I don't know why everyone is so tense in here :D I am planning to write a series of posts about concurrency (not because I am an expert but I want to improve myself by this way), including the executor framework, completable futures, locks and synchronisation. Obviously it is not possible to go into details of all in a single post. For one who knows operating system level details, and new to java I thing this is where you start, simply with Thread API. Even if you are using high-level abstraction this is these are basics to know. It is quite popular in the job interviews. And also it is quite common for people uses the Executor framework daily but does not know how to properly stop a thread, or non-daemon threads etc.