My recommendation is to learn to use a debugger. Step through the code, see what's happening as opposed to what you think is happening.
Exception in thread "main" java.lang.NullPointerException
C:\Users\zulfi\AppData\Local\NetBeans\Cache\8.1\executor-snippets\run.xml:53: Java returned: 1
BUILD FAILED (total time: 7 seconds)
bad operand types for binary operator '-'
first type: String
second type: int