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Trail: Bonus
Lesson: Generics
Generics
Introduction
Defining Simple Generics
Generics and Subtyping
Wildcards
Generic Methods
Interoperating with Legacy Code
The Fine Print
Class Literals as Runtime-Type Tokens
More Fun with Wildcards
Converting Legacy Code to Use Generics
Acknowledgements
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Generics and Subtyping

> Let's test your understanding of generics. Is the following code snippet legal?

> >     List<String> ls = new ArrayList<String>(); // 1
> >     List<Object> lo = ls; // 2

> Line 1 is certainly legal. The trickier part of the question is line 2. This boils down to the question: is a `List` of `String` a `List` of `Object`. Most people instinctively answer, "Sure!"

> Well, take a look at the next few lines:

> >     lo.add(new Object()); // 3
> >     String s = ls.get(0); // 4: Attempts to assign an Object to a String!

> Here we've aliased `ls` and `lo`. Accessing `ls`, a list of `String`, through the alias `lo`, we can insert arbitrary objects into it. As a result `ls` does not hold just `String`s anymore, and when we try and get something out of it, we get a rude surprise.

> The Java compiler will prevent this from happening of course. Line 2 will cause a compile time error.

> In general, if `Foo` is a subtype (subclass or subinterface) of `Bar`, and `G` is some generic type declaration, it is not the case that `G<Foo>` is a subtype of `G<Bar>`. This is probably the hardest thing you need to learn about generics, because it goes against our deeply held intuitions.

> We should not assume that collections don't change. Our instinct may lead us to think of these things as immutable.

> For example, if the department of motor vehicles supplies a list of drivers to the census bureau, this seems reasonable. We think that a `List<Driver>` is a `List<Person>`, assuming that `Driver` is a subtype of `Person`. In fact, what is being passed is a copy of the registry of drivers. Otherwise, the census bureau could add new people who are not drivers into the list, corrupting the DMV's records.

> To cope with this sort of situation, it's useful to consider more flexible generic types. The rules we've seen so far are quite restrictive.

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