11.1 A Review of Dynamic Method Selection
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In previous lectures we have gone over classes extending other classes, and to make sense of this we considered whether the sub class has an "is-a relationship" with the superclass. For example, if we had the two classes:
Dog: Implements bark() method
ShowDog: Extends Dog, overrides bark method
This would be a valid extension as a ShowDog is-a Dog and a Dog is-an Object. These relationships satisfy the condition of a subclass being-an instance of the super class.
This is a particularly tricky problem to solve but the rules to look out for include:
Compliers will allow Memory Boxes to hold any subtype of itself
For example, compliers will allow the Dog memory box to hold a ShowDog object as a ShowDog is a subtype of the Dog Class
Compliers will allow calls based on Static type.
For example, if a variable were declared as a Dog it's static type would be a Dog and the complier would allowed it to call bark()
Overridden non-static methods are selected at run time based on dynamic type.
Everything else is based on static types, inculding overloaded methods.
Every variable in Java has a “compile-time type”, a.k.a. “static type”.
This is the type specified at declaration. Never changes!
Variables also have a “run-time type”, a.k.a. “dynamic type”.
This is the type specified at instantiation (e.g. when using new).
Equal to the type of the object being pointed at.
