I found some strange exception:
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
How it can be possible
No. Every object can be casted to an java.lang.Object, not a String. If you want a string representation of whatever object, you have to invoke the toString() method; this is not the same as casting the object to a String.
java.lang.Object
String
toString()