Can somebody answer this question for me please: why does this code not work?

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interface FooNode {
  public FooNode getSelf();
}
class BarNode {
  public BarNode getSelf() {
    return this;
  }
}
class FooBarNode extends BarNode implements FooNode {
  public FooBarNode getSelf() {
    return this;
  }
}

As far as I can see it should be fine, since FooBarNode is a FooNode and also is a BarNode the compiler should have nothing to complain about. Of course it doesn’t work, and I can see that this is so from the language specification; but the question remains: why? My complaint is really that this breaks the semantic model completely. What, in essence, is being said by the FooNode.getSelf() interface method is that it returns an object which conforms to the FooNode interface. Given that FooBarNode does conform to this interface it should be fine as a return type from this method. I assume that there is some deep reason, probably to do with efficiency of the virtual machine, why the language behaves like this; but having seen some of the design decisions to come out of Sun’s Java group I wouldn’t be too surprised if it was just an accidental fuck up.