Message conversion and a new type system for OO model

In this paper, we propose that the messages should be defined independently to the class definitions in an object-oriented program. The compatibility relation between two message types is also discussed. This kind of relationship is essential and useful for software reuses. A software designer can declare the compatibility relation between two message types if messages are defined independently to the class definitions. A message of one type can be converted to another type automatically when necessary if a compatibility relation between the two message types is specified. A new type system is proposed based on these new concepts in this paper. This type system is more flexible than the traditional ones while keeping the safety of type checking.