

Thank you Junilu! What an oversight from my part! Still learning how queues work with nodes.

[Junilu Lacar]

The problem wasn't so much how queues work with nodes as it was misunderstanding how object references work. On line 27 in main() you pass newStudent to the enqueue() method. Since the reference you passed was never assigned to a different Student instance, you ended up with all nodes that you added to the queue pointing to the same Student object that was created on line 5. The same thing would have happened if you were working with Lists or Maps.