Exception matching

Bruce Eckel's Thinking in Java Contents | Prev | Next

//: Human.java
// Catching Exception Hierarchies
 
class Annoyance extends Exception {}
class Sneeze extends Annoyance {}
 
public class Human {
  public static void main(String[] args) {
    try {
      throw new Sneeze();
    } catch(Sneeze s) {
      System.out.println("Caught Sneeze");
    } catch(Annoyance a) {
      System.out.println("Caught Annoyance");
    }
  }
} ///:~ 

The Sneeze exception will be caught by the first catch clause that it matches, which is the first one, of course. However, if you remove the first catch clause:

    try {
      throw new Sneeze();
    } catch(Annoyance a) {
      System.out.println("Caught Annoyance");
    }

The remaining catch clause will still work because it’s catching the base class of Sneeze. Put another way, catch(Annoyance e) will catch a Annoyance or any class derived from it . This is useful because if you decide to add more exceptions to a method, if they’re all inherited from the same base class then the client programmer’s code will not need changing, assuming they catch the base class, at the very least.

If you try to “mask” the derived-class exceptions by putting the base-class catch clause first, like this:

    try {
      throw new Sneeze();
    } catch(Annoyance a) {
      System.out.println("Caught Annoyance");
    } catch(Sneeze s) {
      System.out.println("Caught Sneeze");
    }

the compiler will give you an error message, since it sees that the Sneeze catch-clause can never be reached.

Exception guidelines

  1. Fix the problem and call the method (which caused the exception) again.
  2. Patch things up and continue without retrying the method.
  3. Calculate some alternative result instead of what the method was supposed to produce.
  4. Do whatever you can in the current context and rethrow the same exception to a higher context.
  5. Do whatever you can in the current context and throw a different exception to a higher context.
  6. Terminate the program.
  7. Simplify. If your exception scheme makes things more complicated, then it is painful and annoying to use.
  8. Make your library and program safer. This is a short-term investment (for debugging) and a long-term investment (for application robustness).


Comments

  • There are no comments yet. Be the first to comment!

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • By now, every company has a business continuity (BC) plan, right? Not quite. Many US businesses aren't equipped to deal with major disruptions. The adoption of BC practices remains low, as many a C-suite folks simply have the "it can't happen to us" mentality or they have a subjective notion of risk, assuming that they can withstand disaster. Read this article bundle to learn why your BC plan can't wait, tips on how to get started with your plan and what it should contain, and much more.

  • Live Event Date: January 28, 2015 @ 11:00 a.m. ET / 8:00 a.m. PT Check out this upcoming live webcast and join Jeff Sloyer, IBM Developer Evangelist and Master Inventor, for a tutorial for building cloud-based applications. Using IBM's platform as a service, Bluemix, Jeff will show you how to architect and assemble cloud-based applications built for cloud scale. Leveraging the power of microservices, developers can quickly translate monolithic applications to a cloud-based microarchitecture. This hour-long …

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date