Get user input using java swing and write to text file

import java.io.*;
import javax.swing.JOptionPane;
import java.util.Scanner;

public class ClientList
{
public static void main (String args[]) throws IOException
	{
	 // Main class ClientList creates Clients.txt file that is to be opened up and appended whenever clients are to be added
		FileWriter outFile = new FileWriter("Clients.txt");
		BufferedWriter outStream = new BufferedWriter(outFile);
		String outString = "Client Listing";
		outStream.write(outString);
		outStream.newLine();
		outStream.close();
		System.out.println("Clients.txt is created\n");

	 // "Primitive" menu asks user what he/she wants to do and accepts commands via keyboard scanner
		Scanner menu = new Scanner(System.in);
		System.out.println("Welcome to ClientList");
		System.out.println();
		System.out.println("To add clients to the file, enter 1");
		System.out.println("To open file and view its contents, enter 2");
	 //	System.out.println("To search for a client in the file, enter 3");
		int command = menu.nextInt();

		switch (command)
		{
		case 1: Clients client = new Clients();
				client.enterData();
	    		break;
	    case 2: Clients client2 = new Clients();
	    		client2.getClients();
	    		break;
	 // case 3: Clients client = new Clients();
	 //			client.searchClients();
	 //   		break;
	    default: System.out.println("Doesn't exist!");
	    		 System.out.print("Enter a valid number ===> ");
	    	     int l = menu.nextInt();
	    	     break;
		}
	}
}

class Clients
	{
	 // Initializing variables for client data
		String firstName;
 		String lastName;
 		String age;
 		String height;
 		String weight;
 		String startDate;
 		String phNum;

		public void enterData() throws IOException
 		{
 	 // Initializing boolean variable isValid to check the number of clients user inputs
 	 // and make sure it really IS a number
 		boolean isValid = false;
 		String number;

 	 // Ask user how many clients he/she wishes to enter data for
 	 	do
 	 	{
		number = JOptionPane.showInputDialog("Enter the number of clients you wish to save:");
	 // Loop to check what the user entered against the allowed charcters (only numbers between 0 and 9 allowed)
		if (number.matches("[0-9]*"))
 	 	{
        isValid = true;
        }
        else
        {
        isValid = false;
        }
 	 	}
 	 	while (isValid != true); // Keeps prompting user for a number until a valid number between 0 and 9 is entered
 	 // Finally, parse String number and change to an integer for use in data input loop
		int numOfClients = Integer.parseInt(number);

	 // Data input prompts are put into a loop that repeats until data for all clients is entered
		for (int k = 0; k