at compile time gives error.but i think that Object is a super class he takes all sub class object .
method set in class Manager cannot be applied to given types;
found: String reason: actual argument String cannot be converted to Object by method invocation conve
The most likely explanation is you actually have your own Object class hanging around somewhere in the same package (either as source code or a compiled code). If you can't find it, try changing all the references to java.lang.Object and see if that changes the result.
subject: why this is not working(subclass to superclass).