What's the best way to return a pair of values in Java? [duplicate]

Answer 1

Some observation of mine:

1. Array is bulit-in, fast and easy to use, although imposible to expand its capacity. What if you want 3 values to be returned after 3 months?

2. ArrayList/other colletions can be good, allows you to increment the capacity(initially 10). Note that Vector can be overkill in comparison to ArrayList when you only want to store 2 values to be fetched later. Map also can be good because it's always sorted and ordered.

3. Some user-defined class: maybe an option if is meaningful(means that the data returned is important-ish to be a Java Bean), and you want to store more than just 2 integers into it. Readibility is better in case you add more notes in its Javadoc. Can be expanded as you like, just add fields in this class. Slower, but safer.

Answer 2

Apache Commons Lang3 provides an abstract Pair class with a couple implementations including ImmutablePair and MutablePair.

Answer 3

Three approaches, all not so great:

1. Roll your own Pair<A, B>. You said you didn't want to do that.
2. Return a Object[]. This is not type safe.
3. Mimic out variables or pointers by supplying single element arrays as parameters.

An example of #3:

public boolean getUserDetails(String userId, String[] lastName, String[] firstName, Date[] dob) {
  assert lastName  != null && lastName.length  == 1;
  assert firstName != null && firstName.length == 1;
  assert dob       != null && dob.length       == 1;
  ...
}

The third option makes life painful for the caller.

So like I said, no nice solution.

As an aside, Scala uses various Tuple classes (up to 21-tuple, from what I remember) to help you with this.

Answer 4

if both are integers then I would advise a java.awt.Point but otherwise just create a container class with two objects x and y

Answer 5

The closest thing I've seen to a "pair" in the standard libraries are the Map.Entry interface and the AbstractMap.SimpleEntry and AbstractMap.SimpleImmutableEntry classes that implement it.

If both objects are the same class an array is easier to use.

Answer 6

I have been told by experts that when faced with the question of pairs, one of two things is true:

1. You need to rethink your structure (this blunt answer doesn't help anyone)
2. You need to build your own class to hold the pair

I would suggest that the second case is not all that abnormal. However, if what you are doing seems too trivial for introducing a new class, then using a Map could work, as others have suggested. If you are simply sending a single response back, then a Map seems like a bit much.

If a list of pairs sounds like it would work, and you need to maintain order, you could try a LinkedHashMap so that order is maintained.