Java: how to transform from List to Map without iterating

Question

I have a list of objects that I need to transform to a map where the keys are a function of each element, and the values are lists of another function of each element. Effec

Answer 1

Now with Java8 you can do it like:

static class Element {
    final int f1;
    final String f2;

    Element(int f1, String f2) {
        this.f1 = f1;
        this.f2 = f2;
    }

    int f1() { return f1;}
    String f2() { return f2; }
}

public static void main(String[] args) {
    List elements = new ArrayList<>();
    elements.add(new Element(100, "Alice"));
    elements.add(new Element(200, "Bob"));
    elements.add(new Element(100, "Charles"));
    elements.add(new Element(300, "Dave"));

    elements.stream()
            .collect(Collectors.groupingBy(
                    Element::f1,
                    Collectors.mapping(Element::f2, Collectors.toList())
                    ))
            .forEach((f1, f2) -> System.out.println("{"+f1.toString() + ", value="+f2+"}"));
}