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Tip of the Day
Language: Java
Expertise: Intermediate
Dec 11, 2001

Exception Restrictions when Overriding


When you extend a class and override a method, Java insists that the new method cannot be declared as throwing checked exceptions of classes other than those that were declared by the original method (in the base class). It may, however, throw exceptions derived from the base-class exceptions. 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:
 
public class Base {
   public void f() throws IOException {
   }
}

public class LegalOne extends Base {
   public void f() throws IOException {
   /* Overridden method throws the same _
exception as in the base-class
version, hence legal. */
   }
}

public class LegalTwo extends Base {
   public void f() throws IOException, _
MalformedURLException {
   /* MalformedURLException is derived from _
IOException, hence it is legal
to throw this exception in f(). */
   }
}

public class IllegalOne extends Base
   public void f() throws IOException, _
IllegalAccessException {
   /* IllegalAccessException is not derived _
from IOException; in fact, it
is the super class of IOException. Hence this _
method declaration is
illegal, since overridden methods can only throw _
either the base-class
exceptions or exceptions derived from the base-class _
exceptions. */
   }
}

public class IllegalTwo extends Base
   public void f() throws Exception {
   /* Exception is the super-class of IOException, _
hence illegal, as
explained above. */
   }
}
Navneet Gupta
 
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