java - How to use a button to call a method from another class? -

i'm pretty new java, i'm trying make basic ui. can use jbuttons call joptionpanes easily, can't use them call class.

currently have main class, ui class, , class handles data.

in class handles data have method performs basic sum on 2 elements of array:

    public void firstmonth(){         string ans;         int num1 = integer.parseint(sorteddata[0][0][0]);         int num2 = integer.parseint(sorteddata[0][0][1]);         int sum = num1 * num2;          ans=string.format("the sum of day1 , day2 is: %s", sum);         joptionpane.showmessagedialog(null, ans, "title", joptionpane.plain_message);      } 

then in ui class try , call when 1 of buttons pressed:

public class userinterface extends jframe {  getdata = new getdata();      private class myhandler implements actionlistener{          public void actionperformed(actionevent event){              if(event.getsource()==button1){                 a.firstmonth();              } } } 

but when press button more exceptions can bear count. ideas?

sorry if basic question.

edit, here exceptions when click button:

exception in thread "awt-eventqueue-0" java.lang.numberformatexception: null     @ java.lang.integer.parseint(integer.java:454)     @ java.lang.integer.parseint(integer.java:527)     @ pkg110_term3.getdata.firstmonth(getdata.java:75)     @ pkg110_term3.userinterface$myhandler.actionperformed(userinterface.java:71)     @ javax.swing.abstractbutton.fireactionperformed(abstractbutton.java:2018)     @ javax.swing.abstractbutton$handler.actionperformed(abstractbutton.java:2341)     @ javax.swing.defaultbuttonmodel.fireactionperformed(defaultbuttonmodel.java:402)     @ javax.swing.defaultbuttonmodel.setpressed(defaultbuttonmodel.java:259)     @ javax.swing.plaf.basic.basicbuttonlistener.mousereleased(basicbuttonlistener.java:252)     @ java.awt.component.processmouseevent(component.java:6505)     @ javax.swing.jcomponent.processmouseevent(jcomponent.java:3321)     @ java.awt.component.processevent(component.java:6270)     @ java.awt.container.processevent(container.java:2229)     @ java.awt.component.dispatcheventimpl(component.java:4861)     @ java.awt.container.dispatcheventimpl(container.java:2287)     @ java.awt.component.dispatchevent(component.java:4687)     @ java.awt.lightweightdispatcher.retargetmouseevent(container.java:4832)     @ java.awt.lightweightdispatcher.processmouseevent(container.java:4492)     @ java.awt.lightweightdispatcher.dispatchevent(container.java:4422)     @ java.awt.container.dispatcheventimpl(container.java:2273)     @ java.awt.window.dispatcheventimpl(window.java:2719)     @ java.awt.component.dispatchevent(component.java:4687)     @ java.awt.eventqueue.dispatcheventimpl(eventqueue.java:729)     @ java.awt.eventqueue.access$200(eventqueue.java:103)     @ java.awt.eventqueue$3.run(eventqueue.java:688)     @ java.awt.eventqueue$3.run(eventqueue.java:686)     @ java.security.accesscontroller.doprivileged(native method)     @ java.security.protectiondomain$1.dointersectionprivilege(protectiondomain.java:76)     @ java.security.protectiondomain$1.dointersectionprivilege(protectiondomain.java:87)     @ java.awt.eventqueue$4.run(eventqueue.java:702)     @ java.awt.eventqueue$4.run(eventqueue.java:700)     @ java.security.accesscontroller.doprivileged(native method)     @ java.security.protectiondomain$1.dointersectionprivilege(protectiondomain.java:76)     @ java.awt.eventqueue.dispatchevent(eventqueue.java:699)     @ java.awt.eventdispatchthread.pumponeeventforfilters(eventdispatchthread.java:242)     @ java.awt.eventdispatchthread.pumpeventsforfilter(eventdispatchthread.java:161)     @ java.awt.eventdispatchthread.pumpeventsforhierarchy(eventdispatchthread.java:150)     @ java.awt.eventdispatchthread.pumpevents(eventdispatchthread.java:146)     @ java.awt.eventdispatchthread.pumpevents(eventdispatchthread.java:138)     @ java.awt.eventdispatchthread.run(eventdispatchthread.java:91) 

given integer.parseint takes string argument, it's safe sorteddata 3d string array. stacktrace, array sorteddata initialized individual elements are not.

the default value object types null. same applys values within object array.

make sure these elements have been assigned values prior invoking integer.parseint.

for (int i=0; < sorteddata.length; i++) {    (int j=0; j < sorteddata[i].length; j++) {       (int k=0; k < sorteddata[i][j].length; k++) {          sorteddata[i][j][k] = ... // string       }    } }