Используйте BasicStroke
и нарисуйте его дважды, толще и тоньше.
import java.awt.*;
import java.awt.image.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.imageio.ImageIO;
import java.io.File;
class PaintThick {
public static void main(String[] args) throws Exception {
int size = 150;
final BufferedImage bi = new BufferedImage(
size,size,BufferedImage.TYPE_INT_RGB);
Graphics2D g = bi.createGraphics();
double pad = 20;
Line2D.Double line1 = new Line2D.Double(
pad,pad,(double)(size-pad),(double)(size-pad));
int cap = BasicStroke.CAP_BUTT;
int join = BasicStroke.JOIN_MITER;
BasicStroke thick = new BasicStroke(15,cap,join);
BasicStroke thinner = new BasicStroke(13,cap,join);
g.setColor(Color.WHITE);
g.fillRect(0,0,size,size);
g.setColor(Color.BLACK);
g.setStroke(thick);
g.draw(line1);
g.setColor(Color.WHITE);
g.setStroke(thinner);
g.draw(line1);
ImageIO.write(bi,"png",new File("img.png"));
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JOptionPane.showMessageDialog(
null, new JLabel(new ImageIcon(bi)));
}
});
}
}