Fonction de retour String.

package client;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.util.Enumeration;
import javax.comm.*;
public class Commande 
{
		/**
		 * @param args
		 * @throws NoSuchPortException
		 * @throws PortInUseException
		 * @throws IOException
		 * @throws UnsupportedCommOperationException
		 * @throws UnsupportedEncodingException
		 * @throws InterruptedException
		 */
	public String executerAT(String cmd) throws UnsupportedCommOperationException, UnsupportedEncodingException, IOException ,PortInUseException
	{
			Enumeration<CommPortIdentifier> enumP = CommPortIdentifier.getPortIdentifiers();
			BufferedReader serial_in=null;
			PrintStream serial_out=null;
			String msg="";
				try
				{
				CommPortIdentifier tmp = enumP.nextElement();
				System.out.println(tmp.getName());
				SerialPort serial = (SerialPort) tmp.open("Me",2000);
				serial.setSerialPortParams(460800, SerialPort.DATABITS_8,SerialPort.STOPBITS_1,  SerialPort.PARITY_NONE);
				serial_in=new BufferedReader(new InputStreamReader(serial.getInputStream()));
				serial_out=new PrintStream(serial.getOutputStream(),true,"ISO-8859-1");
				serial_out.print(cmd);
				serial_out.print("\r");
				msg=msg+serial_in.readLine()+"\n";
				msg=msg+serial_in.readLine()+"\n";
				msg=msg+serial_in.readLine()+"\n";
				}catch(PortInUseException e)
				{
				e.printStackTrace();
				System.exit(0);
				}
			return msg;
		}
	public static void main(String[] args) throws NoSuchPortException, PortInUseException, IOException, UnsupportedCommOperationException, InterruptedException
	{
		Commande t=new Commande();
		t.executerAT("AT");
	}
	}