Query with Java 8 parameter to generate random numbers using Collectors.toList()

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Query with Java 8 parameter to generate random numbers using Collectors.toList()

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I have this method to generate random numbers in a ArrayList;

public static List<Integer> gerarAleatorio() {

        List<Integer> aleatorios = new ArrayList<>();

        while(aleatorios.size() < 6) {

            int num = (int) (1  + Math.random() * 60);

            if(!aleatorios.contains(num)) {
                aleatorios.add(num);
            }

        }
        return aleatorios;
    }

My question is why I can’t do this method:

public static void gerarAleatorio(List<Integer> lista) {
lista = aleatorios.stream().collect(Collectors.toList());

and receive the random list at the end, with the Collectors.

1

The second code is incomplete. Try to complete it so the answer can be appropriate.

– E.Thomas

2016/12/22 at 19:00
It gives some compilation error saying that aleatorios was not declared in its second method? Moreover, where does its second method call the Math.random()?

– Victor Stafusa

2016/12/22 at 20:28

1 answer

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by Victor Stafusa • **63,338** points · Answer 1 · 2016-12-22T20:34:10+00:00

Try to do so:

private static int gerarNumeroAleatorio() {
    return (int) (1 + Math.random() * 60);
}

public static List<Integer> gerarAleatorio() {
    return Stream.generate(EstaClasse::gerarNumeroAleatorio)
        .distinct()
        .limit(6)
        .collect(Collectors.toList());
}

Explanation:

The method gerarNumeroAleatorio() is self-explanatory.

Already the method gerarAleatorio(), begins with...

... an infinite sequence of elements provided by the method gerarNumeroAleatorio(), ...
... but without repeating elements, ...
... limited to only 6 elements (and therefore no longer infinite) and ...
... collect the result of this turning it into a list.