🔸 TLDR

▪️ Need two aggregations? teeing() does it in one collect with a clean immutable merge.

🔸 THE PROBLEM

Ever needed two aggregations (ex: count + sum) on the same stream?

The “classic” approach often streams the data twice — simple, but not always ideal.

🔸 CODE: OLD VS MODERN

🔸 SEE A PROBLEM WITH THIS CODE? LET US KNOW 👀

Hint: there’s a subtle one depending on what items is… 😉

🔸 WHY THE MODERN WAY WINS

▪️ ⚡ Single pass: stream once, aggregate twice

▪️ 🧩 Composable: mix any 2 collectors (min/max, summary stats, grouping, etc.)

▪️ 🔒 Immutable result: merge straight into a record / value object

🔸 HOW IT WORKS (IN 1 SENTENCE)

Collectors.teeing() routes each element to two downstream collectors, then merges both results using your merger function (Stats::new here).

🔸 TAKEAWAYS

▪️ Prefer teeing() when you want two results from one stream without mutable accumulators

▪️ Great fit for Stats/DTO/record building

▪️ Still keep readability in mind: for tiny lists, two passes can be totally fine ✅

#Java #Java12 #Streams #Collectors #FunctionalProgramming #CleanCode

#Performance #BackendDevelopment #JVM

https://javaevolved.github.io/collections/collectors-teeing.html

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