import java.awt.*;
import java.applet.*;

public class thickLine extends Applet {

 public void init( ) { }

 public void paint( Graphics g ) {
  drawThickLine
    (g, 0, 0, getSize().width, getSize().height, 5, new Color(0).black);
  drawThickLine
    (g, 0, getSize().height, getSize().width, 0, 5, new Color(0).red);

  drawThickLine
    (g, getSize().width/2, 0, getSize().width/2, getSize().height, 8,
     new Color(0).green);
  drawThickLine
    (g, 0, getSize().height/2, getSize().width, getSize().height/2, 12,
     new Color(0).blue);
  }

 public void drawThickLine(
  Graphics g, int x1, int y1, int x2, int y2, int thickness, Color c) {
  // The thick line is in fact a filled polygon
  g.setColor(c);
  int dX = x2 - x1;
  int dY = y2 - y1;
  // line length
  double lineLength = Math.sqrt(dX * dX + dY * dY);

  double scale = (double)(thickness) / (2 * lineLength);

  // The x,y increments from an endpoint needed to create a rectangle...
  double ddx = -scale * (double)dY;
  double ddy = scale * (double)dX;
  ddx += (ddx > 0) ? 0.5 : -0.5;
  ddy += (ddy > 0) ? 0.5 : -0.5;
  int dx = (int)ddx;
  int dy = (int)ddy;

  // Now we can compute the corner points...
  int xPoints[] = new int[4];
  int yPoints[] = new int[4];

  xPoints[0] = x1 + dx; yPoints[0] = y1 + dy;
  xPoints[1] = x1 - dx; yPoints[1] = y1 - dy;
  xPoints[2] = x2 - dx; yPoints[2] = y2 - dy;
  xPoints[3] = x2 + dx; yPoints[3] = y2 + dy;

  g.fillPolygon(xPoints, yPoints, 4);
  }
}

public void paint(Graphics g){
    Graphics2D g2d = (Graphics2D)g;
    int width = 10;
    g2d.setStroke(new BasicStroke(width));
    g2d.drawLine(x1, y1, x2, y2);
}

int width = 10;
BasicStroke bs = new BasicStroke(width);
JLabel l = new JLabel();
l.getGraphics().setStroke(bs);
l.drawLine(0,0,100,100);

import javax.swing.*;
import java.awt.*;
import java.awt.geom.*;

public class TestLine extends JFrame{
 private MyPanel panel;
 public TestLine() {
    setSize(200, 200);
    panel = new MyPanel();
    getContentPane().add( panel, "Center" );
    }

 public static void main( String [] args ){
    TestLine tl = new TestLine();
    tl.setVisible( true );
    }
}

class MyPanel extends JPanel {
    final static BasicStroke stroke = new BasicStroke(2.0f);
    final static BasicStroke wideStroke = new BasicStroke(8.0f);

    public MyPanel(){}

    public void paintComponent( Graphics g ){
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint
          (RenderingHints.KEY_ANTIALIASING, 
            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setStroke( stroke );
        g2.draw(new Line2D.Double(10.0, 10.0, 100.0, 10.0));
        g2.setStroke( wideStroke );
        g2.draw(new Line2D.Double(10.0, 50.0, 100.0, 50.0));
        }
}