FunctionalFooBarQix.java

package fr.mga.foobarqix;

import java.util.HashMap;
import java.util.Map;
import java.util.stream.IntStream;

import static java.lang.System.out;
import static java.util.Collections.unmodifiableMap;
import static java.util.stream.Collectors.joining;

public class FunctionalFooBarQix {

    private static final Map<Integer,String> digitReplacements = digitReplacements();

    private static Map<Integer, String> digitReplacements() {
        Map<Integer,String> digitReplacements = new HashMap<>();
        digitReplacements.put(3,"FOO");
        digitReplacements.put(5,"BAR");
        digitReplacements.put(7, "QIX");
        return unmodifiableMap(digitReplacements);
    }


    public static String fooBarQix(int number) {
        String result = "";
        result += digitReplacements.entrySet().stream()
                .filter(replacement -> number % replacement.getKey() == 0)
                .map(Map.Entry::getValue)
                .collect(joining());

        result += String.valueOf(number).codePoints()
                .mapToObj(Functional::handleDigit)
                .collect(joining());

        return result.isEmpty() ? String.valueOf(number) : result;
    }

    /**
     * @param digitChar NOT the numeric value of the value, rather the character representing it (
     * for instance digitChar=49 represents char '1')
     * @return
     */
    private static String handleDigit(int digitChar) {
        int digit = Character.getNumericValue(digitChar);
        return digitReplacements.getOrDefault(digit, "");
    }
    
    public static void main(String[] args) {
        IntStream.rangeClosed(1, 100)
                .mapToObj(Functional::fooBarQix)
                .forEach(out::println);
    }
}

FunctionalFooBarQixTest.java

package fr.mga.foobarqix;

import org.junit.Test;

import java.util.stream.IntStream;

import static org.fest.assertions.api.Assertions.assertThat;

public class FunctionalFooBarQixTest {

    @Test
    public void should_work_as_ugly_foobarqix() {
        IntStream.range(1, 100).forEach(number -> assertThat(
                Functional.fooBarQix(number)
            ).isEqualTo(uglyFooBarQix(number))
        );
    }

    private static String uglyFooBarQix(int i) {
        String result = "";
        if(i %3 == 0) result += "FOO";
        if(i %5 == 0) result += "BAR";
        if(i %7 == 0) result += "QIX";

        String integer = String.valueOf(i);
        for (int j = 0; j < integer.length(); j++) {
            char element = integer.charAt(j);
            if(element == '3') result += "FOO";
            if(element == '5') result += "BAR";
            if(element == '7') result += "QIX";
        }

        return result.isEmpty() ? integer : result;
    }
}