Java Reference
In-Depth Information
Let us first examine the key listener. The getKeyCode method returns an integer that is
associated with the pressed key. We could not use the getKeyChar method because there are
no characters that are associated with the arrow keys (and even if there were, we do not know
how to refer to them). The KeyEvent class stores the code of most virtual keys as constants.
For example, KeyEvent.VK LEFT is the left arrow. The code changes the coordinates of the
paddle by calling the moveLeft or moveRight method and then it refreshes the panel.
Note that we could have also rewritten the code as follows.
String s= KeyEvent . getKeyText ( e . getKeyCode () ) ;
if (s.equals( "Left" )) {
The getKeyText method converts the key code to text. However, this is an inferior solution
because it assumes that the string Left will be returned. For example, nothing stops Java
from returning the string LEFT in future implementations.
Note that the panel needs to be made focusable in order for the key listener to work.
Only components that have the focus can receive key events and by default the panel is not
focusable. The call setFocusable(true) makes the panel focusable.
The mouse listener is a little more interesting than the key listener. It saves the old
position of the mouse cursor and it compares it with the new position of the mouse cursor.
The paddle is moved by the value of the difference of the two numbers. This means that if
we move the mouse fast, then the paddle will be moved fast as well. Of course, we do not
know the old value of the cursor the first time we move the mouse. Therefore, we need an
extra variable and an if statement to address this special case. Finally, note that we can
use the mouse to move the paddle only when the cursor is inside the panel. The reason is
that mouse events are reported to a component only while the mouse cursor is over that
If we run the program that we have so far, we will note that everything works except
that the ball goes right though the paddle. The reason is that we have not added code
that bounces the ball off the paddle. We will add this code in the time listener inside the
BreakoutPanel class. Our code will create a virtual ball that predicts were the ball will go
next without actually moving the ball. We will then check to see if the virtual ball intersects
the paddle. The reason a virtual ball is created is because we want to change the trajectory
of the ball before it overlaps with the paddle. If the virtual ball intersects the paddle, then
we will check if it intersects the left or right part of the paddle. If it intersects the left part
of the paddle, then the ball will bounce to the left. Alternatively, if it intersects the right
part of the paddle, then the ball will bounce to the right. Here is the new version of the
Ball class that supports the extended interface.
public class Ball extends BreakoutShape
private static final int SIZE = 10;
private static final int START X = 200;
private static final int START Y = 400;
private BreakoutPanel panel ;
private int dx = 1;
private int dy = 1;
public Ball (Color color ,BreakoutPanel panel) {
super ( new Ellipse2D.Double(Ball .STARTX, B a l l . START Y, Ball .SIZE ,
Ball .SIZE) , color , true );
this . panel = panel ;
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