Dynamic Swing Refresh on JLabel

Question

I am doing exercises to learn programming in Java. I created a main class where I simply generate random numbers and add each number in an array list. Every time I insert a new number in the array list, I calculate the average of the array list which changes.

I am stuck at the following part: I want to display live on a JFrame each time I calculate a new average (every time a random number is added). I created a JFrame and a JLabel for that. JLabel then needs to be constantly updated.

What directions do I need to go to from there? Swing worker? Swing timer? Action listener? I read a few posts already on the necessity to have it run in the background, multiple threads but I am a bit lost now so any help will be appreciated.

public class TestSwing {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {

    TestFrame frame = new TestFrame(); 
    frame.setBounds(300, 200, 700, 400);

    // Variables
    Random rn = new Random();
    ArrayList<Integer> elements = new ArrayList<>();

    //Generate random numbers
    for(int i =0; i < 100; i++)
    {
        int answer = rn.nextInt(10) + 1;

        // Add random numbers to array
        elements.add (answer);

            //Get average of array every time a new number is added to the array
            int sum = 0;
            for(int j=0; j < elements.size() ; j++)
            {
                sum = sum + elements.get(j); 
            }
            //calculate average value
            double average = sum / elements.size();

   //Show Frame
   frame.setVisible(true);
   // This is where I am stuck
    }      
} 

}

Answer 1

You left out quite a few things. Here is a better version of your code, you will need to add a timer like LuxxMiner suggested.

import javax.swing.*;
import java.awt.*;
import java.util.*;

public class TestSwing {

/**
 * @param args the command line arguments
 */

    public static void main(String[] args) {

        JFrame frame = new JFrame();
        frame.setBounds(300, 200, 700, 400);

        //Set layout
        frame.setLayout(new BorderLayout());


        // Variables
        Random rn = new Random();
        ArrayList<Integer> elements = new ArrayList<>();

        //Generate random numbers
        for(int i =0; i < 100; i++)
        {
            int answer = rn.nextInt(10) + 1;

            // Add random numbers to array
            elements.add (answer);

                //Get average of array every time a new number is added to the array
                int sum = 0;
                for(int j=0; j < elements.size() ; j++)
                {
                    sum = sum + elements.get(j); 
                }
                //calculate average value
                double average = sum / elements.size();
        }  
        //Add label
        JLabel label = new JLabel(String.valueOf(average));
        frame.add(label, BorderLayout.CENTER);

        //Show Frame
        frame.setVisible(true);   
} 
}

Answer 2

A java.util.Timer is simple and will do well. Please read this and this for more information.

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
new Timer().scheduleAtFixedRate(new TimerTask() {
    int counter = 0;

    @Override
    public void run() {
        counter++;
        if (counter >= 10) { // If ran 10 times, stop.
            this.cancel();
        }
        int answer = rn.nextInt(10) + 1;
        elements.add(answer);
        int sum = 0;
        for (int j = 0; j < elements.size(); j++) {
            sum = sum + elements.get(j);
        }
        double average = sum / elements.size();
        label.setText(String.valueOf(average)); // Set the text of the
                                                // label (automatically
                                                // repaints panel)
        System.out.println(String.valueOf(average));
    }
}, 500, 1000); // Run every second (= 1000 milliseconds), wait 500
                // milliseconds before starting it.