Exception matching

Thursday Mar 1st 2001
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When an exception is thrown, the exception-handling system looks through the “nearest” handlers in the order they are written. When it finds a match, the exception is considered handled, and no further searching occurs.

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When an exception is thrown, the exception-handling system looks through the “nearest” handlers in the order they are written. When it finds a match, the exception is considered handled, and no further searching occurs.

Matching an exception doesn’t require a perfect match between the exception and its handler. A derived-class object will match a handler for the base class, as shown in this example:

//: 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

Use exceptions to:

  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).
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