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I'm writing a program writes and read the file. it has 50 numbers and if users enter number more than 50 it should ask the users to enter it again. my question is for the error it should show the message box but again return to the text field for input. Also one more question, for example, user1 is entering 1 and user2 is also entering 1, it should tell the user2 "please enter another number it's already entered." how can I do it in writing and reading files.
this is my code:

public class JavaFXApplication24 extends Application 
 private int seatInput;
 @Override
 public void start(Stage primaryStage) 
 StackPane pane = new StackPane();

 HBox hbox = new HBox();
 Label num = new Label("Please enter the number from 1 to 50: ");

 TextField numInput = new TextField();

 File file = new File("NumberBook.txt");
 Button ok = new Button("ok");
 ok.setOnAction((new EventHandler<ActionEvent>()
 public void handle(final ActionEvent event ) NumberFormatException e) 

 
 try 
 PrintWriter output = new PrintWriter(file);

 catch (IOException ex) 
 System.out.printf("error", ex);
 
 try 
 Scanner input1 = new Scanner(file);

 System.out.printf("Entered: %s", seatInput);

 catch (FileNotFoundException ex) 
 System.out.printf("error", ex);
 

 

 ));
 hbox.getChildren().addAll(num,numInput,ok);



 pane.getChildren().addAll(hbox);

 Scene secondScene = new Scene(pane, 600, 400);

 // New window (Stage)
 Stage newWindow = new Stage();
 newWindow.setTitle("Booking and Timing");
 newWindow.setScene(secondScene);
 newWindow.show();

 

From a quick glance here are some problems with your code:

1) if ((numInput.getText().equals(1)&& numInput.getText().equals(50))). There are 2 problems here. Firstly, as fabian pointed out, this statement will always evaluate to false. Secondly, from your question I think you want the user to enter a number from 1 to 50, but here you're checking if they enter 1 and 50 (which also means it's always false).

2) JOptionPane.showInputDialog("Please enter again") Because your application is JavaFX, you should use the built in Dialog instead.

Now to answer your questions, you can call requestFocus on the text field right after the error message is displayed. Your second question seems like you want some networking functionality in the application, which adds a whole lot of complexity to it. You might need to do some research on how to do it in Java.