Can I define the Negatable interface in Java?

Question

Asking this question to clarify my understanding of type classes and higher kinded types, I'm not looking for workarounds in Java.

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In Haskell, I could write something like

class Negatable t where     negate :: t -> t  normalize :: (Negatable t) => t -> t normalize x = negate (negate x) 

Then assuming Bool has an instance of Negatable,

v :: Bool v = normalize True 

And everything works fine.

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In Java, it does not seem possible to declare a proper Negatable interface. We could write:

interface Negatable {     Negatable negate(); }  Negatable normalize(Negatable a) {     a.negate().negate(); } 

But then, unlike in Haskell, the following would not compile without a cast (assume MyBoolean implements Negatable):

MyBoolean val = normalize(new MyBoolean()); // does not compile; val is a Negatable, not a MyBoolean 

Is there a way to refer to the implementing type in a Java interface, or is this a fundamental limitation of the Java type system? If it is a limitation, is it related to higher-kinded type support? I think not: it looks like this is another sort of limitation. If so, does it have a name?

Thanks, and please let me know if the question is unclear!

Answer 1

Actually, yes. Not directly, but you can do it. Simply include a generic parameter and then derive from the generic type.

public interface Negatable<T> {     T negate(); }  public static <T extends Negatable<T>> T normalize(T a) {     return a.negate().negate(); } 

You would implement this interface like so

public static class MyBoolean implements Negatable<MyBoolean> {     public boolean a;      public MyBoolean(boolean a) {         this.a = a;     }      @Override     public MyBoolean negate() {         return new MyBoolean(!this.a);     }  } 

In fact, the Java standard library uses this exact trick to implement Comparable.

public interface Comparable<T> {     int compareTo(T o); }