How to pass Graphics objects between classes and methods to draw lines

Question

I am making a gui program with the mouseListener and mouseMotionListener. I have the following Line class

public class Line {

private int x1, x2, y1, y2;
private Color color;

public Line(int x1, int x2, int y1, int y2, Color color)
{
    this.x1 = x1;
    this.x2 = x2;
    this.y1 = y1;
    this.y2 = y2;
    this.color = color;
}

public void draw(Graphics page)
{
    page.drawLine(x1, y1, x2, y2);
    page.setColor(color);
}

}

here is my mouseReleased where i get the final points of the desired line.

  public void mouseReleased (MouseEvent event)
  { // ending points
     moving = false;
     Point p2 = event.getPoint();
     x2 = p2.x;
     y2 = p2.y;  
     line = new Line(x1,x2,y1,y2,currentColor);
     lineList.add(line);
     canvas.paintComponent(??????????);

Here is the canvas method that should draw all of these lines in the array list "lineList". to the canvas

private class CanvasPanel extends JPanel
{
  //this method draws all shapes specified by a user
  public void paintComponent(Graphics page)
   {
    super.paintComponent(page);
    setBackground(Color.WHITE);
    for(int i = 0; i <lineList.size()-1;i++)
    {
        line.draw(page);
    }

However I do not know how to pass the graphics object to the canvas class in order to actually draw my lines on the JPanel. Assuming i have all other info correct(initial line points, JPanel set correctly, and buttons set up) how do i pass these to actually make it draw the lines to the canvas. Thank you!

Answer 1

No, you don't want to pass a Graphics object anywhere, and in fact you don't paint from within the MouseListener or MouseMotionListener. Instead you change fields from within those classes, call repaint() and then use the field results in your paintComponent method.

In fact in your code, if CanvasPanel has access to the lineList, all you need to do is call repaint() on it after adding a new line into the lineList collection. That's it.

Also, don't set background within paintComponent but rather within the constructor. Also you need to swap your method calls in the Line's draw method. You need to set the color before drawing the line.

e.g.,

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.List;
import javax.swing.*;

@SuppressWarnings("serial")
public class Drawing extends JPanel {
    public static final Color BG = Color.WHITE;
    public static final Color LINE_COLOR = Color.RED;
    public static final Color CURRENT_LINE_COLOR = Color.LIGHT_GRAY;
    public static final int PREF_W = 800;
    public static final int PREF_H = PREF_W;
    private List<Line> lineList = new ArrayList<>();
    private Line currentLine = null;
    private CanvasPanel canvasPanel = new CanvasPanel();

    public Drawing() {
        MyMouse myMouse = new MyMouse();
        canvasPanel.addMouseListener(myMouse);
        canvasPanel.addMouseMotionListener(myMouse);

        setLayout(new BorderLayout());
        add(canvasPanel);
    }

    private class CanvasPanel extends JPanel {
        public CanvasPanel() {
            setBackground(BG);
        }

        public void paintComponent(Graphics page) {
            super.paintComponent(page);
            // setBackground(Color.WHITE); // !! no, not here

            for (Line line : lineList) {
                line.draw(page);
            }

            if (currentLine != null) {
                currentLine.draw(page);
            }
        }

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

    private class MyMouse extends MouseAdapter {
        private int x1;
        private int y1;

        @Override
        public void mousePressed(MouseEvent e) {
            if (e.getButton() != MouseEvent.BUTTON1) {
                return;
            }
            x1 = e.getX();
            y1 = e.getY();
            currentLine = null;
            canvasPanel.repaint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (e.getButton() != MouseEvent.BUTTON1) {
                return;
            }
            Line line = createLine(e, LINE_COLOR);
            lineList.add(line);
            currentLine = null;
            canvasPanel.repaint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            currentLine = createLine(e, CURRENT_LINE_COLOR);
            repaint();
        }

        private Line createLine(MouseEvent e, Color currentColor) {
            int x2 = e.getX();
            int y2 = e.getY();
            return new Line(x1, x2, y1, y2, currentColor);
        }


    }

    private static void createAndShowGui() {
        JFrame frame = new JFrame("Drawing");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new Drawing());
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGui();
            }
        });
    }

}

class Line {

    private int x1, x2, y1, y2;
    private Color color;

    public Line(int x1, int x2, int y1, int y2, Color color) {
        this.x1 = x1;
        this.x2 = x2;
        this.y1 = y1;
        this.y2 = y2;
        this.color = color;
    }

    public void draw(Graphics page) {

        // swap these calls!
        page.setColor(color); //!! This first!
        page.drawLine(x1, y1, x2, y2);  // **Then** this
        // !! page.setColor(color);
    }
}