When a program violates the constraints of Java programing language , Java virtual machine(JVM) throws an error signal to the program. These signals are known as Exception in Java programing language. Excpetions are of two types. Checked (Compile-time) exception and un-checked(run-time) exception. JVM has a collection of exceptions that will be thrown at compile time with the exception message and the print stack trace. A developer can create their own exception which is closely related to the application he or she is developing. A Custom exception can be used to form a better customized message to the user of your application. Have a look.