Your code is almost correct. Try
class SequentialNumber
{
private static int currentNumber = 3000;
public static String GetNextNumber()
{
currentNumber++;
return "B"+currentNumber;
}
}
public class program
{
public static void main(String args[])
{
for (int n=0; n<20; ++n)
{
String sn = SequentialNumber.GetNextNumber();
System.out.printf("sequential_number[%d] = %s\n", n, sn);
}
}
}