Why, when I change the size of the window, do the drawn components disappear?

I built an application using the windowbuilder of eclipse. In it, when clicking with the mouse, a figure is drawn according to the last button selected.

However, when I change the size of the window, all the drawings disappear.

After reading this question , that tutorial , and this article ; I suppose I should override the paint(Graphics g) method and add fields to a class that stores the drawings.

But it is not clear to me where I would do this. Because I really do not understand what's going on behind the scenes of the code.

Class created with windowbuilder

public class PintarFiguras {

    private JFrame frame;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    PintarFiguras window = new PintarFiguras();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the application.
     */
    public PintarFiguras() {
        initialize();
    }

    /**
     * Initialize the contents of the frame.
     */
    private void initialize() {
        frame = new JFrame();
        frame.setBackground(Color.GRAY);
        frame.getContentPane().setBackground(Color.GRAY);
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel canvas = new JPanel();
        canvas.setBackground(Color.WHITE);
        canvas.setForeground(Color.BLACK);
        canvas.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                // pegar coordenadas
                int x = e.getX();
                int y = e.getY();

                Graphics g = canvas.getGraphics(); 

                Artista.desenhar(g,x,y);
            }
        });
        frame.getContentPane().add(canvas, BorderLayout.CENTER);
        canvas.setLayout(new CardLayout(0, 0));

        JToolBar toolBar = new JToolBar();
        frame.getContentPane().add(toolBar, BorderLayout.NORTH);

        JButton btnQuadrado = new JButton("quadrado");
        btnQuadrado.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                Artista.setModo(MODOS.QUADRADO);
            }
        });
        toolBar.add(btnQuadrado);

        Component horizontalStrut = Box.createHorizontalStrut(20);
        toolBar.add(horizontalStrut);

        JButton btnCirculo = new JButton("circulo");
        btnCirculo.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                Artista.setModo(MODOS.CIRCULO);
            }
        });
        toolBar.add(btnCirculo);

        Component horizontalStrut_1 = Box.createHorizontalStrut(20);
        toolBar.add(horizontalStrut_1);

        JButton btnNada = new JButton("nada");
        btnNada.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                Artista.setModo(MODOS.NADA);
            }
        });
        toolBar.add(btnNada);

        Component horizontalStrut_2 = Box.createHorizontalStrut(20);
        toolBar.add(horizontalStrut_2);

        JButton btnLimpar = new JButton("limpar");
        btnLimpar.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                Graphics g = canvas.getGraphics();
                g.clearRect(0, 0, canvas.getWidth(), canvas.getHeight());
                frame.setBackground(Color.WHITE);
            }
        });
        toolBar.add(btnLimpar);
    }

}

Enum of the figures to draw

public enum MODOS {
QUADRADO, CIRCULO, NADA}

Class artist, who does the drawings

public abstract class Artista {
    private static MODOS modo = MODOS.NADA;

    public static MODOS getModo() {
        return modo;
    }

    public static void setModo(MODOS modo) {
        Artista.modo = modo;
    }

    /**
     * pinta um objeto de acordo com o modo
     * https://docs.oracle.com/javase/tutorial/2d/basic2d/index.html
     */
    public static void desenhar(Graphics g, int x, int y) {     
        switch(modo){   

        case QUADRADO:
            pintarQuadrado(g,x,y);
            break;

        case CIRCULO:
            pintarCirculo(g,x,y);
            break;

        case NADA:
            break;
        }       
    }

    private static void pintarQuadrado(Graphics g, int x, int y) {
        g.fillRect(x, y, 20, 10);
    }

    private static void pintarCirculo(Graphics g, int x, int y) {
        g.drawOval(x, y, 20, 20);
    }

}