How to display date and time "at runtime"?

Question

I would like to know how to display the current date and time of the computer, and the time should be updated at runtime. I thought of passing an object to a label, but it did not. I’m not going to save this date, I just want it to be displayed, I wanted to put it on a label so I can position it anywhere after.

package teste;

import static java.awt.Frame.MAXIMIZED_BOTH;
import java.text.SimpleDateFormat;
import java.util.GregorianCalendar;
import java.util.Locale;
import javax.swing.JFrame;
import static javax.swing.JFrame.EXIT_ON_CLOSE;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class TelaSistema extends JFrame {

    public JLabel label = new JLabel();
    public JPanel painel = new JPanel();

    public TelaSistema() {
        setExtendedState(MAXIMIZED_BOTH);
        setTitle("Teste");

        GregorianCalendar calendar = new GregorianCalendar();

        label = new JLabel("Data aqui");

        painel.add(label);
        add(painel);

        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    public static void main(String args[]) {

        TelaSistema tela = new TelaSistema();
    }
}

Answer 1

Note: Always start the screen inside the Event-Dispatch-Thread, because swing is not Thread-Safe, and the entire GUI needs to start in this one Thread. In this reply explains better the reason for this and any problems that may occur. This other reply shows some ways to start the application within this thread.

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Using the package classes java.time, you can do as below:

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

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

public class TelaSistema extends JFrame {

    public JLabel label = new JLabel();
    public JPanel painel = new JPanel();

    public TelaSistema() {
        setExtendedState(MAXIMIZED_BOTH);
        setTitle("Teste");

        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("dd/MM/yyyy HH:mm:ss");
        LocalDateTime timePoint = LocalDateTime.now();

        label = new JLabel(timePoint.format(fmt));

        painel.add(label);
        add(painel);

        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    public static void main(String[] args) {

        java.awt.EventQueue.invokeLater(() -> new TelaSistema()); 

    }
}

The class DateTimeFormatter is responsible for formatting the date and class LocalDateTime is one of the new API classes for working with time.

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And to get the date updated in real time, you can create a Timer which forces the label to update each time, in this case, 1 second. To make it easier to work with the label, I used a class by part that extends from JLabel:

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import javax.swing.JLabel;
import javax.swing.Timer;

public class ClockLabel extends JLabel {

    public ClockLabel() {
        Timer t = new Timer(1000, e -> setText(getDateTime()));
        t.setInitialDelay(0);
        t.start();
    }

    private String getDateTime() {
        return LocalDateTime.now().format(DateTimeFormatter.ofPattern("dd/MM/yyyy HH:mm:ss"));
    }
}

Add this class to your project and then just instate it:

label = new ClockLabel();

See a working example:

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I recommend reading of this question to learn how to work with these new classes, they are optimized to work with time, and also reading of this other answer explaining some reasons to avoid working with old classes to manipulate dates.