Groupify

Groupify.java

public final class Groupify {
    public static String groupify(final String input, final int groupSize) {
        if (groupSize < 1) {
            throw new IllegalArgumentException("groupSize must be bigger than zero, was " + groupSize);
        }

        final String inputPadded = input + "x".repeat((groupSize - input.length() % groupSize) % groupSize);

        final String[] split = inputPadded.split("(?<=\\G.{" + groupSize + "})");

        return String.join(" ", split);
    }
}

GroupifyTest.java

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

class GroupifyTest {
    @Test
    void groupify() {
        assertEquals("HI TH ER Ex", Groupify.groupify("HITHERE", 2));
        assertEquals("DISC ORDx", Groupify.groupify("DISCORD", 4));
        assertEquals("OHMY", Groupify.groupify("OHMY", 4));
        assertEquals("test1234567xxxxxxxxx", Groupify.groupify("test1234567", 20));
        assertEquals("w o w t h i s i s s u c h a l o n g s e n t e n c e . . .", Groupify.groupify("wowthisissuchalongsentence...", 1));
        assertEquals("", Groupify.groupify("", 1));
        assertThrows(NullPointerException.class, () -> Groupify.groupify(null, 1));
        assertThrows(IllegalArgumentException.class, () -> Groupify.groupify("noo", -1));
        assertThrows(IllegalArgumentException.class, () -> Groupify.groupify("yikes", 0));
    }
}