LargestNumber.java

import java.util.Arrays;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class LargestNumber
{
    public static void main(String[] args)
    {
        String number = IntStream.range(1, 31)
            .mapToObj(String::valueOf)
            .collect(Collectors.joining());

        int[] values = number.chars()
            .map(x -> x - '0')
            .toArray();

        int len = number.length();

        System.out.println(number);
        System.out.println(Arrays.toString(values));
        System.out.println(len);

        int max = 0;
        for (int i = 0; i < len; i++)
            for (int j = i + 1; j < len; j++)
                for (int k = j + 1; k < len; k++)
                    for (int l = k + 1; l < len; l++)
                        for (int m = l + 1; m < len; m++)
                            for (int n = m + 1; n < len; n++)
                                for (int o = n + 1; o < len; o++)
                                    max = Math.max(max, join(values, i, j, k, l, m, n, o));

        System.out.println(max);
    }

    private static int join(int[] values, int i, int j, int k, int l, int m, int n, int o)
    {
        return values[i] * 1_000_000
            + values[j] * 100_000
            + values[k] * 10_000
            + values[l] * 1000
            + values[m] * 100
            + values[n] * 10
            + values[o];
    }
}