src/main/java/com/cedarsoftware/util/io/JsonWriter.java
public static boolean ensureJsonPrimitiveKeys(Map map)
    {
        for (Object o : map.keySet())
        {
            if (!(o instanceof String))
            {
                return false;
            }
        }
        return true;
    }
public static boolean ensureJsonPrimitiveKeys(Map map)
    {
        for (Object o : map.keySet())
        {
            if (!(o instanceof String))
            {
                return false;
            }
        }
        return true;
    }
Ensure that all keys within the Map are String instances
@param map Map to inspect that all keys are primitive.  This allows the output JSON
to be optimized into {"key1":value1, "key2": value2} format if all the
keys of the Map are Strings.  If not, then a Map is written as two
arrays, an @keys array and an @items array.  This allows support for Maps
with non-String keys.