Using Pairs or 2-tuples in Java [duplicate]

Question

This question already has an answer here:

• A Java collection of value pairs? (tuples?) 17 answers

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

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

Answer 1

I will start from a general point of view about tuples in Java and finish with an implication for your concrete problem.

1) The way tuples are used in non-generic languages is avoided in Java because they are not type-safe (e.g. in Python: tuple = (4, 7.9, 'python')). If you still want to use something like a general purpose tuple (which is not recommended), you should use Object[] or List<Object> and cast the elements after a check with instanceof to assure type-safety.

Usually, tuples in a certain setting are always used the same way with containing the same structure. In Java, you have to define this structure explicitly in a class to provide well-defined, type-safe values and methods. This seems annoying and unnecessairy at first but prevents errors already at compile-time.

2) If you need a tuple containing the same (super-)classes Foo, use Foo[], List<Foo>, or List<? extends Foo> (or the lists's immutable counterparts). Since a tuple is not of a defined length, this solution is equivalent.

3) In your case, you seem to need a Pair (i.e. a tuple of well-defined length 2). This renders maerics's answer or one of the supplementary answers the most efficient since you can reuse the code in the future.

Answer 2

Android Tuple Utils

This object provides a sensible implementation of equals(), returning true if equals() is true on each of the contained objects.

Answer 3

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> { 
  public final X x; 
  public final Y y; 
  public Tuple(X x, Y y) { 
    this.x = x; 
    this.y = 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 4

Another 2 cents : Starting with Java 7, there is now a class for this in standard Lib : javafx.util.Pair.

And Yes, It is standard Java, now that JavaFx is included in the JDK :)

Answer 5

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 6

Though the article is pretty old now, and though I understand that I'm not really very helpful, I think the work done here: http://www.pds.ewi.tudelft.nl/pubs/papers/cpe2005.pdf, would have been nice in mainstream Java.

You can do things like:

int a;
char b;
float c;
[a,b,c] = [3,'a',2.33];

or

[int,int,char] x = [1,2,'a'];

or

public [int,boolean] Find(int i)
{
  int idx = FindInArray(A,i);
  return [idx,idx>=0];
}

[idx, found] = Find(7);

Here tuples are:

• Defined as primitive types - no templates/generics
• Stack-allocated if declared locally
• Assigned using pattern-matching

This approach increases

• Performance
• Readability
• Expressiveness