java - Is it possible to have a JLabel changing its Text depending on a variable value? -

i'created jlabel should display "texta" if variable count == -1,
"text b" if variable count == 0 , "textc" if variable count == 1.

i've used swing create interface, can see below

tempconverter

the red rectangle shows jlabel should be.

i have tried creating 3 jlabels , changing setvisible(boolean) whenever variable count value condition applies. didn't work because got following error:

exception in thread "main" java.lang.nullpointerexception @ tempconverterui.tempconverter.main(tempconverter.java:354) c:\users\x\appdata\local\netbeans\cache\8.1\executor-snippets\run.xml:53: java returned: 1

and jlabels not placed in same location in gui (overlapping not possible).

i've tried using jlabel.settext() change text displayed in jlabel, whenever variable condition applied. got similar error 1 above (if not same).

i've read other posts , researched further , found people suggested actionlisteners set unsure these work simple variable, opposed component in gui.

my code follows:

package tempconverterui;  import javax.swing.joptionpane; import messageboxes.userdata; import com.sun.jna.library; import com.sun.jna.native; import com.sun.jna.wstring;  public class tempconverter extends javax.swing.jframe {  public interface somelib extends library {       public int engstart();        public int endstop();       public int engcount();       public wstring enggetlasterror();       public int engsetattribute(wstring aszattributeid, wstring aszvalue);  }  /**  * creates new form tempconverter  */ public tempconverter() {     initcomponents(); }  /**  * method called within constructor initialize form.  * warning: not modify code. content of method  * regenerated form editor.  */ @suppresswarnings("unchecked") // <editor-fold defaultstate="collapsed" desc="generated code">                           private void initcomponents() { 

the layout created here, followed temperature convertion methods , unrelated component's functionality (which believe not relevant in case)

/**  * @param args command line arguments  */ public static void main(string args[]) {  /**this login form gets created*/                   userdata.popup();  /**after library functions called, return variable count value*/         somelib lib = (somelib) native.loadlibrary("somelib", somelib.class);      int startresult = lib.engstart();      system.out.println(startresult);     if (startresult < 0)     {         system.out.println(lib.enggetlasterror());     }      system.out.println(userdata.getacinput());     int setatresult = lib.engsetattribute(new wstring("code"), userdata.getacinput());     system.out.println(setatresult);     if (setatresult < 0)     {         system.out.println(lib.enggetlasterror());     } 

and next piece of code should control jlabel text display

    int count = lib.engcount();     system.out.println(count);     if (count == -1)     {         system.out.println(lib.enggetlasterror());      }     else if (count == 0)     {      }     else     {      }      new tempconverter().setvisible(true);   }  // variables declaration - not modify                      private javax.swing.jpanel bottompanel; private javax.swing.jbutton convertbutton; private static javax.swing.jbutton button; private javax.swing.jtextfield from; private javax.swing.jcombobox<string> fromcombo; private javax.swing.jlabel fromlabel; private javax.swing.jlabel title; private javax.swing.jtextfield to; private javax.swing.jcombobox<string> tocombo; private javax.swing.jlabel tolabel; private javax.swing.jpanel toppanel; // end of variables declaration                    

}

any appreciated. if include simple code example well, fantastic new java (and programming, in general).

issues:

1. don't set jlabel visible, rather add gui, leave visible default, , set text via settext(...).
2. give class holds jlabel public methods allow outside classes ability set label's text. public void setlabeltext(string text), , in method call settext(text) on jlabel.
3. debug nullpointerexception other npe -- @ stacktrace, find line throws it, , code see why key variable on line null.
4. when , how change jlabel depend on event want listen for. if user input, want respond input, actionlistener added jbutton or jtextfield, or itemlistener added jradiobutton.
5. if want instead listen change in state of variable, no matter how variable changed, make "bound property" (tutorial) using propertychangesupport , propertychangelistener.

for example of latter:

import java.awt.dimension; import java.awt.event.actionevent; import java.awt.event.actionlistener; import java.beans.propertychangeevent; import java.beans.propertychangelistener;  import javax.swing.*; import javax.swing.event.swingpropertychangesupport;  @suppresswarnings("serial") public class showcount extends jpanel {     private static final int timer_delay = 1000;     private jlabel countlabel = new jlabel("                 ");     private countmodel model = new countmodel();      public showcount() {         model.addpropertychangelistener(countmodel.count, new modellistener(this));          setpreferredsize(new dimension(250, 50));         add(new jlabel("count:"));         add(countlabel);          timer timer = new timer(timer_delay, new timerlistener(model));         timer.start();     }      public void setcountlabeltext(string text) {         countlabel.settext(text);     }      public static void main(string[] args) {         swingutilities.invokelater(() -> createandshowgui());     }      private static void createandshowgui() {         showcount mainpanel = new showcount();         jframe frame = new jframe("showcount");         frame.setdefaultcloseoperation(jframe.dispose_on_close);         frame.add(mainpanel);         frame.pack();         frame.setlocationbyplatform(true);         frame.setvisible(true);     } } 

class countmodel {     public static final string count = "count"; // count "property"      // support object notify listeners of change     private swingpropertychangesupport support = new swingpropertychangesupport(this);     private int count = 0;      public int getcount() {         return count;     }      public void setcount(int count) {         int oldvalue = this.count;         int newvalue = count;         this.count = count;          // notify listeners count has changed         support.firepropertychange(count, oldvalue, newvalue);     }      // 2 methods allow listeners register support object     public void addpropertychangelistener(propertychangelistener listener) {         support.addpropertychangelistener(listener);     }      public void addpropertychangelistener(string propertyname, propertychangelistener listener) {         support.addpropertychangelistener(propertyname, listener);     }  } 

class modellistener implements propertychangelistener {     private showcount showcount;      public modellistener(showcount showcount) {         super();         this.showcount = showcount;     }      @override     public void propertychange(propertychangeevent evt) {         int newvalue = (int) evt.getnewvalue();         showcount.setcountlabeltext(string.format("%03d", newvalue));     } } 

class timerlistener implements actionlistener {     private countmodel model;      public timerlistener(countmodel model) {         super();         this.model = model;     }      @override     public void actionperformed(actionevent e) {         int oldcount = model.getcount();         int newcount = oldcount + 1;         model.setcount(newcount);     } }