"JOptionPane java.lang.NullPointerException" error in Java

Question

This is my full code:

import javax.swing.JOptionPane;

public class Lab5bug {

public static void main(String[] args) {

    int x=0;
    String str;
    for (;;)
    {
        str = JOptionPane.showInputDialog("Enter a grade 0-100\n(Hit Cancel to abort)");
        if (str==null || str.equals(""))
            break;
        x = Integer.parseInt(str);
        if (x>=0 && x<=100)
            break;
        JOptionPane.showMessageDialog( null, "Invalid grade, try again");
    }
    if (str != null  & !str.equals(""))   //<===========BUG:  NEEED TO USE && 
        //&,||.| are all lead to bug if we press Cancel.
        //str can be null it does not pass 1st condition
        //but still pass 2nd condition
        //str can be "" pass first condition but not second condition
        //and therefre it still show the message The grade is 0
        //But why 
        JOptionPane.showMessageDialog( null, "The grade is " + x);
}
}

When I run the program and press Cancel at the first dialog box, then the program returns a error:

Exception in thread "main" java.lang.NullPointerException
at Lab5bug.main(Lab5bug.java:19)

I have already located the problem, at this line if (str != null & !str.equals("")) But why only && works? I do not understand the logic behind this.

Answer 1

Replace

if (str != null & !str.equals(""))

with

if (str != null && !str.equals(""))

& stands for bitwise AND. Therefore when str is null, the expression !str.equals("") throws a NullPointerException. When the logical AND operator && is used, the !str.equals("") expression won't be reached because the first condition str != null is false.