Question

What is the difference between wait() and wait(timeout).Anyway wait() needs to wait for a notify call but why we have wait(timeout) ?

So what is the difference between sleep(timeout) and wait(timeout) ?

Answer 1

wait(timeout) will return if the thread is still waiting after the timeout has elapsed. This is for hang notification, for low-power polling, etc etc. Sleep(timeout) won’t wake up until the timeout has elapsed; wait(timeout) is either the notify() call or the timeout, whichever comes first.

Quoting from the JavaDoc:

This method causes the current thread (call it T) to place itself in
the wait set for this object and then to relinquish any and all
synchronization claims on this object. Thread T becomes disabled for
thread scheduling purposes and lies dormant until one of four things
happens:

• Some other thread invokes the notify method for this object and thread T happens to be arbitrarily chosen as the thread to be
  awakened.
• Some other thread invokes the notifyAll method for this object.
• Some other thread interrupts thread T.
• The specified amount of real time has elapsed, more or less. If timeout is zero, however, then real time is not taken into
  consideration and the thread simply waits until notified.