How to read data only once in Java Swing?

Question

I am new to Swing. I want to write a program which reads a file and draws something depending on the data read from the file.

I checked the tutorials and they call the drawing methods in paint(). So I added the code to read the file too in the paint() method. However, I noticed that the paint() method may be called multiple times. So the file would be read each time the paint() method is called.

I want to read the file only once. If I read the file in main() method how do I get access to the Graphics object? Or is there any other approach to solve this problem?

import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Test extends JPanel {

    public void paint(Graphics g) {
    // Code to read a file
    // Code to draw something depending on the data read from the file
    }


    public static void main(String[] args) {

        JFrame frame = new JFrame("Java 2D Skeleton");
        frame.add(new Test());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(280, 240);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

Answer 1

Read the file in main (or anywhere else in the normal control flow of your program). Put the data into an instance variable accessible to the JPanel.

In paint(), read the instance variable and draw from the data stored therein.

For example:

public class Foo extends JPanel {
    private SomeDataStorageClass myData;
    public static void main(String[] args) {
        Foo f = new Foo();
        f.myData = readSomeData();
    }
    public void paint(Graphics g) {
        doSomeDrawing(myData);
    }
}

Answer 2

I checked the tutorials and they call the drawing methods in paint().

That may be an old AWT tutorial. That is NOT the way painting is done in Swing. When using Swing you should be overriding the paintComponent(...) method. Read the section from the Swing tutorial on Custom Painting for an example and more information.

However, I noticed that the paint() method may be called multiple times

Swing will determine when a component needs to be repainted and invoke the painting methods.

I want to read the file only once

Painting should be as efficient as possible. So add the code to the constructor of your class to read the file.

Answer 3

Look into something called lazy instantiation or lazy loading.

http://en.wikipedia.org/wiki/Lazy_loading

The general idea is to have a field that is initially null, then instantiate it when it is needed (and initialize), then subsequent calls will not recreate it since it checks if it is created.

Example:

if (myFieldObject == null)
{
// instantiate it and load in your file
}

I should note that if you plan on overriding a component's painting, there is a specific method called paintComponent for Swing components.

I should also note that my lazy loading idea may or may not be the best approach but it is something to consider.