Threads seem to have been an important part of the SCJP exam questions in past versions of that accreditation exam. Why are threads considered important topics?
A thread in the Information Technology world is the term used to define a process in a program. A thread does not exist by itself rather, it is interconnected with other threads so that when the program needs sequential processes to be run, each thread acts according to what it was designed to do by the software programmer. In the case of SCJP (or Sun Certified Java Programmer) threads, the software programmer will be using Java language to code threads.
A thread usually needs just milliseconds of time to wait its turn to run in a program. If the program is designed for multi-threading, that means it can implement running of different threads at the same time. You may find in your studies that majority of the operating systems being used nowadays rely on multi-threading for system applications. If your operating system does support multi-threading, this means essentially that your CPU-hungry program will have to share CPU space with other programs in the same multi-threading application.
One important aspect of SCJP questions on threads that may come up would be thread safety. This pertains to setting the safety requirements and expected behavior of that thread in real-time use. You being the programmer of a Java language thread – are required to create these thread safety standards and delineate its behavior because you are the one creating code for the thread.
This is why all SCJP exams have a section devoted to threads, because to neglect threads would be like neglecting the English alphabet when trying to teach the English language to a child. Threads can be said to be part of the building blocks of SCJP accreditation exams and the Java language itself.