Question

I am upgrading the following code:

return this.getMethod().execute(mp);

Where the execute method has signature:

public Map<String,Object> execute(Object mp)

I have code that expects the return of the method to be Map<String, List<Map<String, String>>>, but the compiler is choking on the conversion. So can I / how do I get this to cast properly? And is there any change between Java 5 and 6 that would make this now a compile time issue?

Answer 1

As long as you’re sure that the returned map will only contain values of type List<Map<String, String>>, then you need to use a double cast:

Map<String, Object> map = this.getMethod().execute(mp);
@SuppressWarnings("unchecked") //thoroughly explain why it's okay here
Map<String, List<Map<String, String>>> mapWithNarrowedTypes =
        (Map<String, List<Map<String, String>>>)(Map<?, ?>)map;
return mapWithNarrowedTypes;