Applying a Multi-catch Block - Sybex CSG 11 p.420 Kindle Edition

[Rade Koncar]

Boyarsky, Jeanne; Selikoff, Scott. OCP Oracle Certified Professional Java SE 11 Developer Complete Study Guide (p. 420). Wiley. Kindle Edition.

In a subchapter: Applying a Multi-catch Block


Under this code:

public static void main(String args[]) {
   try {
      System.out.println(Integer.parseInt(args[1]));
   } catch (ArrayIndexOutOfBoundsException e) {
      System.out.println("Missing or invalid input");
   } catch (NumberFormatException e) {
      System.out.println("Missing or invalid input");
   }
}

Authors wrote this:

How can you reduce the duplicate code? One way is to have the related exception classes all inherit the same interface or extend the same class. For example, you can have a single catch block that just catches Exception. This will catch everything and anything.


I don't understand how would inheriting the same interface help here? I cannot use interface in the catch clause. If they extend the same exception, then they will be handled by the same block, I can understand that.

There is too much left untold and left to the user to ponder how one would solve it with interfaces...

[Paul Clapham]

Rade Koncar wrote:I cannot use interface in the catch clause.

Why not?

[Rade Koncar]

Paul Clapham wrote:Why not?

Hi Paul, I try this code:

public class ExTest {
  public static void main(String[] args) {

    ExTest mua = new ExTest();

    try {
      mua.bla();
      /*
        Required type: Throwable
        Provided: MyInt
       */
    } catch (MyInt c) {

    }

  }

  private void bla()throws FirstException, SecondException {

  }

  interface MyInt {}

  class FirstException extends Exception implements MyInt {

  }

  class SecondException extends Exception  implements MyInt{

  }

}

My IDE objects if I use interface in the catch clause.

[Paul Clapham]

And if you write

  private void bla()throws MyInt {

?

(I too haven't ever encountered the idea of an exception implementing an interface, I'm just suggesting what to try if you wanted to do that. If it's even possible.)

[Rade Koncar]

No, I get the same error.

  /*
  Required type: Throwable
  Provided: MyInt
 */
  private void bla() throws MyInt {

  }

Since exceptions are defined as classes they can implement an interface, but I wonder what the authors meant by this...

It is a very uncommon practice, this I why I got interested in the first place

[Paul Clapham]

Rade Koncar wrote:Since exceptions are defined as classes they can implement an interface, but I wonder what the authors meant by this...

Then I'd agree with you. I never thought of having a class extend Exception and also implement an interface, but there's no rule against it so why not? But apparently that isn't useful, at least not for catching those exceptions. Unless both of us are missing something, which is still possible.

[Campbell Ritchie]

No, you can make an exception implement an interface (e.g. Serializable, which most exceptions already implement). But you cannot catch it as Serializable; there is no way to know that a Serializable is an exception class.

[Rade Koncar]

Hey Campbell, thanks for chipping in! Perhaps you can help us unveil what the authors had in mind when they suggested using interfaces?

[Paul Clapham]

In that example you can certainly reduce the code like this:

   try {
      System.out.println(Integer.parseInt(args[1]));
   } catch (ArrayIndexOutOfBoundsException | NumberFormatException e) {
      System.out.println("Missing or invalid input");
   }

In fact I though that's what "multi-catch" actually referred to. No idea where interfaces can be used here, though.

[Campbell Ritchie]

I don't think you can use an interface; there is no such thing as an interface that is a subtype of Throwable, and whatever type you use in the parameters to the catch must be a subtype of Throwable.

Readers are reminded that I said, “subtype,” in its non‑strict sense; every type is a non‑strict supertype and a non‑strict subtype of itself.

[Jeanne Boyarsky]

Rade,
Good point. It can't work with an interface. I've added to the errata and given you a cow.

[Rade Koncar]

Thanks, I was distracted by my job for several weeks, this is the best present/reminder to finish prepping for the exam!