unmodifiable-collectors.json

{
  "id": 28,
  "slug": "unmodifiable-collectors",
  "title": "Unmodifiable collectors",
  "category": "collections",
  "difficulty": "intermediate",
  "jdkVersion": "16",
  "oldLabel": "Java 8",
  "modernLabel": "Java 16+",
  "oldApproach": "collectingAndThen",
  "modernApproach": "stream.toList()",
  "oldCode": "List<String> list = stream.collect(\n    Collectors.collectingAndThen(\n        Collectors.toList(),\n        Collections::unmodifiableList\n    )\n);",
  "modernCode": "List<String> list = stream.toList();",
  "summary": "Collect directly to an unmodifiable list with stream.toList().",
  "explanation": "Java 10 added toUnmodifiableList(), toUnmodifiableSet(), and toUnmodifiableMap() to replace the verbose collectingAndThen wrapper. For lists specifically, Java 16's stream.toList() provides an even simpler alternative — no collect() call at all. Use toUnmodifiableSet() and toUnmodifiableMap() for other collection types.",
  "whyModernWins": [
    {
      "icon": "📏",
      "title": "Shortest yet",
      "desc": "stream.toList() needs no collect() or Collectors import at all."
    },
    {
      "icon": "🔒",
      "title": "Immutable",
      "desc": "Result cannot be modified — no accidental mutations."
    },
    {
      "icon": "📖",
      "title": "Readable",
      "desc": "Reads naturally as the terminal step of any stream pipeline."
    }
  ],
  "support": {
    "state": "available",
    "description": "Widely available since JDK 16 (March 2021)"
  },
  "prev": "collections/reverse-list-iteration",
  "next": "strings/string-isblank",
  "related": [
    "collections/immutable-map-creation",
    "collections/immutable-set-creation",
    "streams/stream-tolist"
  ],
  "docs": [
    {
      "title": "Stream.toList()",
      "href": "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/stream/Stream.html#toList()"
    },
    {
      "title": "Collectors.toUnmodifiableList()",
      "href": "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/stream/Collectors.html#toUnmodifiableList()"
    }
  ]
}