Drawing straight lines with Java is easy. Just call
g.drawLine(x1, y1, x2, y2)
where (x1, y1) and (x2, y2) are the
endpoints of your lines and g is the Graphics object you're drawing with.
This program draws a line diagonally across the applet.
import java.applet.*;
import java.awt.*;
public class SimpleLine extends Applet {
public void paint(Graphics g) {
g.drawLine(0, 0, this.getSize().width, this.getSize().height);
}
}