practicing paintComponent method

Question

I want to practice paintComponent method. My program is consist of two classes(test and paintClass) in different files. I want to divide the area into 25 squares by drawing lines vertically and horizontally by using paintComponent method. My constructor only has paintComponent now. I know it is not very efficient but I will add buttons, labels and other things in the future so I need to use the program like this. When I run the program I get a nullpointerexception error.Can you help?

EDIT : I changed the page into g and added some lines into code. Still has the same error.

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

public class test
{

    public static void main(String[] args) 
    {

        JFrame frame = new JFrame("buttons");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        paintClass paint = new paintClass();//This line has error.
        paint.repaint();//ADDED
        frame.add(paint);
        frame.pack();
        frame.setVisible(true);
    }
}



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

public class paintClass extends JPanel
{
    private Graphics g;
    private int interval,side,number;

    public paintClass() 
    {
        this.repaint();//ADDED
        paintComponent(g);//This line has error.
    }

    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);//ADDED and has an error.
        this.repaint();//ADDED
        side = 250;
        number = 5;
        interval = side / number;
        g.drawRect(0,0, side, side);

        for(int i = 0; i <= number - 1; i++)
        {
            for(int j = 0; j <= number - 1; j++)
            {
                 g.drawLine(i * interval, 0, i * interval, side);
            }
            g.drawLine(0, i * interval, side, i * interval);
        }

    }   
}

Answer 1

You are calling paintComponent in the paintClass's constructor with a Graphics object that has not been instantiated. For simple drawing, you shouldn't need to call this method directly - it will be called by the EDT (with the appropriate Graphics object) when necessary (in other words, remove that line and don't hold onto a reference to the Graphics object).

//constructor of Class - note Classnames should start with uppercase
public paintClass(){
    //no need to call paintComponent or repaint here
}

You can request a repaint by calling the repaint() method defined by JComponent (the parent class of JPanel). You should also call the parent method

@Override
public void paintComponent(Graphics g){
    super.paintComponent(g);
    //do your custom drawing here, and never call repaint from this method
}

I would recommend, if you have not yet, is to study the Oracle tutorials about Custom painting: http://docs.oracle.com/javase/tutorial/uiswing/painting/