Threads and Concurrent Programming
Threads may be seen as methods that execute at "the same time" as other methods. Normally, we think sequentially when writing a computer program. From this perspective, only one thing executes at a time. However, with today's multi-core processors, it is possible to literally have several things going on at the very same time while sharing the same memory. There are lots of ways that this is done in the real world, and this chapter goes over them in a way that you can apply to your own projects.
14.7 CASE STUDY: The Game of Pong
The KeyListener Interface
The implementation of the PongFrame class is shown in figure 14.33. The
frame’s main task is to manage the drawing of the ball and paddle and
to handle the user’s key presses. Handling keyboard events is a simple
matter of implementing the KeyListener interface. This works in much
the same way as the ActionListener interface, which is used to handle
button clicks and other ActionEvents. Whenever a key is pressed, it
generates KeyEvents, which are passed to the appropriate methods of
the KeyListener interface.
There’s a bit of redundancy in the KeyListener interface in the sense
that a single key press and release generates three KeyEvents: A keytyped event, when the key is pressed, a key-released event, when the key
is released, and a key-pressed event, when the key is pressed and released.
While it is important for some programs to be able to distinguish between a key-typed and key-released event, for this program, we will take
action whenever one of the arrow keys is pressed (typed and released).
Therefore, we implement the keyPressed() method as follows:
Each key on the keyboard has a unique code that identifies the key.
The key’s code is gotten from the KeyEvent object by means of the
getKeyCode() method. Then it is compared with the codes for the uparrow and down-arrow keys, which are implemented as class constants,
VK UP and VK DOWN, in the KeyEvent class. If either of those keys were
typed, the appropriate paddle method, moveUP() or moveDown(), is
Note that even though we are not using the keyPressed() and
keyReleased() methods in this program, it is still necessary to provide
implementations for these methods in the frame. In order to implement
an interface, such as the KeyListener interface, you must implement
all the abstract methods in the interface. That is why we provide trivial implementations of both the keyPressed() and keyReleased()