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Hey I know that it is simple to draw oval/rectangle and fill it using
g.fillOval(30, 40, 20, 20); but how to draw a triangle? It would be the best if it would have random coordinates.
There are at least two basics ways you can achieve this, based on your needs.
You could use Polygon or you could make use the 2D Graphics Shape API
Which you might choose comes down to your requirements. Polygon requires you to know, in advance the position of the points within 3D space, where the Shape API gives you more freedom to define the shape without caring about the position in advance.
This makes the Shape API more flexible, in that you can define the shape once and simply translate the Graphics context as needed and repaint it.
For example...
Red is a Polygon, green is a Shape, which is translated into position...
import java.awt.BorderLayout; import java.awt.Color; import java.awt.Dimension; import java.awt.EventQueue; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.Polygon; import java.awt.geom.Path2D; import java.awt.geom.Point2D; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.UIManager; import javax.swing.UnsupportedLookAndFeelException; public class TriangleTest { public static void main(String[] args) { new TriangleTest(); } public TriangleTest() { EventQueue.invokeLater(new Runnable() { @Override public void run() { try { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) { } JFrame frame = new JFrame("Testing"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setLayout(new BorderLayout()); frame.add(new TestPane()); frame.pack(); frame.setLocationRelativeTo(null); frame.setVisible(true); } }); } public class TestPane extends JPanel { private TriangleShape triangleShape; private Polygon poly; public TestPane() { triangleShape = new TriangleShape( new Point2D.Double(50, 0), new Point2D.Double(100, 100), new Point2D.Double(0, 100) ); poly = new Polygon( new int[]{50, 100, 0}, new int[]{0, 100, 100}, 3); } @Override public Dimension getPreferredSize() { return new Dimension(200, 200); } @Override protected void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2d = (Graphics2D) g.create(); g2d.setColor(Color.RED); g2d.fill(poly); g2d.setColor(Color.GREEN); g2d.translate(50, 100); g2d.fill(triangleShape); g2d.dispose(); } } public class TriangleShape extends Path2D.Double { public TriangleShape(Point2D... points) { moveTo(points[0].getX(), points[0].getY()); lineTo(points[1].getX(), points[1].getY()); lineTo(points[2].getX(), points[2].getY()); closePath(); } } } Using the legacy Graphics class, this would be done by using the legacy methoddrawPolygon(int[] x, int[] y, int pointCount).
The newer class Graphics2D supports a much nicer implementation using Path2Ds. You will need either the draw(Shape) or fill(Shape) method. Given the fact that almost everywhere in Java a Graphics object is actually a Graphics2D object, you can cast it so, that is the way to go, IMHO.
Graphics g = ...; Graphics2D g2d = (Graphics2D) g; Path2D.Double triangle = new Path2D.Double(); triangle.moveTo(x1, y1); triangle.pathTo(x2, y2); triangle.pathTo(x3, y3); triangle.closePath(); g2d.fill(triangle);