Add 0 to the left in an ArrayList Integer

You can try this way too, Instead of using everything as "Integer" already start working with "String". But the cat's jump is even in control: String.format("%02d", NumeroQueVoceQuerFormatar) .

private static String sortear() {

        Set<String> numberset = new HashSet<>();

        Random random = new Random();

        while (numberset.size() < 6) {
            String numero = String.format("%02d", (random.nextInt(60) + 1));
            numberset.add(numero);
        }

        ArrayList<String> jogo = new ArrayList<>(numberset);
        Collections.sort(jogo);

        String vetor = Arrays.toString(jogo.toArray());

        return vetor;
    }

This was the result: [08, 21, 25, 33, 34, 48]

Try this:

import java.util.stream.Collectors;
import java.util.Random;
import java.util.SortedSet;
import java.util.TreeSet;

class Teste {
    private static final Random RND = new Random();

    private static String comZero(int x) {
        return (x < 10 ? "0" : "") + x;
    }

    private static String sortear() {
        SortedSet<Integer> numberset = new TreeSet<>();
        while (numberset.size() < 6) {
            numberset.add((RND.nextInt(60) + 1));
        }
        return numberset.stream().map(Teste::comZero).collect(Collectors.toList()).toString();
    }
}

I tested this with this code:

public static void main(String[] args) {
    System.out.println(sortear());
}

Here's the output:

[01, 28, 34, 46, 48, 49]

See here working on ideone.

You can create a string array just to handle this return and use the string format method, like this:

private static String sortear() {
    Set<Integer> numberset = new HashSet<>();
    Random random = new Random();

    while (numberset.size() < 6) {
        numberset.add((random.nextInt(60) + 1));
    }

    ArrayList<Integer> jogo = new ArrayList<>(numberset);
    Collections.sort(jogo);

    List<String> jogoString = new ArrayList<String>();

    for (Integer num : jogo) {
        //%02d diz que vc quer uma string com tamanho de 2 digitos
        jogoString.add(String.format("%02d", num));
    }


    String vetor = Arrays.toString(jogoString.toArray());

    return vetor;

}

  

But for numbers smaller than 10 I'd like to add 0 before   type: 01.02 ... 10.

Only with String.format("%02d", 3); already solves this formatting you need.