How would you automatically downcast in Java?
Look at this example:
public class Parent {}
public class Boy extends Parent {}
public class Girl extends Parent {}
Parent father = new Boy();
Parent mother = new Girl();
Boy kid = (Boy)father;
Girl kiddette = (Girl)mother;
I know I can downcast manually like in example above, but, how would I do it automatically?
So, what I want is a autocast method, something like this:
public ? downcast(? parent) { return ? parent; }
which I would use like this:
Boy kid = downcast(father);
Girl kiddette = downcast(mother);
So, I do not want to perform any explicit casting in the caller's method. I need downcast to hide the casting within itself. Preferrably, I would like to avoid type checks (instanceof) within the downcast method, so, if downcasting can be generalized, that would be great.
What do I need to place instead of ?, ? and ? to make it work.
I remember that I did this exact same thing 10 or so years ago in Java, and I remember that I was puzzled by the code, but it worked and I did not understand how it works just that it does, but I did not need to do this thing in Java again in the last 10 years, so I forgot what I did. And I remember that I managed to solve it without type checks. So, one line of code that worked for no matter how many child classes parent class had.
You can abuse generics to achieve something like this:
private static <F, T extends F> T cast(F value) {
return (T) value;
}
Now you can do
Parent father = new Boy();
Parent mother = new Girl();
Boy kid = cast(father);
Girl kiddette = cast(mother);
This works because the compiler infers the target type from the kid variables. It's also dangerous, as evidenced by the generated warning on (T) value: "Unchecked cast: 'F' to 'T'". In general, you don't want to do this. All you are doing is tricking the compiler into casts that normally wouldn't be allowed.