Can we change the return type of an overridden method?
Return types may vary among methods that override each other if the return types are reference types. The notion of return-type-substitutability supports covariant returns, that is, the specialization of the return type to a subtype.
A method declaration d1 with return type R1 is return-type-substitutable for another method d2 with return type R2, if and only if the following conditions hold:
If R1 is void then R2 is void.
If R1 is a primitive type, then R2 is identical to R1.
If R1 is a reference type then:
R1 is either a subtype of R2 or R1 can be converted to a subtype of R2 by unchecked conversion (§5.1.9), or
R1 = |R2|