java - JComboBox error when items are added -

i try add items jcombobox , receive nullpointerexception error. sample of code :

public adminpanel() {         fillcomboteacher();         initcomponents();     }     public void fillcomboteacher(){         hashset<person> set = cont1.returnteachers();         iterator = set.iterator();         try {             while (it.hasnext()) {                 person p = (person) it.next();                 string name = p.getname();                 comboteacher.additem(name);             }         } catch (exception e) {             system.out.println(e);         }     } 

person class - name, state (with getters , setters)

returnteachers() - returns hashset containing persons state = "teacher"

i don't understand why i'm receiving error , why can't fill combobox, followed tutorial step-by-step , still not well. thanks!

you calling fillcomboteacher before calling initcomponents.

looking @ method's name, assumed initialized comboteacher in latter.

so @ point (comboteacher.additem(name);), comboteacher not yet initialized , hence nullpointerexception.

to fix this, swap 2 method calls in constructor initialize components before trying use them.

few notes:

• a nullpointerexception easy fix. @ stacktrace (it indicates line npe occurs) , see can null , why @ line.

• don't use raw iterator, generic 1 (iterator<person> = set.iterator();), avoid cast object returned next() call.

• since reading elements in set, can use for-each loop for(person p : cont1.returnteachers()) comboteacher.additem(p.getname());