Package tuple
Defining relations between objects is a quite common task for developers, therefore there are a lot of libraries implementing their version of Pair and Triple, the most famous of those libraries are the Apache Commons Lang.
So, why writing my own version of those two classes? As explained in the introduction this library collects the code I have to use more frequently in my everyday coding, so yes, this is just another implementation of the most common classes ever!
How my implementation differs from the one in the Apache Common Lang library?
I always try to keep things as simple as possible. The org.apache.commons.lang3.tuple.Pair implements java.lang.Comparable and java.util.Map.Entry, I cannot understand the reason for this choice, it makes no sense.
- Implementing java.util.Map.Entry adds an useless complexity forcing the Pair class to have a setValue(V) and therefore forcing instances to be mutable.
- Imlementing java.lang.Comparable is even worst because it induces a total order over the paris that has completely no sense.
For example I may have a class ServiceResponse that do not implement Comparable because it makes no sense to define an order over service responses. For some reason I need to define a relation between two responses so I create a Pair of ServiceResponse, the responses are not comparable so why do I need to force an order on a pair of them?
I created my own version of Pair and Triple because I wanted to create a simple, reasonable and easy to use way to define relations between objects.
Classes Pair and Triple
The implementation of this two classes is trivial, it is about POJOs decorated with some useful factory methods to make them easy to use.
Those classes are immutable because the aim of a pair is to define a relation between two objects, and modifying such relation makes no sense.
To change the values in the pair equals to define a new relation so it makes more sense to create a new pair instead.
This classes are safe against serialization. You can serialize and de-serialize instances of Pair and Triple in any way.
Classes ComparablePair and ComparableTriple
There can be some cases where you may want to define a comparable relation between two objects. For example you create pairs of numeric values and want to sort those pairs. For this purpose I created the comparable versions of Pair and Triple.
For obvious reasons this classes are more complex and unfortunately cannot be safe for serialization. You may be able to serialize a ComparablePair but when you try to deserialize it you may get a exception.
To create a ComparablePair or a ComparableTriple the values need to be comparable or you need to provide a suitable Comparator. There are some factory methods to cover the most common use cases and, if you need to define a very specific combination of comparable and non-comparable values, you can use the builders.
For example if you need to define a triple <a,b,c> where class A is comparable, class B is not comparable but you want to define an ascending order and class C is not comparable and you want to define a descending order.
You can create a ComparableTriple like this:
Comparator<B> compB = ...; Comparator<C> compC = ...; ComparableTriple triple = ComparableTriple .left( a ) .middle( b, compB ) .right( c, compC );