How to paint in Java Swing in every concurrent thread?

Question

The goal of my program is to draw squares from randomly generated points. I want to display a set of squares as soon as they are generated by each thread. However, only one set of squares displays, once all the threads are done running. I have used swinginvoke, and am curious as if there is a problem with repaint() since all the threads reach repaint but don't paint until the final thread is done, and end up overlapping each other. I also don't want the "storedata" variable to be shared between each thread but every instance of it keeps the data in it. I tried to fix this by clearing it, but it hasn't worked. The program consists of a custom thread class, a main class where the threads get started, a GUI class, and a custom jpanel where the squares are drawn.

import java.awt.*;
import java.util.Random;
import java.util.concurrent.CopyOnWriteArrayList;

class Action extends Thread {

        private Random rand = new Random();
        private static CopyOnWriteArrayList<Point> storedata = new CopyOnWriteArrayList<>();
        static volatile CopyOnWriteArrayList<CopyOnWriteArrayList<Point>> finallist = new CopyOnWriteArrayList<>();
        private GUI g;

        Action(GUI g) {
            this.g = g;

        }

    private void generatePoint() {
        int x = rand.nextInt(500);
        int y = rand.nextInt(500);
        Point p = new Point(x,y);
        storedata.add(p);

    }

    public void run() {
        for (int i = 0; i < 5; i++) {
            generatePoint();
        }
        CopyOnWriteArrayList<Point> copy = new CopyOnWriteArrayList<>(storedata);
        finallist.add(copy);
        SwingUtilities.invokeLater(() -> {
            try {
                Thread.sleep(500);
                System.out.println("ARRAY = " + copy.toString());
                g.setSolution(copy);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        storedata.clear();
    }
}

public class Main {
    public static void main(String[] args) {
        GUI g = new GUI();
        g.setVisible(true);

        Thread[] threads = new Thread[5];
        for (int i = 0; i < threads.length; i++) {
            threads[i] = new Action(g);
            threads[i].start();
        }

    }


import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.CopyOnWriteArrayList;

public class GUI extends JFrame implements ActionListener  {
    private CustomPanel c;
    GUI()  {
        initialize();
    }


    private void initialize()  {
        this.setLayout(new FlowLayout());
        c = new CustomPanel(500,500);
        this.setSize(1000,1000);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.add(c);
        this.setTitle("Seat Placement Program");
    }

    @Override
    public void actionPerformed(ActionEvent e) {

    }

    public void setSolution(CopyOnWriteArrayList<Point> room) {
        c.setRoom(room);
        c.setPaint(true);
        c.repaint();
    }

import javax.swing.*;
import javax.swing.border.Border;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.CopyOnWriteArrayList;

public class CustomPanel extends JPanel implements ActionListener {

    private Border blackline = BorderFactory.createLineBorder(Color.black);
    private boolean paint;
    private CopyOnWriteArrayList<Point> room;

    public CustomPanel(int h, int w) {
        room = new CopyOnWriteArrayList<>();
        paint = false;
        this.setPreferredSize(new Dimension(w,h));
        this.setBorder(blackline);
    }

    @Override
    public void actionPerformed (ActionEvent e) {

    }


    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if(paint) {
            for(Point p : room) {
                g.drawRect((int) p.getX(),(int) p.getY(),20,20);
            }
        }
    }

    void setPaint(boolean b) {
        paint = b;
    }

    public void setRoom(CopyOnWriteArrayList<Point> room) {
        this.room = room;
    }


}

Answer 1

In swing, painting should only be done in the Event Dispatching Thread . The way to achieve what you want is when the thread has finished, add the shape (along with the color, possibly) to a collection in the JComponent that you want it in (you need to create a custom JComponent that has such a collection, and you should synchronize around it when adding shapes and in paintComponent when painting the shapes). Then in your non-EDT thread, call myJComponent.repaint(). It is okay to call this in all of your non-EDT threads.

Example:

public class MyPanel extends JPanel {
  Collection<Shape> myShapes = ...;
  public void paintComponent(Graphics g) {
    ... // do any other non-shape related painting
    Graphics2D g2 = (Graphics2D) g;
    synchronized (myShapes) {
      for (Shape s : myShapes) {
        //TODO: do you want to set a color?
        g.draw(s);
        //TODO: do you want to fill the shapes?
        g.fill(s);
      }
    }
  }
}

Then in your threads:

public void run() {
  ...
  Shape newShape = ...;
  synchronized (myPanel.myShapes) {
    //add newShape to the collection
  }
  myPanel.repaint();
}