MultiImageFileSource.java

@Override
    public synchronized void setValueWithKey(String key) throws IOException, IllegalArgumentException {
        final List<String> keys = getKeys();
        if (keys.contains(key)) {
            index = keys.indexOf(key);
            loadImage(paths.get(index));
        } else {
            throw new IllegalArgumentException(key + " was an illegal key");
        }
    }

    @Override
    public List<String> getKeys() {
        return Collections.unmodifiableList(paths.stream().map(path -> Files.getNameWithoutExtension(path)).collect(Collectors.toList()));
    }