Body Mass Index Calculator - error

Question

I am creating a body mass index calculator to practice creating a GUI. However, I cannot figure out why I am getting the error shown below. I'm thinking that I'm trying to display the value of BMI incorrectly. Can someone help please?

Exception in thread "main" java.lang.NullPointerException at Source.(Source.java:21) at Main.main(Main.java:5)

import javax.swing.JFrame; 

public class Main {
public static void main (String args []) {
    Source sourceObject = new Source();
    sourceObject.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    sourceObject.setSize(275,180); 
    sourceObject.setVisible(true); 

}

}

import java.awt.FlowLayout; 
import javax.swing.JFrame; 
import javax.swing.JLabel; 
import javax.swing.JOptionPane;

public class Source extends JFrame { 

private JLabel item1;
private JLabel item2;
private JLabel item3;
private String weight, height;
private int BMI;

public Source () {
    super("Title"); 
    setLayout(new FlowLayout()); 
    item1 = new JLabel("Text"); 
    item1.setToolTipText("This appears on hover"); 
    weight = JOptionPane.showInputDialog(null, "Weight: ");
    height = JOptionPane.showInputDialog(null, "Height: ");
    item3.setText(String.valueOf(BMI));
    add(item1);
    add(item2);
    add(item3);

}
int BMICalc() {
    int weig = Integer.parseInt(weight);
    int heig = Integer.parseInt(height);
    int BMI = (weig)/(heig * heig);
    return BMI;

}


}

Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — xenteros, Jan 21 '17 at 22:35

davidxxx · Answer 1 · 2017-01-21T21:53:42.773

1

Actually item2 and item3 are declared but never instantiated.

But the actual problem is you are calling a method on item3.

Instantiate all the fields that have a known value in the Source constructor.

edited Jan 21 '17 at 21:53

answered Jan 21 '17 at 21:45

davidxxx

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Where do you see me calling a method on item2? – helloumarian Jan 21 '17 at 21:53
I would say item3 sorry – davidxxx Jan 21 '17 at 21:53
I still dont understand what is wrong, can you please explain a little bit more? – helloumarian Jan 21 '17 at 21:57
item3 is null and you call a method on it. A variable referencing a null object should be first instantiated if you want to perform method invocations on. In the constructor of Source, instantiate the label : `item3 = new JLabel();` – davidxxx Jan 21 '17 at 22:08
Any ideas why the result is always 0? – helloumarian Jan 21 '17 at 22:33
The solution is hidden in my answer below ;) – Andreas Dolk Jan 21 '17 at 22:33
I tried different calculations like weight * height just to make sure its not the fault of me applying wrong equation. – helloumarian Jan 21 '17 at 22:34
@helloumarian you have a `int BMI` field declared in the `Source` class but you don't value it with the call of the BMICalc() method. So `BMI` has always its default value for an int : `0`. Before this instruction `item3.setText(String.valueOf(BMI));` do that : `BMI = BMICalc();` it should solve your problem. – davidxxx Jan 21 '17 at 22:48

score 0 · Answer 2 · answered Jan 21 '17 at 22:33

You missed to create the JLabels 2 and 3. Try this:

public Source () {
    super("Title"); 
    setLayout(new FlowLayout()); 
    item1 = new JLabel("Text"); 
    item1.setToolTipText("This appears on hover"); 

    item2 = new JLabel("Text");

    weight = JOptionPane.showInputDialog(null, "Weight: ");
    height = JOptionPane.showInputDialog(null, "Height: ");
    BMICalc();        

    item3 = new JLabel("Text");
    item3.setText(String.valueOf(BMI));
    add(item1);
    add(item2);
    add(item3);
}

Body Mass Index Calculator - error

2 Answers2