Type Security Test

Object o1 = new Object ();
Object o2 = new B ();
Object o3 = new D ();

Object o4 = o3;
B b1 = new B ();
B b2 = new D ();

D d1 = new D ();
// B b3 = new Object (); CTE
// D d2 = new Object (); CTE

B b4 = d1;
// D d3 = b2; CTE

D d4 = (D) d1;
D d5 = (D) b2;

// D d6 = (D) b1; RTE
// B b5 = (B) o1; RTE
B b6 = (D) b2;

Here, CTE indicates an error during compilation and RTE indicates an error during running.

Remember two points to cover all the above conversions:
1. An object can be converted to any of its base types without any additional syntax. This is a safe implicit conversion.
2. It is mandatory to convert an object to its derived type because an exception may occur during running -- RTE