CODEWITHSUNDEEP
javanullpointerexceptionpaintcomponent
hackerboy
hackerboy

Reputation: 113

practicing paintComponent method

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);
        }

    }   
}

Upvotes: 1

Views: 614

Answers (4)

MadProgrammer
MadProgrammer

Reputation: 347244

First rule of painting in Swing, you don't control the paint process. Swing will paint when and if it wants. The best you can do is provide hints to the system that you would like something updated.

  • Never maintain a reference to a Graphics or to you didn't create yourself
  • Never call repaint directly or indirectly from inside any paint method, this will set up a recursive paint cycle which will eventually consume your CPU
  • It's pointless to call repaint on a component which isn't displayable on the screen

For example

public class paintClass extends JPanel
{

    private int interval,side,number;

    public paintClass() 
    {
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400, 400);
    }

    @Override
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);

        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);
        }

    }   
}

Upvotes: 1

copeg
copeg

Reputation: 8348

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/

Upvotes: 2

Titus
Titus

Reputation: 22474

You need to Override the paintComponent(Graphics g) not to create one. Also, if you want the paintComponent(...) to be executed, call repaint() on that component, do not call paintComponent(...) directly.

Upvotes: 0

flafoux
flafoux

Reputation: 2110

In your class "paintClass", you should instantiate the page var :

public paintClass() 
{
    page = new Graphics();
    paintComponent(page);
}

Upvotes: -2

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