java - Adding an ActionListener to a JButton from another class is giving a NullPointerException? -

i want able add acitonlistener jbutton class keep code neat. problem brings nullpointerexception when trying add it.

i'm adding actionlistener through handler class defined 'h'.

in display class:

public class display {      private handler h; //my handler object      private jframe frame;     private jbutton btncalculate;      /**      * launch application.      */     public static void createdisplay() {         eventqueue.invokelater(new runnable() {             public void run() {                 try {                     display window = new display();                     window.frame.setvisible(true);                 } catch (exception e) {                     e.printstacktrace();                 }             }         });     }      /**      * create application.      */     public display() {         initialize();     }      /**      * initialize contents of frame.      */     private void initialize() {         frame = new jframe();         frame.setbounds(100, 100, 450, 300);         frame.setdefaultcloseoperation(jframe.exit_on_close);         frame.getcontentpane().setlayout(null);          btncalculate = new jbutton("calculate");         btncalculate.setbounds(66, 208, 89, 23);         btncalculate.addactionlistener(h.getcalculatelistener()); //actionlistener added here.         frame.add(btncalculate);     }      //getter jbutton     public jbutton getbtncalculate() {         return btncalculate;     }  } 

here's handler class includes getters. helps make lot more simple having getters in handler class instead of having them spread out in multiple different classes:

public class handler {     private display display; //my display object     private calculateactionlistener calculate; //my calculateactionlistener object      public handler(display display, calculateactionlistener calculate) {         this.display = display;         this.calculate = calculate;     }      public jbutton getbutton() {         return display.getbtncalculate();     }      public actionlistener getcalculatelistener() {         return calculate.getcalculatelistener();     } } 

and finally, here's calculateactionlistener class contains actual actionlistener:

public class calculateactionlistener {      //here's actual actionlistener     private actionlistener calculatelistener = new actionlistener() {         public void actionperformed(actionevent e) {             joptionpane.showmessagedialog(null, "working!");         }     };      //getter actionlistener     public actionlistener getcalculatelistener() {         return calculatelistener;     } } 

note imports removed there in code. imports not problem.

you not creating new instance of handler. before use h, need create instance of it. add line before create button in display class constructor. this:

h = new handler(this, new calculateactionlistener());