Bdtjtk

import java.util.*;

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

import java.awt.geom.*;









class game extends JFrame {

    public game(){   //this is constructor

        JFrame frame  = new JFrame();

        frame.setVisible(true);

        frame.setSize(500,500);

        frame.setTitle("Hello world");

    }



    public void paint(Graphics g){

        Graphics2D g2 = (Graphics2D) g;

        Line2D line = new Line2D.Double(60,90,150,100);

        g2.draw(line);





    }





    public static void main(String args) {



        game l = new game();



    }





}

The above code is compiling in java but on running the code it only displays the frame and its title, but does not include any of the lines being drawn using the Graphics2D and Line2D, what is the mistake that is being made??? The frame being displayed does not show any content, why is that???

First, you are creating and displaying a JFrame which is not an instance of game, so there is no chance that it paints what you have in the paint method of game .

You usually don't want to create a subclass of JFrame for custom painting anyway, just create a subclass of JPanel, and set it as the content pane of the frame.

Also don't override paint, but paintComponent, which is the method responsible for painting the current component.

You should also call the parent method of paintComponent, to make sure that all the usual cleaning takes place correctly.

Also by convention, class names should start with an upper case letter.

One last thing, make the frame visible only once you have added all your components, or you may encounter visual glitches some day.

Putting it all together :

import java.awt.Color;

import java.awt.Graphics;

import java.awt.Graphics2D;

import java.awt.geom.Line2D;



import javax.swing.JFrame;

import javax.swing.JPanel;



class Game extends JPanel {



    @Override

    public void paintComponent(final Graphics g) {



        super.paintComponent(g);

        Graphics2D g2 = (Graphics2D) g;

        g2.setColor(Color.BLACK);

        Line2D line = new Line2D.Double(60, 90, 150, 100);

        g2.draw(line);



    }



    public static void main(final String args) {



        Game l = new Game();



        JFrame frame = new JFrame();



        frame.setSize(500, 500);

        frame.setTitle("Hello world");



        frame.setContentPane(l);



        frame.setVisible(true);



    }



}

In your constructor call method of JFrame class using this keyword because you extends JFrame class in your class.

public game(){   //this is constructor

/*JFrame frame  = new JFrame();

frame.setVisible(true);

frame.setSize(500,500);

frame.setTitle("Hello world");*/

    this.setVisible(true);

    this.setSize(500,500);

    this.setTitle("Hello world");

}

This solve your problem.

you dont need to create instance of JFrame class ,
modified you constructor as shown below

public game(){   //this is constructor

   setVisible(true);

   setSize(500,500);

   setTitle("Hello world");

}