Question

Basic JAVA Question

When we try to run this program, a problem occurs. What is this problem called? And why does this problem occur here?
public class Main {
   public static void main(String[] args) {
   PhoneNumber pn = new PhoneNumber();
       System.out.println(pn.isTollFree());
   }
}

class PhoneNumber {
String number;
  
public boolean isTollFree() {
return number.charAt(1) == '8';
}
}

Answer 1

-The problem is called the NullPointer Exception.

-The problem occurs here because String number hasn't been assigned any value .It contains Null. But an object reference is being used here :

number.charAt(1) == '8'

If String Number contained some value, this error wouldn't occur.

Proof:

Without Assignment :

With Assignment :

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