i was restricted not to use Thread concept
This is a rather vague restriction. Does this mean you can't use threads? If so, you can't start a JVM (which uses threads), and therefore, the problem is outside the scope of Java.
Does it mean you can't use the java.lang.Thread API? If so, perhaps your teacher is hinting at the use of Object wait/notify/notifyAll.
It is important to note that you can
NEVER have a thread pause for a specific period of time - you can however, have a thread pause for at least a specific period of time. When that thread comes out of 'sleep' state it is put into 'ready for execution' state - not 'running' state. It is up to the indeterminate thread scheduler to decide when the thread is shifted to 'running' state.