Hierarchy in Exceptions

[Nina Binde]

I have the following code which does not compile. Gives a compilation error: C:\JavaExamples\SL275Lab\mod01\examples\PassA.java:25: exception Level2Exception is never thrown in body of corresponding try statement
catch (Level2Exception e) {
^
1 error

class Level1Exception extends Exception {}
class Level2Exception extends Level1Exception {}
class Level3Exception extends Level2Exception {}
class Purple {
  public static void main(String args[]) {
    int a,b,c,d,f,g,x;
    a = b = c = d = f = g = 0;
    x = 3;
    try {
  try {
switch (x) {
  case 1: throw new Level1Exception();
  case 2: throw new Level2Exception();
  case 3: throw new Level3Exception();
}
a++;
  }
  catch (Level1Exception e) {
  b++;
  }
  finally {
  c++;
  }
    }
    catch (Level2Exception e) {
d++;
}
    catch (Exception e) {
f++;
}
    finally {
g++;
}
    System.out.print(a+","+b+","+c+","+d+","+f+","+g);
  }
}

When I interchange the catch statements, it compiles fine like below:

class Level1Exception extends Exception {}
class Level2Exception extends Level1Exception {}
class Level3Exception extends Level2Exception {}
class Purple {
  public static void main(String args[]) {
    int a,b,c,d,f,g,x;
    a = b = c = d = f = g = 0;
    x = 3;
    try {
  try {
switch (x) {
  case 1: throw new Level1Exception();
  case 2: throw new Level2Exception();
  case 3: throw new Level3Exception();
}
a++;
  }
  catch (Level2Exception e) {
  b++;
  }
  finally {
  c++;
  }
    }
    catch (Level1Exception e) {
d++;
}
    catch (Exception e) {
f++;
}
    finally {
g++;
}
    System.out.print(a+","+b+","+c+","+d+","+f+","+g);
  }
}

I fail to understand why in the first case was it complaining that Level2Exception is never thrown in the body of the corresponding try statement when I clearly have this in the try code:

case 2: throw new Level2Exception();

Or is it throwing the compilation error because the exception heirarchy is violated since Level1Exception is caught before Level2Exception is caught? In that case, the error message should be different than what I get.

Any thoughts on this is appreciated. Thank you!

[Vipin Das]

In the first case even if a level2exception is thrown it will be caught by the catch clause. Since it is the superclass all exceptions will be caught. In a sense no exception will come downwards. When u interchange the code if a level1exception comes the first catch clause cant catch it and it will come downwards. That is why the compiler is not complaining. But i dont know y the " Level2Exception has already been caught" exception is not coming. or y there is no error if you give Level2Exception in both catch clauses

[Mathangi Shankar]

Hi,

Please use the following code in the main try block:

throw new Level2Exception();

Now the code will compile fine. The problem is not due to hierarchy of exceptions.

The Level2Exception was never thrown in the main try block.

Mathangi.

[Nina Binde]

hmmm..I do throw the Level2Exception in the inner try block which is part of the main(enclosing) try block, isn't it? It should still consider it as thrown and not give me compile error.
Why then would it work for Level1Exception as seen in case 2?

Thanks!

[Barry Gaunt]

I'm talking about the first posted code example.
In the innermost try you are catching Level1Exception. That means Level1Exceptions and its subclasses will be caught.
So a thrown Level2Exception will never be thrown up to the outer try's catch clauses, because it is already handled by the innermost try's catch clause.


If after the "b++;" you put "throw e;" you will get a successful compilation.
[ November 04, 2004: Message edited by: Barry Gaunt ]

[Nina Binde]

Got you Barry! Thanks..