What scope does the variable has to have?
There is three possibilities here.
A) the variable is just a local variable. In that case you nearly have already the answer... you just need to declare a type-parameter for the enclosing method for that type:
interface ItfA { Number propA(); };
interface ItfB { Number propB(); };
class Main {
private <T extends ItfA & ItfB> T getT() {
return null;
}
private <TT extends ItfA & ItfB> void doStuffWithT() {
TT theT = getT();
System.err.println(theT.propA());
System.err.println(theT.propB());
}
}
B) The scope is the live of an object and in that case is a member field.
The obvious answer is to make the class generic and the type-parameter would
have the same & constraint:
interface ItfA { Number propA(); };
interface ItfB { Number propB(); };
class Main<T extends ItfA & ItfB> {
T theT;
public void setT(T newT) {
theT = newT;
}
public void doStuffWithT() {
System.err.println(theT.propA());
System.err.println(theT.propB());
}
}
C) The scope is the live of the program, then the variable is a static class member. Here you don't have a generics solution.
C.1) Obviously if the class of the values that you are going to handle is known you would just use that class as the field type.
C.2) If not, you could constraint the code to handle only classes that implement an interface that extends ItfA and ItfB. That interface, say ItfAB. Would be to field type.
C.3) Now, what about not imposing that constraint? What about allow the code to handle objects from any class that implement those interfaces?
Unfortunately there is no a clean-cut solution to that:
C.3.a) You could either type the field Object and provide methods to access it as an ItfA or as a ItfB (basically hiding the casting).
C.3.b) Or, instead of holding directly a reference to the object, you use a proxy object that implements those interfaces and delegates calls to those interfaces methods to the original "T" typed value. The class for that proxy could itself be a generic accepting an arbitrary <T extends ItfA & ItfB> value (similar to the B. example above).
