Threads and timing

Threads: a brief introduction

• A thread is a single sequential flow of control within a program. Every Java program has at least one thread running, but you can create your own threads by instantiating the Thread class.
• Multiple threads are called for in a program in which two separate processes should be running simultaneously. In this situation, you can assign a new thread to one of the processes when it starts.
• After a thread is created with one of the Thread constructors, it is still not actually running. It starts running when its start method is called. This method in turn calls the thread's run method.
• The simplest way to incorporate threads into a program is to create a class that is a subclass of the Thread class. In this case, the run method that is called by start is defined in this class; this definition overrides run in Thread.
• A thread dies when its run method exits.
• A thread may be put to sleep with the static Thread method sleep(long), where the argument specifies the number of milliseconds to sleep (long is an integer type allowing numbers with greater absolute values than int). Note that this method throws an InterruptedException, which has to be caught in the method definition.
• Here is a simple demo using threads from the JavaTutorial.

Using timers

• In order to control the timing of events, for example, for animation, you may be able to avoid doing complex thread programming and use the built-in Timer class instead (the one discussed here is the one in the Swing package: javax.swing.Timer).
• A timer has a delay and one or more action listeners listening to it. Once the timer has been started, it sends an action event to its listeners every delay milliseconds. The action listeners respond to the events according to what is in their actionPerformed method.
• The constructor for Timer is of the form Time(int delay, ActionListener listener). delay represents the delay in milliseconds, and listener is an action listener that is automatically registered as listening to the timer.
• The main Timer methods are start() and stop(), with the obvious meanings.
• Here is a simple demo using the Timer.

© 2004. Indiana University and Michael Gasser. www.indiana.edu/~gasser/Q530/Notes/threads.html 6 Dec 2004