Why would anyone want Person to be a Pair<F,Pair<B,Z>> ?
On Wed, Feb 16, 2011 at 7:35 AM, Alexey Zinger <[email protected]>wrote:
> Don't know how many people have rolled their own structured typing system
> in plain old Java with generics, but I've been finding myself dragging the
> same Pair class from one project to another and using it in some new and
> interesting circumstances. Here's the basic gist of it (leaving out the
> obvious):
>
> public class Pair<H, T>
> {
> public final H head;
> public final T tail;
>
> public Pair(H head, T tail) { ... }
> public static <H, T> Pair<H, T> of(H head, T tail) { return new Pair<H,
> T>(head, tail); }
> public int hashCode() { ... }
> public boolean equals(Object obj) { ... }
> }
>
> The above gives you essentially structured typing in Java. By nesting
> these pairs, you can now create ad-hoc strongly typed object graphs and pass
> them around. Gets a bit ugly fast when you see things like Pair<Foo,
> Pair<Bar, Baz>>, but it does work. But here's an interesting side-effect
> that can be used as a 'good' pattern:
>
> One of the problems in Java with defining new classes to represent data
> structures is that we have to correctly implement hashCode and equals and
> too often they're really easy to mess up in strange and subtle ways
> depending on their intended use. So sometimes we resort to stuffing things
> into collections, seeing as collections implement that logic for a given,
> well, collection of elements. But Java collections rely on a common parent
> type, so their use can quickly devalue the type system and the power of
> generics. Aside from simply passing around bare tuple-based object graphs,
> which adds a ton of bracket hell to the code, it's possible to define data
> structure classes as extensions of said tuple-based object graphs:
>
> public class Person extends Pair<String, Pair<String, Integer>>
> {
> public Person(String first, String last, int age) { super(first,
> Pair.of(last, age)); }
> public String getFirst() { return head; }
> public String getLast() { return tail.head; }
> public int getAge() { return tail.tail; }
> }
>
> I retain strong typing AND get to be assured of hashCode and equals
> correctness (as long as Pair's implementations are right, which isn't hard
> to ensure using Guava). I'm even being a good boy and making my data
> structure an immutable bean.
>
> Alexey
> http://azinger.blogspot.com
> http://bsheet.sourceforge.net
> http://wcollage.sourceforge.net
>
>
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