Java: One function with many return types... Is it possible with generics?

Question

I have a few procedures that, for simplicity's sake, look like the following:

public String fetchValueAsString(String key);
public DateTime fetchValueAsDateTime(String key);

I want something like

public <X is a String or a DateTime> X fetchValue(String key);  // pseudo-code

that I could call like this (without casting; the type is implied by the passed parameters):

String str = fetchValue("subject");
DateTime dt = fetchValue("startDate");

I know I could just have one method that returns the Object type and just do a casting conversion, but I'm wondering if there is a way I can have just one method called, and somehow use generics to determine the return value. So, is it possible in Java (it is in C#)?

Answer 1

Because of type erasure, in Java the return type would have to be passed in explicitly:

public <X> X fetchValue(Class<X> type, String key);

If you know the value is going to be of the correct type no matter what, you don't have to pass in the Class<X>, and you can simply do a cast to (X) in the implementation. However, you'll get a compiler warning. With the Class<X> passed in you can do type.cast(value) safely, and more importantly make an attempt at converting it to the correct type if is not already of that type.

Answer 2

Sure, but it will only work if the datatype is implied by the passed parameter.

I can declare the following and get no error:

public static <T> T parseString(String s) {
    return (T) new Date();
}

public static void test() {
    Date dt = parseString("date");
}

But the upshot to this kind of thing is the interior of your parsing function is going to have to be able to look at the string and determine the target type and its going to have to do a lot of casting inside its body to work properly.

Your question leaves somewhat ambiguous whether this is the kind of thing you want, or if what you really want is to have the type of the receiver determine either the method called or the behavior inside the method. This is, AFAIK not possible. Once you're in the method, all you have to work with is the string.