CODEWITHSUNDEEP
javaswingawt
0x6B6F77616C74
0x6B6F77616C74

Reputation: 2629

Draw a line mixing swing and awt

I've tried to draw a line using Graphics class and then put it inside JFrame object:

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

class Window
{
    private JFrame frame;
    private Graphics g;

    public void Window()
    {
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocation(15, 0);
        frame.setSize(600, 200);
        frame.setVisible(true);

    }        

    public void DrawCross()
    { 

        g.setColor(Color.BLACK);
        g.drawLine(300,0, 200, 0);
        frame.paint(g);  
    }        

}

...

run:
Exception in thread "main" java.lang.NullPointerException
    at Window.DrawCross(Window.java:33)
    at Main.main(main.java:21)
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

Where am I wrong and how to fix my mistake? It's impossible to initialise an object of Graphics class that is abstract.

Upvotes: 0

Views: 153

Answers (3)

0x6B6F77616C74
0x6B6F77616C74

Reputation: 2629

Working code, created basing on JoopEggen and Reimeus answers:

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

class Window
{
    private JFrame frame;

    public Window()
    {
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocation(15, 0);
        frame.setSize(600, 200);
        frame.setVisible(true);
    }

    public void drawCoordinateSystem()
    {                
        frame.add( new CoordinateSystem());
    }        

} 



 public class CoordinateSystem extends JPanel 
{
    @Override 
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Dimension size = this.getSize();

        g.setColor(Color.BLACK);
        g.drawLine(0,size.height/2,size.width, size.height/2);

        g.drawLine(size.width/2, 0, size.width/2, size.height);

    }
}        


class Main 
{    
    public static void main(String [] args)
    {
     Window h = new Window();

     h.drawCoordinateSystem();      
    }        

}

Upvotes: 0

Reimeus
Reimeus

Reputation: 159754

The Graphics object has not been instantiated resulting in the NPE being thrown.

This approach is just plain wrong. Don't call paint directly. Also don't do any custom painting on a JFrame, instead add a sub-class of JComponent and override paintComponent. Here you will have a readily instantiated Graphics object.

Painting in AWT and Swing

Upvotes: 2

Joop Eggen
Joop Eggen

Reputation: 109547

The main error is that it should be public Window() without void. That is the constructor notation in java. As you called new Window() you did not call the public void function Window, hence frame remained null. Hence the NullPointerException.

Upvotes: 2

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