Question

public class INode
{
    private int value;
    private INode right, down;
    private int row, col;

    public INode(int value)
    {
        this.value = value;
    }

    public int getValue() 
    {
        return value;
    }

    public void setValue(int value) 
    {
        this.value = value;
    }

    public INode getRight() 
    {
        return right;
    }

    public void setRight(INode right) 
    {
        this.right = right;
    }

    public INode getDown() 
    {
        return down;
    }

    public void setDown(INode down)
    {
        this.down = down;
    }

    public int getRow() 
    {
        return row;
    }

    public void setRow(int row) 
    {
        this.row = row;
    }

    public int getCol() 
    {
        return col;
    }

    public void setCol(int col) 
    {
        this.col = col;
    }


}

I can get value of a = 8 but for head, even though I use constructor to set up, still give me value = null… dont know why.

And the driver is:

import java.util.*;
public class List
{
public static INode head;
public List()
{
    head = new INode(8);

}
public static void main (String[] args)
{
    INode a = new INode(8);
    int data = a.getValue();
    System.out.println(data);
    System.out.println(head.getValue());
}


}

Please help me a hand guys. Dont understand why when I use a constructor, I cant assign the value to the node, yet when I create an instance, i can…

Thank guys, love you folks! Great help!

Answer 1

You do not instantiate the class List. Change your code to

public INode head; // remove static
public List() {
    head = new INode(8);
}

And modify your main method:

public static void main (String[] args) {
    INode a = new INode(8);
    int data = a.getValue();
    System.out.println(data);

    List l = new List(); // create new List instance
    System.out.println(l.head.getValue()); // get the head from List instance
}

Another valid alternative would be to change just one line:

public static INode head = new INode(8); // create instance during class initialization

I recommend to look at the difference between class (static) and instance variables, e.g. in the Java Tutorials (extract follows):

• Instance Variables (Non-Static Fields) Technically speaking, objects store their individual states in “non-static fields”, that is,
  fields declared without the static keyword. Non-static fields are also
  known as instance variables because their values are unique to each
  instance of a class (to each object, in other words); the currentSpeed
  of one bicycle is independent from the currentSpeed of another.

• Class Variables (Static Fields) A class variable is any field declared with the static modifier; this tells the compiler that there
  is exactly one copy of this variable in existence, regardless of how
  many times the class has been instantiated. A field defining the
  number of gears for a particular kind of bicycle could be marked as
  static since conceptually the same number of gears will apply to all
  instances. The code static int numGears = 6; would create such a
  static field. Additionally, the keyword final could be added to
  indicate that the number of gears will never change.