In Java, how can I join a LocalDate and a time formatted String into one observable property of LocalDateTime?

Question

My attempt is to unify the values of a DatePicker (that handles the date selection, but not the time) and a TextField (that handles the time) into an observable LocalDateTime of both joined.

I've set up observable properties for both in the model, but I am having difficulties joining them.

So far I managed to make a few tries with Bindings.createObjectBinding(), but I don't seem to have much success there.

I would like to know at least if I am on the right path or should I go about this differently?

Answer 1

By using LocalDateTime#of(LocalDate,LocalTime) you can create a LocalDateTime from a LocalDate and a LocalTime. What you need now is a way to get an instance of both a LocalDate and a LocalTime. Fortunately, the DatePicker control gives you its value as a LocalDate so we're done there. Next is finding a way to get a LocalTime from a TextField. This is possible by using a TextFormatter and a StringConverter which knows how to convert a String to a LocalTime and vice versa. There's a built-in StringConverter for this use case: LocalTimeStringConverter.

Once we have both the DatePicker and the TextFormatter we need to create a binding which creates a LocalDateTime from the two values. Since both DatePicker and TextFormatter have a value property, which holds a LocalDate and, in this case, a LocalTime, respectively, creating the binding is relatively simple with Bindings#createObjectBinding(Callable,Observable...).

DatePicker dp = new DatePicker();
// Have to associate the TextFormatter with a TextField
TextFormatter<LocalTime> tf = new TextFormatter<>(new LocalTimeStringConverter());

ObjectBinding<LocalDateTime> binding = Bindings.createObjectBinding(() -> {
    LocalDate ld = dp.getValue();
    LocalTime lt = tf.getValue();
    return ld == null || lt == null ? null : LocalDateTime.of(ld, lt);
}, dp.valueProperty(), tf.valueProperty());

Here's a full example:

import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.beans.binding.ObjectBinding;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.DatePicker;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.control.TextFormatter;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import javafx.util.converter.LocalTimeStringConverter;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.format.DateTimeFormatter;

public class App extends Application {

  @Override
  public void start(Stage primaryStage) {
    DatePicker datePicker = new DatePicker();
    datePicker.setValue(LocalDate.now());

    TextField timeField = new TextField();
    TextFormatter<LocalTime> timeFieldFormatter =
        new TextFormatter<>(new LocalTimeStringConverter());
    timeField.setTextFormatter(timeFieldFormatter);
    timeFieldFormatter.setValue(LocalTime.now());

    HBox dateTimeBox = new HBox(10, datePicker, timeField);
    dateTimeBox.setAlignment(Pos.CENTER);

    ObjectBinding<LocalDateTime> ldtBinding = Bindings.createObjectBinding(() -> {
      LocalDate date = datePicker.getValue();
      LocalTime time = timeFieldFormatter.getValue();
      return date == null || time == null ? null : LocalDateTime.of(date, time);
    }, datePicker.valueProperty(), timeFieldFormatter.valueProperty());

    Label ldtLabel = new Label();
    ldtLabel.textProperty().bind(Bindings.createStringBinding(() -> {
      LocalDateTime dateTime = ldtBinding.get();
      return dateTime == null ? null : dateTime.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME);
    }, ldtBinding));

    VBox root = new VBox(15, dateTimeBox, ldtLabel);
    root.setAlignment(Pos.CENTER);
    root.setPadding(new Insets(25));

    primaryStage.setScene(new Scene(root));
    primaryStage.show();
  }
}

The above binds the text of a Label to the ObjectBinding<LocalDateTime>. The value of the TextFormatter will update whenever the text is "committed" (e.g. by pressing Enter while the TextField has focus).

The way I constructed the LocalTimeStringConverter means it will use my Locale and FormatStyle.SHORT for both parsing and formatting the LocalTime. As an example, for me that means something like 3:30 PM or 11:25 AM. This is customizable—see the various constructors of LocalTimeStringConverter.