GridPanel.java

// GridPanel.java
import java.awt.*;
import java.awt.geom.*;
import javax.swing.JPanel;
import javax.swing.JFrame;

public class GridPanel extends JPanel
{
    final static int  resolution = 96;
    AffineTransform tf = new AffineTransform();

    protected void show_size(Graphics g)
    {
	int width = getWidth();
	int height = getHeight();
	String str = String.format("width %d height %d",width,height);
	FontMetrics fm = g.getFontMetrics();
	Rectangle2D rect = fm.getStringBounds(str,g);
	width -= rect.getWidth();
	height -= rect.getHeight();
	g.drawString(str,width/2,height/2);
    }

    protected void draw_grid(Graphics g)
    {
	Graphics2D g2 = (Graphics2D) g;
	Paint savePaint = g2.getPaint();
	g2.setPaint(new Color(220,220,255));
	Stroke saveStroke = g2.getStroke();
	g2.setStroke(new BasicStroke(1.0f/resolution));
	double u = 4.0;
	for (int i=-4; i<=4; i++) {
		double v = i;			
		g2.draw(new Line2D.Double(-u,v,4,v));
		g2.draw(new Line2D.Double(v,-u,v,u));
	}
	// locate origin
	boolean show_origin = false;
	if (show_origin) {
  	   g2.setPaint(Color.BLACK);
	   u = 0.05;
	   g2.draw(new Line2D.Double(-u,u,u,-u));
	   g2.draw(new Line2D.Double(-u,-u,u,u));
	}
	g2.setStroke(saveStroke);
	g2.setPaint(savePaint);
    }

    // override to draw content on the grid pattern
    public void drawContent(Graphics g)
    {
 	Graphics2D g2 = (Graphics2D) g;
	Paint savePaint = g2.getPaint();
 	g2.setPaint(Color.BLACK);
	double u = 0.1;
	g2.draw(new Line2D.Double(-u,u,u,-u));
	g2.draw(new Line2D.Double(-u,-u,u,u));
	g2.setPaint(savePaint);
    }

    public void paintComponent(Graphics g)
    {
	super.paintComponent( g );
	Graphics2D g2 = (Graphics2D) g;
	AffineTransform saveXform = g2.getTransform();
	Paint savePaint = g2.getPaint();
	int width = getWidth();
	int height = getHeight();
	g2.translate(width/2,height/2);
	g2.scale(resolution,resolution);
	try {
	     tf .setToTranslation(width/2,height/2);
	     tf.scale(resolution,resolution);
                      tf.invert();
	}
	catch(Exception e) {
	     System.err.println( e);
	}

	draw_grid(g);
	Stroke saveStroke = g2.getStroke();
	g2.setStroke(new BasicStroke(2.0f/resolution,BasicStroke.CAP_ROUND,BasicStroke.JOIN_ROUND));

	drawContent(g);

	g2.setTransform(saveXform);
	g2.setPaint(savePaint);
	g2.setStroke(saveStroke);
	//show_size(g);
    }


   public void showTransform() {
	double[] mtx = new double[6];
	tf.getMatrix(mtx);
	System.out.println("Transform:");
	System.out.format(" %g %g %g\n",mtx[0],mtx[2],mtx[4]);
	System.out.format(" %g %g %g\n",mtx[1],mtx[3],mtx[5]);
  }
   

    public static void main(String[] args)
    {
        EventQueue.invokeLater(() -> {
		GridPanel panel = new GridPanel();
		JFrame app = new JFrame("GridPanel");
		app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		app.add(panel);
		app.setSize(496,358);
		app.setVisible(true);
	});
    }
}