Code Quality Improvement - Session 65: Collection Isn't Just List

When implementing a bulkQuery() function that retrieves large amounts of user data in bulk, code quality varies significantly depending on how the argument and return value types are set. The existing approach of taking List<UserCategory> as an argument and returning List<Set<User>> has an implicit relationship, requiring results and categories to be matched via index.

This structure is hard to understand because it requires matching results using zip() and similar methods, making it a fragile structure where relationships can easily break. To improve this, the following changes are recommended.

• Change argument type: Change List<UserCategory> to Set<UserCategory> to prevent passing duplicate categories and to explicitly indicate that duplicates don't matter.
• Change return type: Change List<Set<User>> to Map<UserCategory, List<User>>. This explicitly expresses the relationship between categories and user data, improving the code's readability and reusability.

Additionally, when the order of data matters, it's better to use List instead of Set. In particular, performing sorting at the database layer has performance benefits, and adding a comment about the sort order can further improve code readability.