How do I pull out fields from a custom class using streams? (Without using for/while loops)

I would like to know how to pull out field and make it become a list using streams. For example, if I have a list of Pairs, Pair[2] { Pair<3,5>,Pair<5,7>}, I would like to make it become Integer[3] {3,5,7}. (After using Stream.distinct() to remove the dupicate 5) Pair class is provided if that is needed.

public class Pair<T,U> {
    private final T first;
    private final U second;

    public Pair(T first, U second){
        this.first = first;
        this.second = second;
    }

    T first() {
        return this.first;
    }

    U second(){
        return this.second;
    }

    @Override
    public String toString(){
        return "<" + this.first + ", "+ this.second+">";
    }
}

Answer

You want to turn your stream of pairs into a stream of numbers; to do so, any given pair gets mapped into a variable number of items in the stream. .map can only map one thing to one other thing. flatMap, on the other hand, can map one thing to any number (even 0) things.

flatMap turns an element of the stream, into a stream of whatever you want – these streams are then concatenated together to form a new single stream.

Thus, all you need to do is to turn a Pair<Integer, Integer> instance into a stream representing the 2 integers. Java will do the rest:

pairs.stream().flatMap(pair -> Stream.of(p.first(), p.second())).distinct().forEach(System.out::println);

That would end up printing 3, 5, and 7, as desired.

NB: Pair is bad java style. Java is extensively nominal: Its designed around the notion that things are named. Those pairs of numbers represent something more specific than ‘a pair of numbers’. You should make a class that models that exact thing. The alternative is that we do away with, say, a Person class and instead make a Tuple<String, Tuple<Integer, Integer, Integer>, String> instead of a class with a field String name;, LocalDate birthDate;, and String studentId; or whatnot. Clearly a bad idea.