Contents | Prev | Next

Exception restrictions

When you override a method, you can throw only the exceptions that have been specified in the base-class version of the method. This is a useful restriction, since it means that code that works with the base class will automatically work with any object derived from the base class (a fundamental OOP concept, of course), including exceptions.

This example demonstrates the kinds of restrictions imposed (at compile time) for exceptions:

//: StormyInning.java
// Overridden methods may throw only the 
// exceptions specified in their base-class 
// versions, or exceptions derived from the 
// base-class exceptions.

class BaseballException extends Exception {}
class Foul extends BaseballException {}
class Strike extends BaseballException {}

abstract class Inning {
  Inning() throws BaseballException {}
  void event () throws BaseballException {
   // Doesn't actually have to throw anything
  }
  abstract void atBat() throws Strike, Foul;
  void walk() {} // Throws nothing
}

class StormException extends Exception {}
class RainedOut extends StormException {}
class PopFoul extends Foul {}

interface Storm {
  void event() throws RainedOut;
  void rainHard() throws RainedOut;
}

public class StormyInning extends Inning 
    implements Storm {
  // OK to add new exceptions for constructors,
  // but you must deal with the base constructor
  // exceptions:
  StormyInning() throws RainedOut, 
    BaseballException {}
  StormyInning(String s) throws Foul, 
    BaseballException {}
  // Regular methods must conform to base class:
//! void walk() throws PopFoul {} //Compile error
  // Interface CANNOT add exceptions to existing
  // methods from the base class:
//! public void event() throws RainedOut {}
  // If the method doesn't already exist in the
  // base class, the exception is OK:
  public void rainHard() throws RainedOut {}
  // You can choose to not throw any exceptions,
  // even if base version does:
  public void event() {}
  // Overridden methods can throw 
  // inherited exceptions:
  void atBat() throws PopFoul {}
  public static void main(String[] args) {
    try {
      StormyInning si = new StormyInning();
      si.atBat();
    } catch(PopFoul e) {
    } catch(RainedOut e) {
    } catch(BaseballException e) {}
    // Strike not thrown in derived version.
    try {
      // What happens if you upcast?
      Inning i = new StormyInning();
      i.atBat();
      // You must catch the exceptions from the
      // base-class version of the method:
    } catch(Strike e) {
    } catch(Foul e) {
    } catch(RainedOut e) {
    } catch(BaseballException e) {}
  }
} ///:~

In Inning, you can see that both the constructor and the event( ) method say they will throw an exception, but they never do. This is legal because it allows you to force the user to catch any exceptions that you might add in overridden versions of event( ). The same idea holds for abstract methods, as seen in atBat( ).

The interface Storm is interesting because it contains one method ( event( ))that is defined in Inning, and one method that isn’t. Both methods throw a new type of exception, RainedOut. When StormyInning extends Inning and implements Storm , you’ll see that the event( ) method in Storm cannot change the exception interface of event( ) in Inning. Again, this makes sense because otherwise you’d never know if you were catching the correct thing when working with the base class. Of course, if a method described in an interface is not in the base class, such as rainHard( ), then there’s no problem if it throws exceptions.

The restriction on exceptions does not apply to constructors. You can see in StormyInning that a constructor can throw anything it wants, regardless of what the base-class constructor throws. However, since a base-class constructor must always be called one way or another (here, the default constructor is called automatically), the derived-class constructor must declare any base-class constructor exceptions in its exception specification.

The reason StormyInning.walk( ) will not compile is that it throws an exception, while Inning.walk( ) does not. If this was allowed, then you could write code that called Inning.walk( ) and that didn’t have to handle any exceptions, but then when you substituted an object of a class derived from Inning, exceptions would be thrown so your code would break. By forcing the derived-class methods to conform to the exception specifications of the base-class methods, substitutability of objects is maintained.

The overridden event( ) method shows that a derived-class version of a method may choose to not throw any exceptions, even if the base-class version does. Again, this is fine since it doesn’t break any code that is written assuming the base-class version throws exceptions. Similar logic applies to atBat( ), which throws PopFoul, an exception that is derived from Foul thrown by the base-class version of atBat( ). This way, if someone writes code that works with Inning and calls atBat( ), they must catch the Foul exception. Since PopFoul is derived from Foul, the exception handler will also catch PopFoul.

The last point of interest is in main( ). Here you can see that if you’re dealing with exactly a StormyInning object, the compiler forces you to catch only the exceptions that are specific to that class, but if you upcast to the base type then the compiler (correctly) forces you to catch the exceptions for the base type. All these constraints produce much more robust exception-handling code. [43]

It’s useful to realize that although exception specifications are enforced by the compiler during inheritance, the exception specifications are not part of the type of a method, which is comprised of only the method name and argument types. Therefore, you cannot overload methods based on exception specifications. In addition, because an exception specification exists in a base-class version of a method doesn’t mean that it must exist in the derived-class version of the method, and this is quite different from inheriting the methods (that is, a method in the base class must also exist in the derived class). Put another way, the “exception specification interface” for a particular method may narrow during inheritance and overriding, but it may not widen – this is precisely the opposite of the rule for the class interface during inheritance.

[43] ANSI/ISO C++ added similar constraints that require derived-method exceptions to be the same as, or derived from, the exceptions thrown by the base-class method. This is one case in which C++ is actually able to check exception specifications at compile time.

Contents | Prev | Next