Swing Hacks

Add a Watermark to a Text Component #5

Chapter 1, Basic JComponents

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HACK

H A C K

#5

Add a Watermark to a Text Component

Hack #5

This hack will show how to create a custom image background for the

JTextField, a complex Swing component that does not already support

backgrounds or icons by default.

One of Swing’s most underused features is the ability to partially override

drawing code. Most programs enhance widgets by using renderers or com-

pletely overriding the paint code. By only partially overriding the drawing,

however, you can create some very interesting effects that blend both new

and existing drawing commands.

Some components, like

JList and JTable, use renderers to customize their

look. To put a background in a

JTextField, however, requires more. The

plan is to subclass

JTextField, prepare the resources for drawing a back-

ground (loading the image, etc.), and then draw a new background while

preserving the normal

JTextField drawing code for the text and cursor.

The actual drawing will be done with a

TexturePaint. Java2D allows you to

fill any area with instances of the

Paint interface. Typically you use a color,

which is an implementation of

Paint, but it is possible to use something else,

such as a texture or gradient. This class will use a

TexturePaint to tile an

image across the component’s background.

The first step is to create a

JTextField subclass (shown in Example 1-10).

Example 1-10 creates a class called

WatermarkTextField. It is a subclass of

JTextField with a custom constructor that accepts a File object containing

an image. It also defines two member variables:

img and texture. After the

usual call to

super( ), the constructor reads the file into the BufferedImage

variable, img. If the file isn’t a valid image—or can’t be read for some other

reason—the method will throw an exception (hence the

throws IOException

clause on the constructor definition).

Example 1-10. Preparing a field for watermarking

public class WatermarkTextField extends JTextField {

BufferedImage img;

TexturePaint texture;

public WatermarkTextField(File file) throws IOException {

super( );

img = ImageIO.read(file);

Rectangle rect = new Rectangle(0,0,

img.getWidth(null),img.getHeight(null));

texture = new TexturePaint(img, rect);

setOpaque(false);

}

}

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Chapter 1, Basic JComponents

#5 Add a Watermark to a Text Component

HACK

After the image is loaded successfully, the constructor creates a TexturePaint.

TexturePaints must be created with a source image and a rectangle. The

rectangle defines the portion of the source to be tiled. In this case, you want

the entire image to be used, so the rectangle is the same size as the image.

If you wanted to use just a portion of the image, you could

make the rectangle smaller. This would also give you the

ability to store all of your textures in a single large image,

which could save loading time and memory.

The last thing the WatermarkTextField constructor does before returning is

call

setOpaque(false). As you have seen earlier in this chapter (and will see

again), the

setOpaque( ) method is one of the core tools for hacking Swing.

In this case, it is used to turn off the default background of the

TextField,

allowing you to substitute your own.

With the subclass created, you can add a method to do the actual drawing:

public void paintComponent(Graphics g) {

Graphics2D g2 = (Graphics2D)g;

g2.setPaint(texture);

g.fillRect(0,0,getWidth(),getHeight( ));

super.paintComponent(g);

}

WatermarkTextField overrides the parent class’s paintComponent( ) method with

its own version. The actual drawing is pretty simple: cast to a

Graphics2D

object (which understands how to work with Paint classes), then fill in the

background with the texture paint and call

super( ).

Earlier, I said that you will override the parent class partially rather than

completely. This is because the code still calls the parent class’s

paintComponent( ) method, but it does it after painting the new background.

Because the

opaque property is set to false, the parent class will not draw its

own background, allowing your custom one to show through. The compo-

nent will draw the text, selections, and cursors as normal on top of the cus-

tom background.

With the class ready, it’s time to pull together an example—Example 1-11.

Example 1-11. Trying out the watermarked text field

public static void main(String[] args) throws Exception {

JFrame frame = new JFrame("Watermark JTextField Hack");

JTextField textfield = new WatermarkTextField(new File("red.png"));

textfield.setText("A Text Field");