Re: light weight types

Joshua Cranmer <Pidgeot18@verizon.invalid>
Sat, 03 Oct 2009 21:34:46 -0400
On 10/03/2009 08:12 PM, Roedy Green wrote:

Can you really explain the difference between
<?> vs<Dog> vs<? extends Dog> vs<E extends Dog> vs<? super T>
in a few paragraphs so that at least 50% of your readers will follow?

It helps to explain generics by thinking in terms of containers--most
generified types are containers in some form or other.

Generified types can be declared thusly:

public class Container<T> {}

The `T' represents a type. The implementer of Container has no idea what
type it is, it's just some type, any type. Since we don't know what it
is, all we can guarantee is that it is an Object of some type (all
non-primitive types are a subclass of Object, and generics don't use
primitive types), so wherever `T' is found in the implementation, you
can mentally replace it with `Object' for the same effect.

Sometimes, though, we don't want to let people include any old object.
To do this, we declare an upper bound on the type like so:
public class Container<T extends Number> {}

Now only numeric types can be stored in the container. Also, since we
know that any given value of `T' has to be a subclass of Number in some
fashion, we can replace it with `Number' instead of `Object'.

You can also specify a /lower/ bound on the type, by specifying `super'
instead of `extends'. The rationale behind this will be made clear
slightly later.

To use generics, you merely have to replace the appropriate parameter
with your class. So an integer container is declared as type
`Container<Integer>' (again primitive types are not valid here; this is
due to implementation necessities. But don't forget that Java 5+
autoboxes primitive types).

An important fact to realize is that subtyping of generic types isn't
quite what would be expected at first: if A is a subclass of B,
Container<A> is not a subtype of Container<B>. Remember that a container
stores it type: you can store any B into a Container<B>, so being able
to cast from Container<A> to Container<B> means you can put a B into the

The way around this is to state that you don't know what's contained
inside in the container. We do this with the type `?' (also known as the
wildcard type): Container<?> stores /something/, but we don't know what.
Since Container<A> stores objects of type `A', it stores objects of any
type, so it is a subtype of (and thus convertible to) Container<?>.
Conversely, since we don't know what type a Container<?> holds, we can't
store stuff inside the container.

However, since we know that all Containers must store Objects, we can
treat whatever is in the Container as an Object. In effect, the wildcard
turns the Container into a read-only object: you can't write into
Container<?>, but you can read from it. This is internally enforced by
the special subtyping rules of generic objects; the exact mechanisms
require some mathematical theory that I will not delve in here.

Similarly to how we limited the storage type of a container with the
`extends Number' relationship, we can limit the type of a wildcard. A
type `Container<? extends Number>' indicates that the object stores
something that is at least of type Number. Once again, that means that
instead of viewing it as containing something that is at least an
Object, we can see it as containing something that is at least a Number.

Types can also be recursive. A recursive type is something of the form
class Foo<T extends Foo<T>>. You generally subclass or implement
interfaces of these form in the fashion class Bar extends Foo<Bar>. The
purpose of a recursive type is to operate on itself in some fashion:
think of Comparable's compareTo method. Classes implementing Comparable
only compare objects of the same class as them, not of other classes
implementing Comparable.

Methods can also be generified. The rationale here is for utility
methods operating on generic container classes. Here is an example of a
generic method definition:
   public static <T> T move(Container<T> from, Container<? super T> to);

Here, you also saw an example of the use of the lower bounds. We want to
be able to take an Integer stored in a Container<Integer> and put it
into a Container<Number>. Using upper bounds is also possible here, but
if we were to do that, we'd be given a Number and not an Integer. The
object being moved is already known to be a Number, so we should we
unnecessarily restrict the type of return?

The other main use of lower bounds is in recursive types. Specifying <T
extends A<T>> is generally incorrect: <T extends A<? super T>> is
typically more correct.

How was that?

The best I have seen has been explanations of what a few specific
examples do. When you find people resorting to examples to explain,
You know it is difficult material.

I think that examples are not so much needed to explain, but more to
motivate why the developers did things the way they did. Generics are
actually rather simple once you understand what they actually mean (what
I gave above is roughly how I came to realize their underlying
mechanisms), which explains why some operations that would seemingly be
safe are actually not safe. Well, rare types (mixing raw types and
generics) don't actually make sense in many cases. Suffice to say that I
am personally extremely aggravated that List.class is of type
Class<List> and not Class<List<?>>, which makes for some particularly
annoying typing situtations.

You might measure a feature in a language by the power it gives you
divided by the effort you need to expend to learn it and use it.
Generics have a fairly low payback number. I don't know if the
complexity was unavoidable, or as a side effect of insisting on type

I wouldn't say that they have fairly low payback. What they increase
substantially is compile-time type safety. Having spent considerable
amounts of time in languages that have very little compile-time type
safety, trust me when I say that the more the compiler complains to you
about, the better.

Generics lack the elegance of other Java features. The syntax is ad
hoc, unshaved, irregular, Perlish, unfinished.

Actually, the syntax is cribbed from C++ templates. Blame Bjarne Stroustrup.

With most features, you have and ah ha moment, and all falls into
place. Everything is as it should be. It could be no other plausible
way. You "grok" them in fullness, and no longer need to read a manual
to write code. Generics don't do that, at least not yet.

To me, generics has had that moment. The best way I can think to explain
them is to think of a generic type as containing an object of some type;
wildcard objects are ways of saying that you don't know what it
contains, but you may have some information.

Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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