🔸 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
// ✕ Java 8: two passes long count = items.stream().count(); double sum = items.stream() .mapToDouble(Item::price) .sum(); var result = new Stats(count, sum); // ✓ Java 12+: one pass with teeing() var result = items.stream().collect( Collectors.teeing( Collectors.counting(), Collectors.summingDouble(Item::price), Stats::new ) );
🔸 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
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