java - Calling in Child class to wait for RFID Tag -

so after long period of research, able make rfid scanner work , detect ports of computer. had split code 2 class files because of 2 jar files having different features:

one reading id , other reading port.

now had them, had call them main gui project. issue facing right child wont wait id scanned , instead give me null value in return. want make work can call child classes main project.

here codes:

rfid_reader.java

import javax.swing.joptionpane; import jssc.serialport; import jssc.serialportevent; import jssc.serialporteventlistener; import jssc.serialportexception;  public class rfid_reader {     static serialport serialport;     static string output;     public string finaloutput;      //this redundant , willing remove it.     public void checkconnection(){         rfid_scan_hw jcom = new rfid_scan_hw();         serialport = new serialport(jcom.collect_ports(""));         startreading();     }      //configuring serialport     public void startreading(){         try {             serialport.openport();             serialport.setparams(serialport.baudrate_9600,serialport.databits_8,serialport.stopbits_1,serialport.parity_none);             //verbose, output no words.             serialport.writebytes("\002v0\003".getbytes());             serialport.closeport();              serialport.openport();             serialport.setparams(9600, 8, 1, 0);             int mask = serialport.mask_rxchar + serialport.mask_cts + serialport.mask_dsr;             serialport.seteventsmask(mask);             serialport.addeventlistener(new serialportreader());         }         catch (serialportexception ex) {             system.out.println(ex);         }     }      //re-scan devices in port. if device not found, try again.     public void rescanconnection(){         rfid_scan_hw jcom = new rfid_scan_hw();         if(jcom.collect_ports("")==""){             joptionpane.showmessagedialog(null, "no scanner found. please try again");         }else{             serialport = new serialport(jcom.collect_ports(""));             startreading();         }     }      //read input device.     class serialportreader implements serialporteventlistener{         @override         public void serialevent(serialportevent event) {                        if(event.isrxchar()){                 if(event.geteventvalue() == 22){                     try{                         byte[] bytes = serialport.readbytes(22);                         string card = new string(bytes);                         string results[] = card.split(",");                          string processed ="";                         char[] cutdown = results[3].tochararray();                         for(int i=0; i<cutdown.length-1; i++){                             processed +=cutdown[i];                         }                          string result = results[2]+"-"+processed;                         finaloutput = result;                     }catch (serialportexception ex) {                         system.out.println(ex);                     }                 }else{                  }             }         }        }  } 

rfid_scan_hw.java

import com.fazecast.jserialcomm.serialport;  public class rfid_scan_hw {     string masterport = "";     public string collect_ports(string x){         serialport ports[] = serialport.getcommports();         string[] portlist = new string[ports.length];         for(int i=0; i<ports.length; i++){             string check = ports[i].getdescriptiveportname();             if(check.startswith("prolific usb-to-serial comm port")==true){                 masterport = ports[i].getsystemportname();             }         }         return masterport;     }     public void displayports(){         serialport ports[] = serialport.getcommports();         for(serialport port : ports){             system.out.println(port.getdescriptiveportname());         }     } } 

and here how called them using button:

private void jbutton8actionperformed(java.awt.event.actionevent evt) {                                                  rfid_reader rf = new rfid_reader();         string id="null";             rf.checkconnection();             id = rf.finaloutput;         joptionpane.showmessagedialog(null, "the id is: "+id);     }  

and result: the id is: null

now here wanted happen.

when press button, button wait scanner before prompting id card.

i'm pretty sure i'm doing wrong please me out.

use threads , synchronize them using synchronized keywords. 1st thread wait connection established, id scanned , available. notifies 2nd thread read/write data rfid device.

also consider using serial communication manager library has many powerful apis may used in project. share details rfid hardware.