Can any one help me writing code which can only creat three digit random number not two digit or one digit number? Following code creates 3 digti but also creats two and one digit number..

public class random
{
public static void main (String [] args)
{
double n = Math.random();
long n3 = Math.round(Math.random()*1000);
System.out.println (n3);
}
}

long n3 = Math.round(Math.random()*(999 - 100) + 100);

You'll need to define a range. One way of doing that is by this:

Math.random() * (Max - Min) + Min

where Max would be the maximum three digit number, and Min the lowest three digit number, hence Max = 999 and Min = 100.

Edit:
A quick verification:

public class random
{
public static void main (String [] args)
{
for (int i=0;i<99999999;i++){
long n3 = Math.round(Math.random()*(999-100)+100);
if (n3 < 100 || n3 >=1000)
System.out.println (n3);
}
}
}

If you want integer values from Random you should use nextInt rather than rounding a random float - see the API doc for Random to appreciate the extra stuff needed to get good random ints.

You're adding the min at the end, because the numbers generated are in range 0 to 899. So, if generated the number 0, adding 100 will give us the lowest three digit number, and if generating 899, adding 100 will give us the 999, meaning the maximum three digit number.

public class Main {
public static void main(String[] args) {
long timeCheck;
long periodOfTime;
ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(2);
long initialDelay = 0; // ...