Today, I will write about a Map anti-pattern that I have sometimes observed.<\/p>\n
I have come across code as follows from time to time:<\/p>\n
Map<String, Object> json = new HashMap<>(); \u00a0 \/\/bad don't do this!!!\n\n\/\/ or\n\nMap<String, Object> parameters = new HashMap<>(); \/\/again don't do this<\/pre>\nI suggested to\u00a0the developer who wrote similar code to use a class instead of a Map. \u00a0I explained\u00a0that a Map should be used for mapping problems, such as associating data with key value pairs.<\/p>\n
The developer was upset, and argued back that\u00a0using a class instead of a map is\u00a0over-engineering. I interpreted this as\u00a0some sort of logical fallacy “Ad Hominem” attack against classes. I exclaimed “Using a map is under-engineering!\u00a0At the very least, we can introduce an interface\u00a0that functions\u00a0a marker class, and let Jackson perform the transformations?” This developer would not budge, and stated that she did not want to write a class for everything that will become serialized as JSON. I stepped back, gave her the benefit of the doubt, looked further at the code, noted the pervasiveness of this anti-pattern, and now I am writing this.<\/p>\n
<\/p>\n
Deeper analysis of\u00a0the code affirmed my conviction that classes should not be substituted with maps. Now I will attempt to objectively make my case.<\/p>\n
My most compelling reason is that it is not cohesive. \u00a0When maps are created, fields are assigned at different places within the code, and ultimately are consumed at different sections of the code. This production and consumption of the data loses its proximity from the location of its original declaration. It would often happen in a different method, a different class, or even a different library. This forces the consumer to search through multiple modules, methods, or classes in order to discover which data resides within the map. This a time-consuming and error prone process that takes much more time than just creating a plain ordinary Java object. Then after time is spent on this lookup, the following problems still exist:<\/p>\n
If you need serialize data\u00a0then introduce a class that represents that data and then use a serialization framework that can perform the serialization for you.<\/p>\n
ObjectMapper mapper = new ObjectMapper();\nString json = mapper.writeValueAsString(anyObjectWithGetters);<\/pre>\nIf you need to read JSON into a general purpose data-structure (situations where you don’t know what class you are parsing, then use Jackson’s JsonNode class.<\/p>\n
If you need to pass data to other classes or methods, then again, introduce a class. You’ll find the code much more cohesive, readable\u00a0and safe.<\/p>\n