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Study guide: Achieve strong performance with threads, Part 1

Brush up on Java terms, learn tips and cautions, review homework assignments, and read Jeff's answers to student questions

By Jeff Friesen, JavaWorld.com, 05/03/02

Glossary of terms

animation

The act of repeatedly drawing on one surface images that slightly differ from each other to achieve a movement illusion.

concurrent

Simultaneous.

current thread

The thread that associates with the Thread (or Thread subclass) object whose reference Thread.currentThread() returns.

daemon thread

A thread that performs housekeeping (such as garbage collection) and other background tasks that probably do not contribute to the application's main work but are necessary for the application to continue its main work.

multithreading

The act of multiple threads executing byte code instruction sequences in the same program.

runnable

An object created from a class that implements the Runnable interface.

thread

An independent path of execution through program code.

user thread

A thread that performs important work for the program's user that must finish before the application terminates.

Tips and cautions

These tips and cautions will help you write better programs and save you from agonizing over why the compiler produces error messages.

Tips

• In several places, "Achieve Strong Performance with Threads, Part 1" refers to the concept of a current thread. If you need access to a Thread object that describes the current thread, call Thread's static currentThread() method. Example: Thread current = Thread.currentThread ();.
• When you face a situation where a class can either extend Thread or implement Runnable, which approach do you choose? If the class already extends another class, you must implement Runnable. However, if that class extends no other class, think about the class name. That name will suggest that the class's objects are either active or passive. For example, the name Ticker suggests that its objects are active—they tick. Thus, the Ticker class would extend Thread, and Ticker objects would be specialized Thread objects. In contrast, Rectangle suggests passive objects—Rectangle objects do nothing on their own. Thus, the Rectangle class would implement Runnable, and Rectangle objects would use Thread objects (for testing or other purposes) instead of being specialized Thread objects.

Cautions

• Sun Microsystems has deprecated a variety of Thread methods, such as suspend() and resume(), because they can lock up your programs or damage objects. As a result, you should not call them in your code. Consult the SDK documentation for workarounds to those methods.
• After a thread calls start(), subsequent calls to that method before the run() method exits cause start() to throw a java.lang.IllegalThreadStateException object.
• Do not attempt to join the current thread to itself because the current thread will wait forever.
• Do not depend on activeCount()'s return value when iterating over an array. If you do, your program runs the risk of throwing NullPointerException objects. Why? Between the calls to activeCount() and enumerate(Thread [] thdarray), one or more threads might possibly terminate. As a result, enumerate(Thread [] thdarray) would copy fewer thread references into its array. Therefore, think of activeCount()'s return value as a maximum value for array-sizing purposes only. Also, think of enumerate(Thread [] thdarray)'s return value as representing the number of active threads at the time of a program's call to that method.
• The setDaemon(boolean isDaemon) method throws an IllegalThreadStateException object if a call is made to that method after the thread starts execution.

Reader questions

Find out what questions your fellow readers are asking and my answers to those questions.