Using Tuples in Java

Question

My Hashtable in Java would benefit from a value having a tuple structure. What data structure can I use in Java to do that?

Hastable<Long, Tuple<Set<Long>,Set<Long>>> table = ...

Answer 1

I don't think there is a general purpose tuple class in Java but a custom one might be as easy as the following:

public class Tuple<X, Y> { 
  private X x; 
  private Y y; 
  public Tuple(X x, Y y) { 
    this.x = x; 
    this.y = y; 
  } 
  public X getX() { 
    return this.x; 
  } 
  public Y getY() { 
    return this.y; 
  } 
} 

Of course, there are some important implications of how to design this class further regarding equality, immutability, etc., especially if you plan to use instances as keys for hashing.

Answer 2

Create a class that describes the concept you're actually modeling and use that. It can just store two Set<Long> and provide accessors for them, but it should be named to indicate what exactly each of those sets is and why they're grouped together.

Answer 3

Here's this exact same question elsewhere, that includes a more robust equals, hash that maerics alludes to:

http://groups.google.com/group/comp.lang.java.help/browse_thread/thread/f8b63fc645c1b487/1d94be050cfc249b

That discussion goes on to mirror the maerics vs ColinD approaches of "should I re-use a class Tuple with an unspecific name, or make a new class with specific names each time I encounter this situation". Years ago I was in the latter camp; I've evolved into supporting the former.