paintComponent is called, but nothing is displayed

Question

I'm new to AWT, this is just a sample program to see how it works.

My problem is that on events like resize/minimize, the window is cleared.

I have overridden paintComponent as specified in many SO questions, and the code is actually called (because if I put a println in there, I see the output), however nothing is displayed.

I've tried this on Windows and Linux and the result is the same, so I guess I'm missing something.

Here is the code:

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.ArrayList;
import javax.swing.JFrame;
import javax.swing.JPanel;

class Site {
    private Integer x;
    private Integer y;
    private Integer width;
    private Integer height;
    private Graphics2D g2d;
    private boolean isOpen = false;

    private Integer id = null;
    private Integer root = null;

    public Site(Integer id, Integer x, Integer y, Integer width, Integer height, Graphics2D g2d) {
        this.id = id;
        this.root = id;
        this.x = x;
        this.y = y;
        this.width = width;
        this.height = height;
        this.g2d = g2d;
    }

    public void Draw() {
        g2d.setPaint(new Color(0, 0, 0));
        g2d.setStroke(new BasicStroke(2));
        Integer x_top_corner = x - (width / 2);
        Integer y_top_corner = y - (height / 2);
        g2d.drawRect(x_top_corner, y_top_corner, width, height);
        System.out.println(x_top_corner);

        if (isOpen) {
            g2d.setPaint(new Color(150, 150, 200));
        } else {
            g2d.setPaint(new Color(200, 200, 200));
        }
        g2d.fillRect(x_top_corner + 1, y_top_corner + 1, width - 2, height - 2);
    }

    public void Open() {
        this.isOpen = true;
    }

    public void Close() {
        this.isOpen = false;
    }
}

class Surface extends JPanel {
    private ArrayList<Site> sites = new ArrayList<Site>();
    private Graphics2D g2d = null;

    private void initSites() {
        Integer width = 30;
        Integer height = 30;
    Integer index = 0;

        for (Integer y = height; y < 800 - height - 30; y += height) {
            for (Integer x = width; x < 800 - width; x += width) {
                sites.add(new Site(index, x, y, width, height, g2d));
                index++;
            }
        }
    Integer x = 20;
    Integer y = 3;
    sites.get(x + (y * 25)).Open();
    sites.get(x + 1 + (y * 25)).Open();
    sites.get(x).Open();
    sites.get(4).Open();
    sites.get(9).Open();
    sites.get(13).Open();
    sites.get(18).Open();
    }

    private void doDrawing(Graphics g) {
        if (g2d == null) {
            g2d = (Graphics2D) g;
        }

        if (sites.size() == 0) {
            initSites();
        }

        for (Site site : sites) {
            site.Draw();
        }
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        doDrawing(g);
    }
}

public class Squares extends JFrame {
    public Squares() {
        initUI();
    }

    private void initUI() {
        add(new Surface());
        setTitle("Squares demo");
        setSize(800, 800);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        Squares ex = new Squares();
        ex.setVisible(true);
    }
}

Answer 1

You're using Graphics2D as a field assuming that one graphics object is as good as the next, and that's not how you do Swing drawing. The Graphics object given into the paintComponent method is not long-lived or stable, and it must be used and not stored into a field for later use. Instead only use the Graphics object dynamically passed into your program from the JVM via the paintComponent's parameter. For example if you give Draw (rename it to draw) a Graphics or Graphics2D parameter, then pass it in from the paintComponent method:

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.ArrayList;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

class Site {
    private Integer x;
    private Integer y;
    private Integer width;
    private Integer height;
    // private Graphics2D g2d;
    private boolean isOpen = false;

    private Integer id = null;
    private Integer root = null;

    // !! public Site(Integer id, Integer x, Integer y, Integer width, Integer
    // height, Graphics2D g2d) {
    public Site(Integer id, Integer x, Integer y, Integer width, Integer height) {
        this.id = id;
        this.root = id;
        this.x = x;
        this.y = y;
        this.width = width;
        this.height = height;
        // this.g2d = g2d;
    }

    public void draw(Graphics2D g2d) {
        g2d.setPaint(new Color(0, 0, 0));
        g2d.setStroke(new BasicStroke(2));
        Integer x_top_corner = x - (width / 2);
        Integer y_top_corner = y - (height / 2);
        g2d.drawRect(x_top_corner, y_top_corner, width, height);
        System.out.println(x_top_corner);

        if (isOpen) {
            g2d.setPaint(new Color(150, 150, 200));
        } else {
            g2d.setPaint(new Color(200, 200, 200));
        }
        g2d.fillRect(x_top_corner + 1, y_top_corner + 1, width - 2, height - 2);
    }

    public void Open() {
        this.isOpen = true;
    }

    public void Close() {
        this.isOpen = false;
    }
}

class Surface extends JPanel {
    private ArrayList<Site> sites = new ArrayList<Site>();
    private Graphics2D g2d = null;
    private int width;
    private int height;

    public Surface(int width, int height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(width, height);
    }


    private void initSites() {
        Integer width = 30;
        Integer height = 30;
        Integer index = 0;

        for (Integer y = height; y < 800 - height - 30; y += height) {
            for (Integer x = width; x < 800 - width; x += width) {
                // sites.add(new Site(index, x, y, width, height, g2d));
                sites.add(new Site(index, x, y, width, height)); // !!
                index++;
            }
        }
        Integer x = 20;
        Integer y = 3;
        sites.get(x + (y * 25)).Open();
        sites.get(x + 1 + (y * 25)).Open();
        sites.get(x).Open();
        sites.get(4).Open();
        sites.get(9).Open();
        sites.get(13).Open();
        sites.get(18).Open();
    }

    private void doDrawing(Graphics g) {
        if (sites.size() == 0) {
            initSites();
        }

        for (Site site : sites) {
            site.draw((Graphics2D) g);
        }
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        doDrawing(g);
    }
}

public class Squares extends JFrame {
    private static final int SIDE_LENGTH = 800;

    public Squares() {
        initUI();
    }

    private void initUI() {
        add(new Surface(SIDE_LENGTH, SIDE_LENGTH));
        setTitle("Squares demo");
        // setSize(800, 800);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        pack();
        setLocationRelativeTo(null);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            Squares ex = new Squares();
            ex.setVisible(true);
        }); 
    }
}

Also,

• run the program from the Swing event thread only
• Don't set sizes directly as you're doing. Override getPreferredSize instead