Most Java developers learn map() pretty quickly when working with Streams.
But things start getting confusing when dealing with nested collections.
For example, imagine a simple backend model:
Customer → Accounts → Transactions
If you try to extract transactions using map(), you might end up with something like:
List<List<List<Transaction>>>
Not exactly helpful.
In this article, I explain how flatMap() solves this problem using practical examples from a banking-style backend system, including:
- Flattening nested collections
- Filtering high-value debit transactions
- Generating transaction statements
- Fraud detection example
- map() vs flatMap() comparison
Interview questions
If you're working with Java Streams, especially with nested data structures, this might help.
👉 Read the full article here:
https://medium.com/@pramod.er90/stop-using-map-for-nested-collections-use-flatmap-instead-2276dbb4fa78
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