Wednesday, January 9, 2008

baseclass dependent on subclass

A parent calls a method overridden by a subclass .. template method. A lot of plugin, callback, hollywood principal uses something like template method.


Say you have just one class. Within it, you call your own methods. Those private and final methods are fine but all other methods are virtual (ie overridable and "subject to overriding"). Problematic if this happens by accident not by design. Now I think baseclass depending on subclass is a very likely mistake.

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The MIT course note (in an earlier blog) points out the danger when a super class method B.m1() calls m2(), which is defined in B, overriden in subclass C.

In this case, "dependent" means "when the subclass C.m2() changes, B may need change, since B's behaviour is affected."

Q: whose m2() runs?
A: the innermost onion's m2(), in this case, C.m2()

Q: how do u specify B's m2() to be used in m1()? B.m2 ?
A: B.m2 will probably fail. B.m2() requires m2 be static.
A: I think u need to provide a private final m2a(), and make m2() call m2a().

If your class B need to call your own method m2(), you need to decide whether to call the m2 defined in this class or a subclass (by dynamic binding) or a parent class. Not sure if it's possible to implement your decision.

For a full example, see Example 60 [[ java precisely ]]

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