Java generic type check for classes with two generic paramters should only hold classes of same type

Question

I have a situation where I want to compare a List of Pairs in Java. I'll be using this Pair -interface from Apache.

A Pair's Left and Right value can be of a type that implements Comparable, so I declare the List as follows:

List<Pair<Comparable, Comparable>> listOfPairs;

The Left and Right value should always be of the same type, eg Pair<String, String> or Pair<Integer, Integer> , and both kinds of Pairs could be added to the same list.

This works:

    Pair<Comparable, Comparable> p1 = new ImmutablePair<>("a", "b");
    Pair<Comparable, Comparable> p2 = new ImmutablePair<>(2, 3);

    listOfPairs.add(p1);
    listOfPairs.add(p2);

This also works:

    Pair<Comparable, Comparable> p3 = new ImmutablePair<>("a", 2);
    listOfPairs.add(p3);

Is it possible to have a type check, to ensure that a Pair's types are of the same type, once it is added to the list, eg fail at compile time?

Answer 1

Yes, you can implement your own class that either extends ImmutablePair or the Pair class.

example:

public class MyPair<T> extends Pair<Comparable<T>, Comparable<T>> {

public MyPair(T key, T value) {

}

}

When you create an instance of such class, the generic T type will always be the same for both arguments.

Answer 2

Yes, if that variable is a local variable, then you can add a type parameter to the method where it's declared.

public <T extends Comparable> void myMethod() {
    ...
    List<Pair<T, T>> listOfPairs;
    ...

If they're fields, you can add a type parameter to the class.

public class MyClass<T extends Comparable> {
    ...
    private List<Pair<T, T>> listOfPairs;
    ...

Another alternative, if neither of these suits your situation, is to define your own Pair generic class something like this with no members, then use it in place of Pair as required. You'll need to write a constructor in this class, which just delegates to the superclass constructor

public class SelfPair<T> extends Pair<T,T> {
    public SelfPair(T first, T second) {
        super(first, second);
    }
}