Supporting the Design Pattern "Object Structures as Plain Values"

We sketch the principles of a type system for an object-oriented language such as Java which allows to statically ensure that an object structure is not modified by a method call, if the primary reference of that object structure is stored in a local variable of the method and this variable does not syntactically occur in the call. The object structure thus behaves like a �plain value�, say a machine number, stored in a local variable in that no hidden side-effects can change it. We call the corresponding design pattern �object structures as plain values�. The model is presented in an informal style; its validity still remains to be shown by a formal definition and soundness proof.