public K firstKey() {
        if (size() == 0) {
            throw new NoSuchElementException("Map is empty");
        }
        return insertOrder.get(0);
    }
Gets the first key in this map by insert order.

@return the first key currently in this map
@throws java.util.NoSuchElementException if this map is empty
jphp-runtime/src/php/runtime/common/collections/map/ListOrderedMap.java