overridden paintComponent(Graphics g) or paint(Graphics g)

NewPanel.java

/**
 *
 * @author teguhteja
 */
import java.awt.*;
import javax.swing.*;

// Extend JPanel to override its paintComponent() method: 

class NewPanel extends JPanel {

  // Set background in constructor. 

  public NewPanel () 
  {
    this.setBackground (Color.cyan);
  }
  

  // Override paintComponent(): 

  public void paintComponent (Graphics g)
  {
    // Always call super.paintComponent (g): 
    super.paintComponent(g);

    // drawString() is a Graphics method. 
    // Draw the string "Hello World" at location 100,100 
    g.drawString ("Hello World!", 100, 100);

    // Let's find out when paintComponent() is called. 
    System.out.println ("Inside paintComponent");
  }

}

class TestSwing4 {

  public static void main (String[] argv)
  {
    // Create a frame 
    JFrame f = new JFrame ();

    // Set the title and other parameters. 
    f.setTitle ("Hello World Test");
    f.setResizable (true);
    // Background is going to be Panel's background. 
    // f.getContentPane().setBackground (Color.cyan); 
    f.setSize (500, 300);

    // Add the panel using the default BorderLayout 
    NewPanel panel = new NewPanel ();
    f.getContentPane().add (panel);

    // Show the frame. 
    f.setVisible (true);
  }

}

NewPanel1.java

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

// Extend JPanel to override paintComponent () 
class NewPanel1 extends JPanel {

  public NewPanel1 () 
  {
    this.setBackground (Color.cyan);
  }

  public void paintComponent (Graphics g)
  {
    super.paintComponent (g);
    
    // Draw a Square: 
    g.drawRect (50,50,50,50);
    g.drawString ("Square", 50, 115);

    // Circle: 
    g.drawOval (200,50,50,50);
    g.drawString ("Circle", 200, 115);

    // Rounded rectangle: 
    g.drawRoundRect (350,50,75,50,20,20);
    g.drawString ("Rectangle", 350, 115);

    // Draw a line across the middle: 
    g.drawLine (0,150,500,150);

    // Now draw some filled shapes: 

    // Square: 
    g.fillRect (50,200,50,50);
    g.drawString ("Square", 50, 265);

    // Circle: 
    g.fillOval (200,200,50,50);
    g.drawString ("Circle", 200, 265);

    // Rounded rectangle: 
    g.fillRoundRect (350,200,75,50,20,20);
    g.drawString ("Rectangle", 350, 265);
  }

}


// Create a new Frame class. 

class NewFrame extends JFrame {

  public NewFrame ()
  {
    // Set the title and other parameters. 
    this.setTitle ("Some Geometric Figures");
    this.setResizable (true);
    this.setSize (500, 300);

    // Create and add the panel. 
    NewPanel1 panel = new NewPanel1 ();
    this.getContentPane().add (panel);

    // Show the frame. 
    this.setVisible (true);
  }
  
}

class TestSwing6 {

  public static void main (String[] argv)
  {
    // Simply fire up the whole application. 
    NewFrame f = new NewFrame ();
  }

}

OvalPaint.java

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

public class OvalPaint extends JPanel {

  public void paintComponent(Graphics g) {
    g.setColor(Color.orange);
    g.fillRect(0, 0, getWidth(), getHeight());
    g.setColor(Color.red);
    g.fillOval(getWidth()/4, getHeight()/4, 
		getWidth()/2, getHeight()/2);
  }

  public static void main(String args[]) {
    JFrame frame = new JFrame("OvalPaint");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    OvalPaint panel = new OvalPaint();
    
    frame.add(panel);

    frame.setSize(300, 200);
    frame.setVisible(true);
  }
}

SmileyApp.java

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

public class SmileyApp extends JPanel {
   @Override
   public void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(Color.YELLOW);
      g.fillOval(10, 10, 200, 200);

      // draw Eyes
      g.setColor(Color.BLACK);
      g.fillOval(55, 65, 30, 30);
      g.fillOval(135, 65, 30, 30);

      // draw Mouth
      g.fillOval(50, 110, 120, 60);

      // adding smile
      g.setColor(Color.YELLOW);
      g.fillRect(50, 110, 120, 30);
      g.fillOval(50, 120, 120, 40);
   }
   public static void main(String[] args) {
      SmileyApp smiley = new SmileyApp();
      JFrame app = new JFrame("Smiley App");
      app.add(smiley, BorderLayout.CENTER);
      app.setSize(300, 300);
      app.setLocationRelativeTo(null);
      app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      app.setVisible(true);
   }
}