You have an interesting definition of readable… You could use the same Map hack in Java too with some extra functions instead of just the if-else, it would still be messy. In Kotlin, I think you’d just need to do this:

(0..100).forEach { i ->
    println(when {
        i % 3 == 0 && i % 5 == 0 -> "FizzBuzz"
        i % 5 == 0 -> "Buzz"
        i % 3 == 0 -> "Fizz"
        else -> "$i"
    })
}

Which is the same as the Java version, except actually more readable. ;)

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For sake of completion, here is your Java equivalent for your Kotlin version:

private class MapBuilder<K, V> {
    List<Map.Entry<K, V>> list = new ArrayList<>();

    public MapBuilder<K, V> put(K key, V value) {
        list.add(new AbstractMap.SimpleEntry<>(key, value));
        return this;
    }

    public Map<K, V> build() {
        Map<K, V> map = new HashMap<>();
        for(Map.Entry<K, V> entry: list) {
            map.put(entry.getKey(), entry.getValue());
        }
        return Collections.unmodifiableMap(map);
    }
}

private <K, V> MapBuilder<K, V> mapBuilder() {
    return new MapBuilder<>();
}

@Test
public void javaFizz() {
    for(int i = 0; i <= 100; i++) {
        System.out.println(this.<Integer, String>mapBuilder()
                .put(0, String.valueOf(i))
                .put(i % 3, "Fizz")
                .put(i % 5, "Buzz")
                .put(i % 15, "FizzBuzz")
                .build().get(0));
    }
}

It’s not as memory efficient as the original Java version.