Converting Int value to List of Integer digits

Question

I have got an easy task to draw three int values. Then build a new int value from the highest digits from drawn values. Is there an easy way to convert an Int to List of Integers? I want to resolve this by streams but I need a List of Integers created from drawn value to find maxValue digit.

For example: 123, 256, 189 = 369

I have done this in a naive way, I'm still learning and start wondering if is an easier and more code efficient way.

public static List<Integer> convertingValue(int value) {
    List<Integer> intList = new ArrayList<>();
    char[] arr = String.valueOf(value).toCharArray();
    for (int i = 0; i < arr.length; i++) {
        intList.add(Character.getNumericValue(arr[i]));
    }
    return intList;
}

public static void main(String[] args) {
    IntSupplier is = () -> new Random().nextInt(999 - 100 + 1) + 100;
    List<Integer> figuresList =
            List.of(is.getAsInt(), is.getAsInt(), is.getAsInt());

    var list = figuresList
            .stream()
            .map(Basic02::convertingValue)
            .map(x -> x.stream().mapToInt(v -> v)
                    .max().orElseThrow(NoSuchElementException::new))
            .collect(Collectors.toList());

    System.out.println(list);
}

Answer 1

1. It would be better to use Random::ints(streamSize, origin, bound) to create a stream of random numbers
2. Convert each number into string, then use String::chars to get stream of characters for each number and find maximal digit for each number
3. To get better representation of the result it is worth to build a String instead of the List<Integer> using custom collector on the basis of StringBuilder . Then this string may be converted then into int/Integer if needed.

String num = new Random().ints(3, 100, 1000)
    .peek(System.out::println) // debug print
    .mapToObj(Integer::toString)
    .map(s -> s.chars().map(Character::getNumericValue).max().getAsInt())
    .collect(Collector.of(StringBuilder::new, StringBuilder::append, StringBuilder::append, StringBuilder::toString));
System.out.println("---\n" + num);

Output:

893
677
436
---
976

---

Existing code may also be refactored to return immediately the maximal digit instead of the list of integers:

public static int getMaxDigit(int value) {
    return String.valueOf(value)
        .chars() // IntStream
        .map(Character::getNumericValue)
        .max() // OptionalInt
        .getAsInt(); // int
}

Then List<Integer> may be collected as:

var list = new Random().ints(3, 100, 1000) // IntStream
    .map(MyClass::getMaxDigit)
    .boxed()
    .collect(Collectors.toList());

Or String may be collected in another way:

var str = new Random().ints(3, 100, 1000) // IntStream
    .map(MyClass::getMaxDigit)
    .mapToObj(Integer::toString)
    .collect(Collectors.joining());

Answer 2

Here is one approach that uses divison / and the remainder
operator % to break down the numbers into their digits.

• first, create an array of 3 values between 100 and 999 inclusive
• the stream each array and find the maximum digit of each value:
  • first, iterate each value divided by 10 .
    eg 248 -> 248, 24, 2
  • then get each digit by using the remainder operator.
    eg 248 % 10 = 8, 24 % 10 = 4, 2 % 10 = 2
• now find the maximum of those three digits.
• and reduce them by converting back to a three digit number.

for (int i = 0; i < 10; i++) {
    int[] values = r.ints(3, 100, 1000).toArray();
    int result = Arrays.stream(values)
            .map(vv -> IntStream
                    .iterate(vv, k -> k > 0, k -> k / 10)
                    .map(a -> a % 10).max().getAsInt())
            .reduce(0, (a, b) -> a * 10 + b);
    
    System.out.println(
            Arrays.toString(values) + " --> " + result);
}

Prints something like the following.

[772, 840, 915] --> 789
[981, 118, 981] --> 989
[924, 415, 507] --> 957
[293, 827, 740] --> 987
[476, 479, 858] --> 798
[414, 555, 163] --> 456
[737, 898, 855] --> 798
[496, 989, 526] --> 996
[369, 944, 279] --> 999
[148, 317, 115] --> 875

Answer 3

Using streams, you can convert numbers to strings, then split these strings into the arrays of digits, then take the maximum digits from the arrays and concatenate them back into a single string, and then finally parse an integer from that string:

public static void main(String[] args) {
    System.out.println(maxDigits(123, 256, 189)); //369
}

public static int maxDigits(int... numbers) {
    String max = Arrays.stream(numbers)
            // IntStream to Stream<String>
            .mapToObj(String::valueOf)
            // split each string into an array
            // of digits and take the maximum
            .map(str -> Arrays.stream(str.split(""))
                    .max(Comparator.naturalOrder())
                    .orElse(""))
            // join max digits into one string
            .collect(Collectors.joining());
    // return integer value
    return Integer.parseInt(max);
}