substring() generates error in Java

substring() generates error in Java

I am getting a error when using this substring(). I am trying to get
initials of first, middle, and last and then convert to upper case.
public String initials() {
String initials = first.substring(0, 1) + middle.substring(0, 1)
+ last.substring(0, 1);
return initials.toUpperCase();
}
That is the code and this is the output it is giving me..
Exception in thread "main" java.lang.NullPointerException
null, null null at name.Name.initials(Name.java:75)
at name.NameDriver.main(NameDriver.java:30)
Java Result: 1
Line 75 is
String initials = first.substring(0, 1) + middle.substring(0, 1)
Line 30 in NameDriver is
System.out.print(name1.initials());