26.4. Formatting Numbers

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24.2. Thread Concepts

A thread is the flow of execution, from beginning to end, of a task in a program. A task is a program unit that is executed independently of other parts of the program. A thread provides the mechanism for running a task. With Java, you can launch multiple threads from a program concurrently. These threads can be executed simultaneously in multiprocessor systems, as shown in Figure 24.1(a).

Figure 24.1. (a) Here multiple threads are running on multiple CPUs. (b) Here multiple threads share a single CPU.

In single-processor systems, as shown in Figure 24.1(b), the multiple threads share CPU time, and the operating system is responsible for scheduling and allocating resources to them. This arrangement is practical because most of the time the CPU is idle. It does nothing, for example, while waiting for the user to enter data.

Multithreading can make your program more responsive and interactive, as well as enhance performance. For example, a good word processor lets you print or save a file while you are typing. In some cases, multithreaded programs run faster than single-threaded programs even on single-processor systems. Java provides exceptionally good support for creating and running threads and for locking resources to prevent conflicts.

When your program executes as an application, the Java interpreter starts a thread for the main method. When your program executes as an applet, the Web browser starts a thread to run the applet. You can create additional threads to run concurrent tasks in the program. In Java, each task is an instance of the Runnable interface, also called a runnable object . A thread is essentially an object that facilitates the execution of a task.