you manage multithreading in multiple ways: using "synchronized" on methods and objects or by synchronizing on static classes for static variables; using more in deep "wait" and "notify"/"notifyAll" to manage also a threads wait set; using semaphores etc.. Multithreading is usefull for using the IDLE time of a CPU core (time for waiting some signal, like I/O), to fasten processing, to avoid collisions on a critical piece of code where a race condition occurs.