Find out how to Carry out Alphabetical Addition in Java

The problem

Your activity is so as to add up letters to at least one letter.

The perform will probably be given an array of single-character Strings, each being a letter so as to add.

Notes:

• Letters will at all times be lowercase.
• Letters can overflow (see second to final instance of the outline)
• If no letters are given, the perform ought to return 'z'

Examples:

addLetters("a", "b", "c") = "f"
addLetters("a", "b") = "c"
addLetters("z") = "z"
addLetters("z", "a") = "a"
addLetters("y", "c", "b") = "d" // discover the letters overflowing
addLetters() = "z"

The answer in Java code

Possibility 1:

public class Answer {

    public static String addLetters(String... letters) {
        String abc = "zabcdefghijklmnopqrstuvwxyz";
        int sum = 0;
        for (int i = 0; i < letters.size; i++) {
            sum += abc.indexOf(letters[i]);
        }
        return String.valueOf(abc.charAt(sum % 26));
    }
}

Possibility 2:

import static java.util.stream.Stream.of;

class Answer {
  static String addLetters(String... letters) {
    int sum = of(letters).mapToInt(l -> l.charAt(0) - 96).sum() % 26 + 96;
    return "" + (char) (sum != 96 ? sum : 122);
  }
}

Possibility 3:

import java.util.Arrays;

public class Answer {
  public static String addLetters(String... letters) {
    int sum = Arrays.stream(letters)
                    .flatMapToInt(String::chars)
                    .map(letter -> letter == 'z' ? 0 : letter - 'a' + 1)
                    .sum();
    int overflow = sum % 26;
    return overflow == 0 ? "z" : Character.valueOf((char)(overflow + 'a' - 1)).toString();
  }
}

Check circumstances to validate our answer

import org.junit.Check;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;
import java.util.Arrays;
import java.util.Random;

public class SolutionTest {
    @Check
    public void fixedTests() {
        assertEquals("f", Answer.addLetters("a", "b", "c"));
        assertEquals("z", Answer.addLetters("z"));
        assertEquals("c", Answer.addLetters("a", "b"));
        assertEquals("c", Answer.addLetters("c"));
        assertEquals("a", Answer.addLetters("z", "a"));
        assertEquals("d", Answer.addLetters("y", "c", "b"));
        assertEquals("z", Answer.addLetters());
    }
    
    non-public String answer(String... letters) {
        int sum = Arrays.stream(letters).mapToInt(s->s.charAt(0)-96).sum() % 26;
        return sum == 0 ? "z" : String.valueOf((char)(sum+96));
    }
    
    @Check
    public void randomTests() {
        Random rnd = new Random();
        for (int i = 0; i < 100; ++i) {
            String[] letters = rnd.ints(97, 123)
                                  .restrict(rnd.nextInt(10) + 1)
                                  .mapToObj(c -> String.valueOf((char) c))
                                  .toArray(String[]::new);
            assertEquals(answer(letters), Answer.addLetters(letters));
        }
    }
}