How To Create A Triangle In Java

I would use a Path2D object, and would place my first point with its moveTo(...) method, and then add additional points with its lineTo(...) method. Then I could draw it or fill it via Graphics2D#draw(...) and Graphics2D#fill(...). Also calling closePath() on it will make sure that your triangle closes appropriately.

For example, the following code produces:

enter image description here

          import java.awt.Color; import java.awt.Dimension; import java.awt.GradientPaint; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.Paint; import java.awt.RenderingHints; import java.awt.geom.Path2D;     import javax.swing.*;  public class Path2DExample extends JPanel {    private static final int PREF_W = 600;    private static final int PREF_H = PREF_W;    private static final Color COLOR_1 = Color.blue;    private static final Color COLOR_2 = Color.red;    private static final Paint GRADIENT_PAINT = new GradientPaint(0, 0, COLOR_1, 20, 20, COLOR_2, true);    private Path2D myPath = new Path2D.Double();     public Path2DExample() {       double firstX = (PREF_W / 2.0) * (1 - 1 / Math.sqrt(3));       double firstY = 3.0 * PREF_H / 4.0;        myPath.moveTo(firstX, firstY);       myPath.lineTo(PREF_W - firstX, firstY);       myPath.lineTo(PREF_W / 2.0, PREF_H / 4.0);       myPath.closePath();    }     @Override    protected void paintComponent(Graphics g) {       super.paintComponent(g);       Graphics2D g2 = (Graphics2D) g;        // to smooth out the jaggies       g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,             RenderingHints.VALUE_ANTIALIAS_ON);       g2.setPaint(GRADIENT_PAINT);  // just for fun!       g2.fill(myPath);  // fill my triangle    }     @Override    public Dimension getPreferredSize() {       if (isPreferredSizeSet()) {          return super.getPreferredSize();       }       return new Dimension(PREF_W, PREF_H);    }     private static void createAndShowGui() {       Path2DExample mainPanel = new Path2DExample();        JFrame frame = new JFrame("Path2DExample");       frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);       frame.getContentPane().add(mainPanel);       frame.pack();       frame.setLocationByPlatform(true);       frame.setVisible(true);    }     public static void main(String[] args) {       SwingUtilities.invokeLater(new Runnable() {          public void run() {             createAndShowGui();          }       });    } }

An additional benefit to use of a Path2D object is that if you want to drag the Shape, it's not hard to do with a MouseListener and MouseMotionListener, say something like:

          private class MyMouseAdapter extends MouseAdapter {   private Point pPressed = null;    @Override   public void mousePressed(MouseEvent e) {      if (e.getButton() != MouseEvent.BUTTON1) {         return;      }      if (myPath.contains(e.getPoint())) {         pPressed = e.getPoint();      }   }    public void mouseDragged(MouseEvent e) {      drag(e);   }    @Override   public void mouseReleased(MouseEvent e) {      drag(e);      pPressed = null;   }    private void drag(MouseEvent e) {      if (pPressed == null) {         return;      }      Point p = e.getPoint();      int tx = p.x - pPressed.x;      int ty = p.y - pPressed.y;      AffineTransform at = AffineTransform.getTranslateInstance(tx, ty);      myPath.transform(at);      pPressed = p;      repaint();   };  }

The whole thing could look like:

          import java.awt.Color; import java.awt.Dimension; import java.awt.GradientPaint; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.Paint; import java.awt.Point; import java.awt.RenderingHints; import java.awt.event.MouseAdapter; import java.awt.event.MouseEvent; import java.awt.geom.AffineTransform; import java.awt.geom.Path2D; import javax.swing.*;  @SuppressWarnings("serial") public class Path2DExample extends JPanel {    private static final int PREF_W = 600;    private static final int PREF_H = PREF_W;    private static final Color COLOR_1 = Color.blue;    private static final Color COLOR_2 = Color.red;    private static final Paint GRADIENT_PAINT = new GradientPaint(0, 0, COLOR_1,          20, 20, COLOR_2, true);    private Path2D myPath = new Path2D.Double();     public Path2DExample() {       double firstX = (PREF_W / 2.0) * (1 - 1 / Math.sqrt(3));       double firstY = 3.0 * PREF_H / 4.0;        myPath.moveTo(firstX, firstY);       myPath.lineTo(PREF_W - firstX, firstY);       myPath.lineTo(PREF_W / 2.0, PREF_H / 4.0);       myPath.closePath();        MyMouseAdapter myMouseAdapter = new MyMouseAdapter();       addMouseListener(myMouseAdapter);       addMouseMotionListener(myMouseAdapter);    }     @Override    protected void paintComponent(Graphics g) {       super.paintComponent(g);       Graphics2D g2 = (Graphics2D) g;       g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,             RenderingHints.VALUE_ANTIALIAS_ON);       g2.setPaint(GRADIENT_PAINT);       g2.fill(myPath);    }     @Override    public Dimension getPreferredSize() {       if (isPreferredSizeSet()) {          return super.getPreferredSize();       }       return new Dimension(PREF_W, PREF_H);    }     private class MyMouseAdapter extends MouseAdapter {       private Point pPressed = null;        @Override       public void mousePressed(MouseEvent e) {          if (e.getButton() != MouseEvent.BUTTON1) {             return;          }          if (myPath.contains(e.getPoint())) {             pPressed = e.getPoint();          }       }        public void mouseDragged(MouseEvent e) {          drag(e);       }        @Override       public void mouseReleased(MouseEvent e) {          drag(e);          pPressed = null;       }        private void drag(MouseEvent e) {          if (pPressed == null) {             return;          }          Point p = e.getPoint();          int tx = p.x - pPressed.x;          int ty = p.y - pPressed.y;          AffineTransform at = AffineTransform.getTranslateInstance(tx, ty);          myPath.transform(at);          pPressed = p;          repaint();       };     }     private static void createAndShowGui() {       Path2DExample mainPanel = new Path2DExample();        JFrame frame = new JFrame("Path2DExample");       frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);       frame.getContentPane().add(mainPanel);       frame.pack();       frame.setLocationByPlatform(true);       frame.setVisible(true);    }     public static void main(String[] args) {       SwingUtilities.invokeLater(new Runnable() {          public void run() {             createAndShowGui();          }       });    } }