Java Exceptions. Cast one type to another

Question

Could someone explain why the first cast does not give a CCE ?

public class Test {

  public static void main(String[] args) throws Throwable {
    Test.<RuntimeException>throwIt(new Exception());
  }

  @SuppressWarnings("unchecked")
  private static <T extends Throwable> void throwIt(Throwable throwable) throws T {
    throw (T) throwable; // no ClassCastException
    throw (RuntimeException) throwable; // ClassCastException(as it should be)
  }
}

P.S. Comment one cast (otherwise it won't compile).

What is your question? Is your code compiling? You will get Unreachable code compilation error. — Jayamohan, Feb 27 '13 at 11:38
Exception is the super class and you cannot cast it to RuntimeException — Biswajit, Feb 27 '13 at 11:40
"as it should be" is the one I understand, the other is the question — Bax, Feb 27 '13 at 11:40
I can tell you that the generic parameter type is erased when the code is compiled. At runtime there isn't even a cast. Won't post an answer until I can find the corresponding JLS section to verify why this is valid behavior. — Perception, Feb 27 '13 at 12:12

Gunnarr · Accepted Answer · 2013-02-27T14:33:45.610

1

This is the feature of Java generics realization, it was realized through type erasure, that's why (T) cast actually cast it to Throwable as leftmost bound. Because, new Exception() produces a Throwable object you can safely throw it.

You can check it in JSL http://docs.oracle.com/javase/specs/jls/se5.0/html/typesValues.html#108979

edited Feb 27 '13 at 14:33

answered Feb 27 '13 at 12:25

Gunnarr

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Java Exceptions. Cast one type to another

1 Answers1