Slow populating of Java JFrame Application with NetBeans 11.0

Question

I'm learning Java so I started off by creating a Digital Clock JFrame Form. It all works, but upon load of the application, it takes about 2/3 second to display my border and my label on the background panel.

File: DigitalClock.java

package digitalclock;

import JForms.DigitalClockJForm;

public class DigitalClock {

    public static void main(String[] args) {
        new DigitalClockJForm().setVisible(true);

    }

}

File: DigitalClockJForm.java

package JForms;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class DigitalClockJForm extends javax.swing.JFrame 
{   
    // Member Variables
    private int second, minute, hour, day, month, year;
    String timeDate;

    // Constructors
    public DigitalClockJForm() {
        initComponents();
        clock();
    }

    public void clock()
    {
        Thread t = new Thread(){

            @Override
            public void run(){

                try {
                    while (true){
                        Calendar cal = new GregorianCalendar();

                        day = cal.get(Calendar.DAY_OF_MONTH);
                        month = cal.get(Calendar.MONTH);
                        year = cal.get(Calendar.YEAR);

                        second = cal.get(Calendar.SECOND);
                        minute = cal.get(Calendar.MINUTE);
                        hour = cal.get(Calendar.HOUR);

                        SimpleDateFormat sdf = new SimpleDateFormat("hh:mm:ss a dd/MM/yyyy");

                        Date date = cal.getTime();

                        timeDate = sdf.format(date);

                        mainLabel.setText(timeDate);
                    }
                }
                catch (Exception e) {
                    e.printStackTrace();
                }
            }
        };

        t.start();
    }

 << Net Beans generated code >>

// Variables declaration - do not modify                     
private javax.swing.JLabel mainLabel;
private javax.swing.JPanel mainPanel;
// End of variables declaration   

The Label and Panel take 2/3 seconds to display on the form.

The form launches like this:

Then after 2/3 seconds, everything, including border is displayed correctly:

I would have expected the label and panel to display as soon as the application is launched.

Answer 1

There is no need to subclass JFrame, or even JPanel in your case, since you are not overriding any of their methods. Just create a JPanel and add your components to it, then add that JPanel to a new JFrame, pack the frame, and display it.

Create a Swing.Timer with a 1000 millisecond delay, and in the actionPerformed method update the displayed time.

It is probably overkill to create a new GregorianCalendar every time the actionPerformed method is called. Updating the display by one second each time should be sufficient, perhaps with a new Calendar every several minutes to re-synch, unless your CPU is extremely busy executing other applications.