CS 3230 - Lab 7

Events Lab

In this week's exciting episode, you'll handle a plethora of different events.

I'm not babying you guys with this lab as much as I did with the previous one. I expect to see a little head scratching this time.

Specifications

Screenshot

Here's what the finished product will look like:

You will need to make two classes, Lab7, derived from JFrame and DrawingPanel derived from JPanel. Put both classes in a file called Lab7.java.

Lab7 class

The Lab7 class will extend JFrame and implement the ActionListener interface.

Please see the Lab6 writeup for details on what you should do in the main method to instantiate the frame, and what you should do in the constructor to arrange the components in the frame.

Handling the button click events

Clicking on the "Clear" button should clear the drawing panel (see below). Clicking on the "Quit" button should close the app. (Hint: call System.exit(0))

To make the buttons work, do this:

1. After you create new buttons (JButtons), call setActionCommand() with a unique command string. (Hint: just use the same string you used as the text for the button.)
2. Next, call addActionListener(this) to tell the button that the frame will listen to its events.
3. Last, override the actionPerformed() method that you inherit from the ActionListener interface. Get the action command from the ActionEvent and use an if / else-if / else construct to test which button was clicked.

You might want to look at the UsesActionCommands.java example.

DrawingPanel class

The DrawingPanel class will extend JPanel and implement the MouseListener, MouseMotionListener, and KeyListener interfaces.

Variables

You will need to declare two variables:

• A Point variable (not Point2D) that will keep track of the last point tracked by mouse movements. (call it lastPoint)
• An ArrayList that will keep track of a list of lines (Line2D objects, actually).

Constructor

• Call addMouseListener(this) to tell the panel to listen for its own mouse events.
• Call addKeyListener(this) to tell the panel to listen for its own keyboard events.
• Call setFocusable(true) so the panel will be able to listen for key events.
• Do not call addMouseMotionListener() in the constructor, we'll do that later.
• Anything else you like, i.e. setting the background color to white.

paintComponent() method

• Cast the Graphics object you receive to a Graphics2D class, just like we did in Lab6.
• Next, iterate through ArrayList, drawing each Line2D object in there. You might want to look at MouseTest.java or Sketch.java for some examples of how to do this.

Note that you are actually handling paint events by overriding the paintComponent() method.

Overriding MouseListener methods

• mousePressed: Save the point where the mouse was clicked by calling the getPoint() method of the MouseEvent object you are passed and assigning the return value to your lastPoint variable. Then call addMouseMotionListener(this) to start listening for drag events.
• mouseReleased: Call removeMouseMitionListener(this) to stop listening for drag events. Next, if you have a value in your lastPoint variable (i.e. lastPoint != null), add a line (actually, a Line2D.Double) to the ArrayList and call the panel's repaint() method to send a paint event. (Calling repaint() effectively makes the paintComponent() method get called.)

You could get by with just the above overrides, but you may want to override the following methods to make your app work a little more effectively.

• mouseExited: Call removeMouseMitionListener(this) to stop listening for drag events and assign lastPoint to null.
• mouseClicked: Add a new line to the ArrayList using the point from the MouseEvent object for both the start and end points (this will effectively draw a dot.) Then, call repaint(). (You may need to offset one of the points by one to get the dot to display.)

Even if you're not interested in the other methods in the MouseListener interface (such as mouseEntered()), you still need to override the methods with empty bodies or the compiler will yell at you.

Overriding MouseMotionListener methods

• mouseDragged: Add a new line to the ArrayList using the lastPoint variable as your starting point and MouseEvent.getPoint() as the end point. Then call repaint().

As before, all methods inherited from the interface must be overridden, even if you don't need to handle the event; just give them empty bodies.

Overriding KeyListener methods

• keyPressed: Get the key code from the KeyEvent object you are passed. If it's a "Q", exit, if it's the ESC key, clear the canvas (explained in the next section).

Note: If you forget to call setFocusable(true) in the Panel's constructor, it will not listen to key events. (Only the component with the focus will receive key events.)

All other methods from the interface must be overridden with empty bodies. You will probably want to look at Sketch.java for an example of how to handle key events.

Convenience methods

You can get by without these, but you will probably find them useful and they will help eliminate duplication of code.

• addLine: This method should take two Point objects (not Point2D) and make a new Line2D.Double which will be added to the ArrayList. Tip: You will probably want to call repaint() in this method. It will eliminate redundant code and make sure that you always repaint when a new line gets drawn. Additionally, you might want to update lastPoint in here.
• clear: This method will simply call the clear() method of the ArrayList and call repaint. You should call this in response to the "Clear" button being pressed and the ESC key being pressed.

Questions and Answers

What packages do I need to import?

These:

import javax.swing.*;
import java.awt.*;
import java.awt.geom.*;
import java.awt.event.*;
import java.util.ArrayList;

Why do I have to hit TAB before ESC to clear the DrawingPanel?

The alert student will note that after you've clicked on the 'Clear' button, then drawn on the panel, then tried to clear it again by hitting ESC, it will not clear. This is because the 'Clear' button has the focus and is the only control listening for kestrokes.

To cure this problem, put the line drawingPanel.grabFocus() after you call drawingPanel.clear() in the event handler for your clear button.

How can I use this stuff in my Application Project?

So glad you asked. You can continue making headway today on your application project by handling various events generated by the mouse, the keyboard, and some of the controls in your frame.

Those of you that are making a drawing program for your application project should find this lab useful.