The Universe of Discourse

Product types in Java
Recently I wanted a Java function that would return two Person objects. Java functions return only a single value. I could, of course, make a class that encapsulates two Persons:

        class Persons2 {
          Person personA, personB;

          Persons2(Person a, Person b) {
            personA = a; personB = b;
          }

          Person getPersonA() { return personA; }
          ...
        }

Haskell functions return only one value also, but this is no limitation, because Haskell has product types. And starting in Java 5, the Java type system is a sort of dented, bolted-on version of the type systems that eventually evolved into the Haskell type system. But product types are pretty simple. I can make a generic product type in Java:

        class Pair<A,B> {
          A a;  B b;

          Pair(A a, B b) { this.a = a; this.b = b; }

          A fst() { return a; }
          B snd() { return b; }
        }

        Pair<Person,Person> findMatch() {
          ...
          return new Pair(husband, wife);
        }

I've been saying for a while that up through version 1.4, Java was a throwback to the languages of the 1970s, but that with the introduction of generics in Java 5, it took a giant step forward into the 1980s. I think this is a point of evidence in favor of that claim.

I wonder why this class isn't in the standard library. I was not the first person to think of doing this; web search turns up several others, who also wonder why this class isn't in the standard library.

I wrote a long, irrelevant coda regarding my use of the identifiers husband and wife in the example, but, contrary to my usual practice, I will publish it another day.

[ Addendum 20090517: Here's the long, irrelevant coda. ]

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