Java - Retrive Indivudual Values from a List in a Map

Question

I have a TreeMap with 3 entries, all from individual ArrayLists.

I use the following code:

Map<String, List<String>> mapOne = new TreeMap<String, List<String>>();
List<String> listFour = Arrays.asList("");

ArrayList<String> listOne = new ArrayList<>();
listOne.add("Writer");
listOne.add("Actor");
listOne.add("Politician");
listOne.add("Dancer");

ArrayList<String> listTwo = new ArrayList<>();
listTwo.add("James");
listTwo.add("Robert");
listTwo.add("Tereza");
listTwo.add("John");

ArrayList<String> listThree = new ArrayList<>();
listThree.add("Joyce");
listThree.add("Redford");
listThree.add("May");
listThree.add("Travolta");

for (int i = 0; i < listOne.size(); i++) {

    String stringOne = listOne.get(i);
    String stringTwo = listTwo.get(i);
    String stringThree = listThree.get(i);

    listFour = Arrays.asList(stringTwo, stringThree);

    mapOne.put(stringOne, listFour);

}

Now I want to obtain the individual String values from the sorted list. like so:

for (Map.Entry<String, List<String>> entry : mapOne.entrySet()) {

    String key = entry.getKey();

    for (String value : entry.getValue()) {

        System.out.println(value);

    }
}

The above code prints a list like

{Robert Redford , John Travolta , Tereza May , James Joyce} 

Is it possible to iterate over the list in a way as to obtain to separate lists, one with the first names and the other with the last names?

listOne = {Robert , John , Tereza, James}
listTwo = {Redford, Travolta, May, Joyce}

Or should I use an entirely different approach?

The whole thing started out with the need to sort one ArrayList and to other accordingly. What seemed trivial at the beginning, turned out to be a real challenge.

I am a sort of a hobby programmer, so the pros out there kindly bear with me.

Answer 1

It seems you're over-engineering somehow.

I will answer with a Stream solution first, just for the sake of trying it.
Note that I'd, personally, prefer the "old" iterative approach (see below).

// You can see by the use of AtomicInteger that this isn't the right road to take!
final AtomicInteger i = new AtomicInteger();
final Collection<List<String>> values1 =
        mapOne.values()
              .stream()
              .flatMap(v -> v.stream())
              .collect(partitioningBy(o -> i.getAndIncrement() % 2 != 0))
              .values();

Output: [[Robert, John, Tereza, James], [Redford, Travolta, May, Joyce]]

---

Iterative approach

final Collection<String> names = new ArrayList<>();
final Collection<String> surnames = new ArrayList<>();

for (final List<String> value : mapOne.values()) {
    names.add(value.get(0));
    surnames.add(value.get(1));
}

Output:

[Robert, John, Tereza, James]
[Redford, Travolta, May, Joyce]

This is safe because you know each inner List has two elements.

---

What JB Nizet is telling you to do (credit to him for writing that), is basically to create an appropriate class

public class Person {
   private String profession;
   private String name;
   private String surname;

   // Getters and setters. JavaBean style
}

And proceed to sort a Collection<Person>.
For example, keeping it as simple as possible

final List<Person> persons = new ArrayList<>();

// Populate list

Collections.sort(persons, (p1, p2) -> {
    // Not null-safe
    return p1.getName().compareTo(p2.getName());
});

This will sort the list by name. It will not return a new List, but simply modify the input one.