swing - JFrame Error java.lang.nullpointerexception -

i have been trying create jframe calculator getting error java.lang.nullpointer.exception. says have problem in line say:

guicalc go = new guicalc(); 

here of code:

package home.personalprojects.jordan;  import java.awt.*; import java.awt.event.*;  import javax.swing.*;  public class guicalc extends jframe {      private jbutton calculate;     private jtextfield num1field, num2field;     private jcombobox operationbox;     private jlabel label1, label2, label3;     private string[] operationposs = {"+", "-", "*", "/"};     string operation;     int num1, num2, answer;      public guicalc(){          super("calculator");         setlayout(new flowlayout());          operationbox = new jcombobox(operationposs);          calculate = new jbutton("calculate");         calculate.settooltiptext("enter 2 numbers , select operation find answer");          label1 = new jlabel("number 1: ");          num1field = new jtextfield("", 10);         label2 = new jlabel("number 2: ");         num2field = new jtextfield("", 10);          operationbox.additemlistener(             new itemlistener(){                 public void itemstatechanged(itemevent event){                     if(event.getstatechange() == itemevent.selected){                          int temp;                         temp = operationbox.getselectedindex();                         switch(temp){                         case 0:                             answer = num1 + num2;                         case 1:                             answer = num1 - num2;                         case 2:                             answer = num1 * num2;                         case 3:                             answer = num1 / num2;                      }                 }             }         });           calculate.addactionlistener(             new actionlistener(){                 public void actionperformed(actionevent event){                     joptionpane.showmessagedialog(null, string.format("the answer: " + answer, event.getactioncommand()));                 }             }         );          add(label1);         add(num1field);         add(label2);         add(operationbox);         add(label3);         add(num2field);         add(calculate);      }      public static void main(string[] args)     {          guicalc go = new guicalc();         go.setdefaultcloseoperation(jframe.exit_on_close);         go.setsize(500,500);         go.setvisible(true);       }  } 

when run code get...

exception in thread "main" java.lang.nullpointerexception     @ java.awt.container.addimpl(container.java:1086)     @ java.awt.container.add(container.java:998)     @ javax.swing.jframe.addimpl(jframe.java:562)     @ java.awt.container.add(container.java:410)     @ testcalc.testcalc.<init>(testcalc.java:79)     @ testcalc.testcalc.main(testcalc.java:87) 

which points

add(label3); 

which suggest label3 has not being initialised.