Make a Java class generic, but only for two or three types

Question

(I was astonished not to be able to find this question already on stackoverflow, which I can only put down to poor googling on my part, by all means point out the duplicate.

Answer 1

You can use so-called "witness" types to make the compiler do what you want.

public interface Reversible< T > {
    public static final class IntReversible implements Reversible< Integer > {}
    public static final class StringReversible implements Reversible< String > {}
    public static final class MagicReversible implements Reversible< MagicValue > {}
}

public abstract class Mirror< T, R extends Reversible< T > > {
    // ...
}

public class IntMirror extends Mirror< Integer, IntReversible > {
    // ...
}

However, the reason your example doesn't make any sense is because you gain virtually nothing from using a generic in this context. What possible algorithm will reverse an integer or a string or a MagicValue without resorting to awful run-time type-checking and casting? The code will be all three reverse algorithms, wrapped with a hideous if-ladder.