An exercise to create a function that, given a string, returns the most commonly-used letter (or letters, in the case of a tie). It should only consider letters and should gracefully handle strings that do not contain letters.

MostCommonLetter.java

import junit.framework.Assert;
import org.junit.Test;

import java.util.*;

public class MostCommonLetterTests {

    public Character[] mostCommonLetter(String string) {
        if (string.isEmpty())
            throw new IllegalArgumentException("Empty string can not be evaluated for most common letter.");

        Map<Character, Integer> counter = new HashMap<>();

        for (int i = 0; i < string.length(); i++) {
            char character = string.charAt(i);

            if (!Character.isLetter(character))
                continue;

            int count = 1;

            if (counter.containsKey(character))
                count += counter.get(character);

            counter.put(character, count);
        }

        if (counter.isEmpty())
            throw new IllegalArgumentException("No letters found in string.");

        String mostCommon = "";
        Integer highestCount = 0;

        for (Map.Entry<Character, Integer> entry : counter.entrySet()) {
            Character character = entry.getKey();
            Integer count = entry.getValue();
            if (count.equals(highestCount)) {
                mostCommon += character.toString();
                highestCount = count;
            }
            else if (count > highestCount) {
                mostCommon = character.toString();
                highestCount = count;
            }
        }

        Character[] result = new Character[mostCommon.length()];

        for (int i = 0; i < result.length; i++) {
            result[i] = mostCommon.charAt(i);
        }

        return result;
    }

    @Test
    public void mostCommonLetterSucceedsA() {
        Character[] character = mostCommonLetter("AABC");
        Assert.assertTrue(character[0].equals('A'));
        Assert.assertEquals(1, character.length);
    }

    @Test
    public void mostCommonLetterSucceedsB() {
        Character[] character = mostCommonLetter("ABBC");
        Assert.assertTrue(character[0].equals('B'));
        Assert.assertEquals(1, character.length);
    }

    @Test
    public void mostCommonLetterSucceedsC() {
        Character[] character = mostCommonLetter("ABCC");
        Assert.assertTrue(character[0].equals('C'));
        Assert.assertEquals(1, character.length);
    }

    @Test
    public void mostCommonLetterSucceedsAB() {
        Character[] character = mostCommonLetter("AABB");
        Assert.assertTrue(character[0].equals('A'));
        Assert.assertTrue(character[1].equals('B'));
        Assert.assertEquals(2, character.length);
    }

    @Test(expected = IllegalArgumentException.class)
    public void mostCommonLetterFailsEmptyString() {
        mostCommonLetter("");
    }

    @Test(expected = IllegalArgumentException.class)
    public void mostCommonLetterFailsNoLetters() {
        mostCommonLetter("!!!");
    }

    @Test(expected = NullPointerException.class)
    public void mostCommonLetterFailsNullValue() {
        mostCommonLetter(null);
    }

}