eclipse - Java Swing Blank JFrame coming up? -

i'm new swing, , wondering why application comes blank, , displays components. seems sporadic. there no exceptions thrown or that. comes blank jframe. @ times when close application , run again, shows components correctly, comes blank. doing wrong in code? i'm using eclipse ide, if matters. here's code:

import java.applet.applet; import java.awt.borderlayout; import java.awt.flowlayout; import java.awt.gridlayout; import java.awt.event.actionevent; import java.awt.event.actionlistener; import java.awt.event.mouseevent; import java.awt.event.mouselistener; import java.io.file; import java.io.filenotfoundexception; import java.util.arraylist; import java.util.scanner;  import javax.swing.*;   public class main extends jframe implements actionlistener {     private static final long serialversionuid = 1l;     jradiobutton randomradiobutton;     jradiobutton uniqueradiobutton;     jradiobutton participationradiobutton;     arraylist<student> allstudents;     jframe mainframe;      public main(){         allstudents = new arraylist<student>();         processallstudents();         mainframe = new jframe();         mainframe.setvisible(true);         mainframe.setsize(250, 400);         mainframe.setdefaultcloseoperation(jframe.exit_on_close);          jpanel componentholder = new jpanel();         componentholder.setlayout(new gridlayout(5,1));          jlabel titletext = new jlabel("                     randomizer");         componentholder.add(titletext);         jbutton picker = new jbutton("pick student");         jfilechooser filepick = new jfilechooser();         filepick.addactionlistener(this);          buttongroup allradiobuttons = new buttongroup();         randomradiobutton = new jradiobutton("completely random");         uniqueradiobutton = new jradiobutton("unique");         participationradiobutton = new jradiobutton("complete participation");          allradiobuttons.add(randomradiobutton);         allradiobuttons.add(uniqueradiobutton);         allradiobuttons.add(participationradiobutton);          componentholder.add(randomradiobutton);         componentholder.add(uniqueradiobutton);         componentholder.add(participationradiobutton);          picker.addactionlistener(this);          componentholder.add(picker);         componentholder.add(filepick);         mainframe.add(componentholder);     }      public void actionperformed(actionevent e){         if(e.getactioncommand().equals("pick student")){             if(randomradiobutton.isselected()){                 student result = getstudentrandom();                 result.increment();                 string resultstring = new string(result.getname() + ", " + result.getfrequency());                 system.out.println(resultstring);                 jlabel resultlabel = new jlabel(resultstring);                 joptionpane.showmessagedialog(mainframe, resultlabel);             }             else if(uniqueradiobutton.isselected()){                 student firststudent = getstudentrandom();                 student secondstudent = getstudentrandom();                 student result;                 if(firststudent.getname().equals(secondstudent.getname())){                     result = secondstudent;                 }                 else{                     result = firststudent;                 }                 result.increment();                 string resultstring = new string(result.getname() + ", " + result.getfrequency());                 system.out.println(resultstring);                 jlabel resultlabel = new jlabel(resultstring);                 joptionpane.showmessagedialog(mainframe, resultlabel);             }             else if(participationradiobutton.isselected()){                 student result = selectstudentparticipant();                 result.increment();                 joptionpane.showmessagedialog(mainframe, result.getname() + ", " + result.getfrequency());             }          } else system.out.println("error.");       }     public void processallstudents(){         file f = new file("test.txt");         scanner scanfile = null;         try {             scanfile = new scanner(f);         } catch (filenotfoundexception e) {             system.out.println("file not found");         }         while(scanfile.hasnext()){             string name = scanfile.next();             int frequency = scanfile.nextint();             student s = new student(name, frequency);             allstudents.add(s);         }     }     public student getstudentrandom(){         int result = (int) (math.random() * allstudents.size());         return allstudents.get(result);     }     public student selectstudentparticipant(){         student temp = null;    //start of bubble sort algorithm          for(int = 0; < allstudents.size() - 1; i++){             student firststudent = allstudents.get(i);             student secondstudent = allstudents.get(i+1);             if(firststudent.getfrequency() > secondstudent.getfrequency()){                 temp = firststudent;                 firststudent = secondstudent;                 secondstudent = temp;             }         }          //end of bubble sort algorithm. data structure sorted increasing          int firstrandom = (int) (math.random() * (allstudents.size()/2) + 0.2 * allstudents.size());    //likely bigger         int secondrandom = (int) (math.random() * (allstudents.size()/2));         int randomindex = 0;    //used represent random index          if(firstrandom > secondrandom){ //more likely. selects first half of list             randomindex = (int) (math.random() * allstudents.size()/2);         }          else if(firstrandom < secondrandom){    //possible, less             randomindex = (int) ((math.random() * allstudents.size()/2) + allstudents.size()/2);         }          else{   //if 2 random numbers same             randomindex = (int) (math.random() * allstudents.size()/2);         }          return allstudents.get(randomindex);      }     public static void main(string[] args){         new main();     }   } 

setvisible(true) has last method called in main method. otherwise nothing may render on jframe