How to implement a snap grid?

Question

I've been watching these forums for a while now, and I don't think I've seen anyone bring this up. I need to implement a toggleable snap grid, basically like might be in a paint or CAD program. However, I have no clue where to start or how I would begin to attempt such a thing in swing. Has anyone done this, or have any suggestions of what API functions might provide what I need? Thanks,
xanatos387

Answer 1

Not sure if you're still having problems...here's some code to allow 'snap to grid' drawing. I've removed alot of stuff for clarity, but you should still get the 'gist' of how it works.
-Brian
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class DotTest extends JPanel implements MouseListener, MouseMotionListener {
     private int dotSize;
     private Shape aShape;
     private boolean inDrag = false;
     private int xOffset, yOffset;
     public static void main(String[] args) {
          int dotSize;
          if (args.length < 0 )
               dotSize = 5;
          else
               dotSize = Integer.parseInt(args[0]);
          DotTest t = new DotTest(dotSize);
          JFrame f = new JFrame("Dot Size Test");
          f.setSize(300, 300);
          f.getContentPane().add(t, BorderLayout.CENTER);
          f.show();
     public DotTest(int d) {
          super();
          dotSize = d;
          buildGUI();
          System.out.println("Using dotSize of :"+dotSize);
          aShape = new Shape(dotSize, dotSize, 3*dotSize, 6*dotSize, Color.blue);
          this.addMouseListener(this);
          this.addMouseMotionListener(this);
     private void buildGUI() {
          setBackground(Color.white);
     public void paintComponent(Graphics g) {
          super.paintComponent(g);
          // custom paint code to 'draw dots'
          Rectangle r = g.getClipBounds();
          g.setColor(Color.gray);
          for(int x = 0;x<r.width;x=x+dotSize) {
               for(int y=0;y<r.height;y=y+dotSize) {
                    g.drawLine(x, y, x, y);
          // draw out 'shape'
          g.setColor(aShape.c);
          g.fillRect(aShape.x, aShape.y, aShape.w, aShape.h);
     public void mousePressed(MouseEvent e) {
          Rectangle r = aShape.getRectangle();
          if (r.contains(e.getX(), e.getY())) {
               inDrag = true;
               xOffset = e.getX() - aShape.x;
               yOffset = e.getY() - aShape.y;
     public void mouseReleased(MouseEvent e) {
          if (inDrag == true) {
               inDrag = false;
     public void mouseClicked(MouseEvent e) {
     public void mouseEntered(MouseEvent e) {
     public void mouseExited(MouseEvent e) {
     public void mouseMoved(MouseEvent e) {
     public void mouseDragged(MouseEvent e) {
          if (inDrag == true) {
               // check to see if the current mouse location falls on a 'dot' location
               // the '%' or 'modulo' operator returns the remainder of a division.
               // if the remainder is 'zero' then the point lies on a 'dot' or 'grid' location.
               int x = (e.getX() - xOffset) % dotSize;
               int y = (e.getY() - yOffset) % dotSize;
               if (x == 0)
                    aShape.x = e.getX() - xOffset;
               if (y == 0) {
                    aShape.y = e.getY() - yOffset;
                    c = true;
               // cause the component to 'update' it's display
               repaint();
     // inner class....just to give us something to 'drag around' the grid
     class Shape {
          int x, y, w, h;
          Color c;
          public Shape(int x, int y, int w, int h, Color c) {
               this.x = x;
               this.y = y;
               this.w = w;
               this.h = h;
               this.c = c;
          public Rectangle getRectangle() {
               return new Rectangle(x, y, w, h);
}