Serializing Canvas objects in Java

Question

I am trying to serialize a Canvas instance (using the Serializable interface) on which I have painted some pixels like the following code suggests:

Canvas board = new Canvas();
Graphics g = board.getGraphics();
g.setColor(Color.BLACK);
g.drawString("txt", 10, 20);
g.dispose();

Then when I serialize board it doesn't save the pixels I've painted. I'm guessing this is because by using getGraphics() I am not saving those pixels to any instance, so I thought that perhaps I should instead paint from within the Canvas 's paint() method. Would serializing the Canvas then save the modified pixels too? If not, what are my options to save/serialize a Canvas with the pixels I've modified? I am guessing I would have to serialize the Graphics object instead of the Canvas ? I am new to Java graphics, any help will be greatly appreciated.

To be more clear, what I'm trying to do is have the pixels I've put on a Canvas instance saved to a file using serialization. Then later I need to reload this Canvas instance from the serialized file I saved earlier, so that when I use it on the screen I see the exact same pixels I modified just before I serialized the Canvas . I know how to serialize objects and all that. I am just unclear where all the pixels are stored.

Update1:

The way the user draws something on the screen is by left-clicking on the Canvas area. Then MouseListener calls the following method, passing along the Point object specifying the mouse xy:

private void drawAt(Point p)
{
    Graphics g = board.getGraphics();
    g.setColor(brushColor);
    g.setFont(brushFont);
    g.drawString(brushText, p.x, p.y);
    g.dispose();
}

Answer 1

Don't serialize Canvas or any other GUI components as you'd be serializing the "View" portion of your program, a risky thing to do (high risk for serialization exceptions due to attempting to serialize and unserialize unserializable sub components) and an inefficient thing to do -- serializing large amounts of data which are built automatically by the JVM and thus don't need serialization.

Instead you will want to serialize the "Model" portion of your data, that portion which holds the logical data of your program. So if your GUI is drawn using data held by an ArrayList, or a collection of ArrayLists, or whatever data it takes, then serialize that data. Then be sure to create your GUI so that can be built using the serialized data.

Or if you need to store an image, then store an image, probably best as a loss-less png file.

Also, I suggest that you draw into a BufferedImage, that you then display that BufferedImage within the paintComponent method override of a JPanel, and that you then save and restore the image. For more on how to draw and save, please have a look at these links. The first two contain my code, the third is that of MadProgrammer's:

• Save image from JPanel after draw ,
• Convert a Graphics2D to an Image or BufferedImage and
• Paint BufferedImage on JFrame and write to File .