JavaFX Application create a miles/kilometers converter

import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.input.KeyCode;
import javafx.scene.layout.GridPane;
import javafx.stage.Stage;

public class Main extends Application {
   final double KILOMETERS_PER_MILE = 1.602307322544464;
   protected TextField tfMile = new TextField();
   protected TextField tfKilometer = new TextField();

   @Override//  ww w . j ava  2s . c om
   public void start(Stage primaryStage) {
      // Set text fields alignment
      tfMile.setAlignment(Pos.BOTTOM_RIGHT);
      tfKilometer.setAlignment(Pos.BOTTOM_RIGHT);

      // Create a gridpane and place nodes into it
      GridPane pane = new GridPane();
      pane.setAlignment(Pos.CENTER);
      pane.add(new Label("Mile"), 0, 0);
      pane.add(tfMile, 1, 0);
      pane.add(new Label("Kilometer"), 0, 1);
      pane.add(tfKilometer, 1, 1);

      // Create and register the handlers
      tfMile.setOnKeyPressed(e -> {
         if (e.getCode() == KeyCode.ENTER && tfMile.getText().length() > 0) {
            double miles = Double.parseDouble(tfMile.getText());
            tfKilometer.setText(String.valueOf(miles * KILOMETERS_PER_MILE));
         }
      });

      tfKilometer.setOnKeyPressed(e -> {
         if (e.getCode() == KeyCode.ENTER && tfKilometer.getText().length() > 0) {
            double kilometers = Double.parseDouble(tfKilometer.getText());
            tfMile.setText(String.valueOf(kilometers / KILOMETERS_PER_MILE));
         }
      });

      // Create a scene and place it in the stage
      Scene scene = new Scene(pane, 250, 60);
      primaryStage.setTitle("java2s.com");
      primaryStage.setScene(scene);
      primaryStage.show();
   }
}