collectors-teeing.json

{
  "id": 26,
  "slug": "collectors-teeing",
  "title": "Collectors.teeing()",
  "category": "collections",
  "difficulty": "intermediate",
  "jdkVersion": "12",
  "oldLabel": "Java 8",
  "modernLabel": "Java 12+",
  "oldApproach": "Two Passes",
  "modernApproach": "teeing()",
  "oldCode": "long count = items.stream().count();\ndouble sum = items.stream()\n    .mapToDouble(Item::price)\n    .sum();\nvar result = new Stats(count, sum);",
  "modernCode": "var result = items.stream().collect(\n    Collectors.teeing(\n        Collectors.counting(),\n        Collectors.summingDouble(Item::price),\n        Stats::new\n    )\n);",
  "summary": "Compute two aggregations in a single stream pass.",
  "explanation": "Collectors.teeing() sends each element to two downstream collectors and merges the results. This avoids streaming the data twice or using a mutable accumulator.",
  "whyModernWins": [
    {
      "icon": "⚡",
      "title": "Single pass",
      "desc": "Process the stream once instead of twice."
    },
    {
      "icon": "🧩",
      "title": "Composable",
      "desc": "Combine any two collectors with a merger function."
    },
    {
      "icon": "🔒",
      "title": "Immutable result",
      "desc": "Merge into a record or value object directly."
    }
  ],
  "support": {
    "state": "available",
    "description": "Widely available since JDK 12 (March 2019)"
  },
  "prev": "collections/sequenced-collections",
  "next": "collections/stream-toarray-typed",
  "related": [
    "collections/copying-collections-immutably",
    "collections/unmodifiable-collectors",
    "collections/stream-toarray-typed"
  ],
  "docs": [
    {
      "title": "Collectors.teeing()",
      "href": "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/stream/Collectors.html#teeing(java.util.stream.Collector,java.util.stream.Collector,java.util.function.BiFunction)"
    }
  ]
}