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Tip of the Day
Language: Java Language
Expertise: Beginner
May 5, 2000

Determining Stack Content Type

Question:
Only Objects can be popped from, or push onto, a stack. That means I have to wrap a primitive data type, such as int, with something akin to Integer before pushing it onto a stack. How can I identify the type of an object popped from a stack?

Answer:
Ideally, when you store objects in any type of container, you already know what their type is, or at least what class they derive from or what interface they implement. One of the most useful features of object oriented programming languages is polymorphism. Polymorphism allows you to define different behaviors for objects, yet access them in the same way.

For example, the AWT Component paint() method will draw a component. Yet a List looks different from a Button. If you are writing one of those rare programs where you need to store multiple objects of undetermined types in a container, you can resort to using the instanceof operator to identify the type of object you are dealing with. For example:

if (obj instanceof Integer) ...
This will tell you if an object is an Integer. The instanceof operator can have a relatively high overhead if used excessively, so you should try to structure your programs so as not to require its use.
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