This is similar to feature request 1866162: http://sourceforge.net/tracker/index.php?func=detail&aid=1866162&group_id=44253&atid=438938
The Dr Java interactions window allows one to omit the explicit downcast required when assigning a subtype variable to reference a supertype object; e.g.
CharSequence cs = "abc";
String s = cs; // should be String s = (String) cs;
Since the omission of this cast is an error when compiling a Java program, it would be helpful to those using Java to teach to have an option to make the interactive interpreter show the same behavior as the compiler; i.e., report an error when the explicit downcast is omitted, so that students are not confused when a piece of code works in the interpreter but not the compiler.