Self-bounding generics
How do I decrypt “Enum<E extends Enum<E>>”?

Self-Bounding generics is quite similar to CRTP(Curiously Recurring Template Pattern), but somewhat different. See the following example:

class Parent<T extends Parent<T>> {

   private int fVal;

   public Parent() { setVal(0); }
   public Parent(int val) { setVal(val); }

   public void setVal(int val) { fVal = val; }
   public int getVal() { return fVal; }

   public boolean compareTo(T other) {
      return getVal() == other.getVal();
   }
}

class Sub extends Parent<Sub> {
}

class Sub2 extends Parent<Sub2> {
}

public class SelfBounding {
   public static void main(String args[]) {
      Sub s = new Sub();
      System.out.println(s.compareTo(new Sub()));
      System.out.println(s.compareTo(new Sub2()));
   }
}

This code raises an error, when compiled: SelfBounding.java|26| compareTo(Sub) in Parent<Sub> cannot be applied to (Sub2).

Let’s see why. Firstly, what is Sub actually? The Sub extends Parent. Specifically, Parent<Sub>. Parent is a parameterized class. So, T should satisfy ‘T extends Parent<T>.’ Here, T is Sub. So, Sub should satisfy ‘Sub extends Parent<T>.’ Obviously, Sub satisfies this constraints because Sub is actually subclassing Parent<Sub>.

Okay, that makes sense. But, why should we bother?

The thing is that the Sub is inhering a method: compareTo(). In the Parent, compareTo is declare to receive an argument of type T. In Sub, T is obviously Sub. Which means that Sub class is inheriting a method whose argument/return type is exactly the same as itself.

Still, there’s an problem. Consider the following:

class Sub extends Parent<Sub> {
}

class Sub2 extends Parent<Sub> {
}

In this case, Sub2 extends Parent<Sub>. So, in Parent, T is Sub. And, obviously, Sub is subclassing Parent<Sub>. Hence, Sub2 inherited a method which gets an argument of type Sub. At this point, everything becomes a mess.

This is why this pattern is hard to apply in practice. There is a hot debate about this issue. Come and play: http://www.artima.com/forums/flat.jsp?forum=106&thread=136394&start=15&msRange=15