Didn't Java once have a Pair class? [duplicate]

Answer 1

No but JavaFX has it.

Cf. Stack Overflow: Java Pair class implementation

Answer 2

A Pair class :

public class Pair<K, V> {

    private final K element0;
    private final V element1;

    public static <K, V> Pair<K, V> createPair(K element0, V element1) {
        return new Pair<K, V>(element0, element1);
    }

    public Pair(K element0, V element1) {
        this.element0 = element0;
        this.element1 = element1;
    }

    public K getElement0() {
        return element0;
    }

    public V getElement1() {
        return element1;
    }

}

usage :

Pair<Integer, String> pair = Pair.createPair(1, "test");
pair.getElement0();
pair.getElement1();

Immutable, only a pair !

Answer 3

Many 3rd party libraries have their versions of Pair, but Java has never had such a class. The closest is the inner interface java.util.Map.Entry, which exposes an immutable key property and a possibly mutable value property.

Answer 4

Map.Entry

Java 1.6 and upper have two implementation of Map.Entry interface pairing a key with a value:

• AbstractMap.SimpleEntry
• AbstractMap.SimpleImmutableEntry

For example

Map.Entry < Month, Boolean > pair = 
    new AbstractMap.SimpleImmutableEntry <>( 
        Month.AUGUST , 
        Boolean.TRUE 
    )
;

pair.toString(): AUGUST=true

I use it when need to store pairs (like size and object collection).

This piece from my production code:

public Map<L1Risk, Map.Entry<int[], Map<L2Risk, Map.Entry<int[], Map<L3Risk, List<Event>>>>>>
        getEventTable(RiskClassifier classifier) {
    Map<L1Risk, Map.Entry<int[], Map<L2Risk, Map.Entry<int[], Map<L3Risk, List<Event>>>>>> l1s = new HashMap<>();
    Map<L2Risk, Map.Entry<int[], Map<L3Risk, List<Event>>>> l2s = new HashMap<>();
    Map<L3Risk, List<Event>> l3s = new HashMap<>();
    List<Event> events = new ArrayList<>();
    ...
    map.put(l3s, events);
    map.put(l2s, new AbstractMap.SimpleImmutableEntry<>(l3Size, l3s));
    map.put(l1s, new AbstractMap.SimpleImmutableEntry<>(l2Size, l2s));
}

Code looks complicated but instead of Map.Entry you limited to array of object (with size 2) and lose type checks...

Answer 5

There is no Pair in the standard framework, but the Apache Commons Lang, which comes quite close to “standard”, has a Pair.

Answer 6

It does seem odd. I found this thread, also thinking I'd seen one in the past, but couldn't find it in Javadoc.

I can see the Java developers' point about using specialised classes, and that the presence of a generic Pair class could cause developers to be lazy (perish the thought!)

However, in my experience, there are undoubtedly times when the thing you're modelling really is just a pair of things and coming up with a meaningful name for the relationship between the two halves of the pair, is actually more painful than just getting on with it. So instead, we're left to create a 'bespoke' class of practically boiler-plate code - probably called 'Pair'.

This could be a slippery slope, but a Pair and a Triplet class would cover a very large proportion of the use-cases.