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

However, using polymorphic method calls as they are intended requires that you have control of the base-class definition because at some point in the extension of your program you might discover that the base class doesn’t include the method you need. If the base class comes from a library or is otherwise controlled by someone else, a solution to the problem is RTTI: You can inherit a new type and add your extra method. Elsewhere in the code you can detect your particular type and call that special method. This doesn’t destroy the polymorphism and extensibility of the program because adding a new type will not require you to hunt for switch statements in your program. However, when you add new code in your main body that requires your new feature, you must use RTTI to detect your particular type.

Putting a feature in a base class might mean that, for the benefit of one particular class, all of the other classes derived from that base require some meaningless stub of a method. This makes the interface less clear and annoys those who must override abstract methods when they derive from that base class. For example, consider a class hierarchy representing musical instruments. Suppose you wanted to clear the spit valves of all the appropriate instruments in your orchestra. One option is to put a ClearSpitValve( ) method in the base class Instrument, but this is confusing because it implies that Percussion and Electronic instruments also have spit valves. RTTI provides a much more reasonable solution in this case because you can place the method in the specific class ( Wind in this case), where it’s appropriate. However, a more appropriate solution is to put a prepareInstrument( ) method in the base class, but you might not see this when you’re first solving the problem and could mistakenly assume that you must use RTTI.



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