Displaying Graphics with Graphics2D

Supporting User Interaction

To allow the user to interact with the graphics you display, you need to be able to determine when the user clicks on one of them. The Graphics2D hit method provides a way for you to easily determine whether a mouse click occurred over a particular Shape. Alternatively you can get the location of the mouse click and call contains on the Shape to determine whether the click was within the bounds of the Shape.

If you are using primitive text, you can perform simple hit testing by getting the outline Shape that corresponds to the text and then calling hit or contains with that Shape. Supporting text editing requires much more sophisticated hit testing. If you want to allow the user to edit text, you should generally use one of the Swing editable text components. If you are working with primitive text and are using TextLayout to manage the shaping and positioning of the text, you can also use TextLayout to perform hit testing for text editing. For more information see the chapter Text and Fonts in the Java 2D Programmer's Guide

Example: ShapeMover

This applet allows the user to drag a Shape around within the applet window. The Shape is redrawn at every mouse location to provide feedback as the user drags it.

The contains method is called to determine whether the cursor is within the bounds of the rectangle when the mouse is pressed. If it is, the location of the rectangle is updated.

public void mousePressed(MouseEvent e){
    last_x = rect.x - e.getX();
    last_y = rect.y - e.getY();
    if(rect.contains(e.getX(), e.getY())) updateLocation(e);
...

public void updateLocation(MouseEvent e){
    rect.setLocation(last_x + e.getX(), last_y + e.getY());
    ...
    repaint();

You can see the complete code for this program in ShapeMover.java and ShapeMover.html includes the applet.

You might notice that redrawing the Shape at every mouse location is slow, because the filled rectangle is rerendered every time it is moved. Using double buffering can eliminate this problem. If you use Swing, the drawing will be double buffered automatically; you don't have to change the rendering code at all. The code for a Swing version of this program is SwingShapeMover.java and an HTML file that includes the applet is here: SwingShapeMover.html.

If you're not using Swing, the Example: BufferedShapeMover section in the next lesson shows how you can implement double buffering by using a BufferedImage. You render into the BufferedImage and then copy the image to the screen.

Displaying Graphics with Graphics2D