nd4j-common/src/main/java/org/nd4j/util/StringUtils.java
public static String join(CharSequence separator, Iterable<?> strings) {
        Iterator<?> i = strings.iterator();
        if (!i.hasNext()) {
            return "";
        }
        StringBuilder sb = new StringBuilder(i.next().toString());
        while (i.hasNext()) {
            sb.append(separator);
            sb.append(i.next().toString());
        }
        return sb.toString();
    }
public static String join(CharSequence separator, Iterable<?> strings) {
        Iterator<?> i = strings.iterator();
        if (!i.hasNext()) {
            return "";
        }
        StringBuilder sb = new StringBuilder(i.next().toString());
        while (i.hasNext()) {
            sb.append(separator);
            sb.append(i.next().toString());
        }
        return sb.toString();
    }
Concatenates strings, using a separator.

@param separator Separator to join with.
@param strings Strings to join.