Parametric Polymorphism

Parametric Polymorphism - Generic Java

Problems:

Collections where you don't know what is in them.
Visitors with highly abstract parameters and returns, also exceptions.
Clone's return type is object even though you know it has some type. This is covariant return types

Solution:

Covariant return types: can return a subtype of what is declared in your super. Example:

figure here

Is this okay? Why does it work? Because... A dog IS AN animal.

Parametric Polymorphism aka Generics

Definition: parametric polymorphism = polymorphic on a type which is parameter.

Ways to think of it:

a class generator - no this is implementation
A type contract

What does this mean about classes and types? Is a list of dogs a list of animals?

figure of UML of generics

No because what if you put a cat in my list of dogs?

Syntax of GJ for generic classes and also limitations.

Now we have a list of data and we know the type, what about the accept method? It doesn't know what type the visitor wants, put it doesn't care. Its contract is that is won't touch what you pass along. Like wise with the return value.

Introduce generic methods Show the singleton hack. This doesn't occur in LRStruct nodes but it does happen in LRStruct state.
Wrap up, discuss exceptions what we want and what we have. Mention special cases and syntax in GJ.

jwalker@cs.oberlin.edu