What are threads In Java?

A program or process can contain multiple threads that execute instructions according to program code. Like multiple processes that can run on one computer, multiple threads appear to be doing their work in parallel. Implemented on a multi-processor machine, they actually can work in parallel. Unlike processes, threads share the same address space; that is, they can read and write the same variables and data structures.

When you're writing multithreaded programs, you must take great care that no one thread disturbs the work of any other thread. You can liken this approach to an office where the workers function independently and in parallel except when they need to use shared office resources or communicate with one another. One worker can speak to another worker only if the other worker is "listening" and they both speak the same language. Additionally, a worker can't use a copy machine until it is free and in a useable state (no half-completed copy jobs, paper jams, and so on). As we work through this article, you'll see how you can get threads to coordinate and cooperate in a Java program much like workers in a well-behaved organization.