Generic pair class

Question

Just attempting this question I found in a past exam paper so that I can prepare for an upcoming Java examination.

Provide a generic class Pair for representing pairs of things. The class should provide a constructor, a method for getting the first member of the pair, a method for getting the second member of the pair, a method for setting the first member of the pair, a method for setting the second member of the pair. The class should be parameterised over two types one for the first member and one for the second member of the pair.

Is this a correct implementation for this question ?

public class Pair<firstThing, secondThing>{
   private firstThing first;//first member of pair
   private secondThing second;//second member of pair

   public Pair(firstThing first, secondThing second){
     this.first = first;
     this.second = second;
   }

   public void setFirst(firstThing first){
    this.first = first;
   }

   public void setSecond(secondThing second) {
     this.second = second;
   }

   public thing getFirst() {
     return this.first;
   }

   public thing getSecond() {
     return this.second;
   }
}

Answer 1

Almost. I'd write it like this:

public class Pair<F, S> {
    private F first; //first member of pair
    private S second; //second member of pair

    public Pair(F first, S second) {
        this.first = first;
        this.second = second;
    }

    public void setFirst(F first) {
        this.first = first;
    }

    public void setSecond(S second) {
        this.second = second;
    }

    public F getFirst() {
        return first;
    }

    public S getSecond() {
        return second;
    }
}

Edit: I agree with @karmakaze's comment. The code should skip the setters and make first and second final to keep it immutable.

Answer 2

The need for a Pair class usually crops up in larger projects - I'm about to (re)implement one for the current project (as previous implementations are not accessible).

Generally I make it an immutable POJO, with a convenience function to create instances. For example:

public class Pair<T,U>
{
    public final T first;
    public final U second;
    public static <T,U> Pair<T,U> of(T first, U second);
}

So that the end-user can write:

return Pair.of (a, b);

and

Pair<A,B> p = someThing ();
doSomething (p.first);
doSomethingElse (p.second);

As mentioned above, the Pair class should also implement hashCode(), equals(), optional-but-useful toString(), as possibly clone() and compareTo() for use where these are supported by T and U - though extra work is required to describe how these contracts are supported by the Pair class.