You _can_ write in Java that very interface you claimed could not be written. interface ThrowingRunnable<E extends Throwable> { void run() throws E;

You _can_ write in Java that very interface you claimed could not be written. interface ThrowingRunnable<E extends Throwable> { void run() throws E; } Only, hardly anybody writes it that way, so people forget that it's possible. Why don't people use exception polymorphism in that way? I am not sure, but I believe it would make code too verbose, since every function containing a virtual method call to something that might throw would have to be parameterized that way.