java.lang.UnsupportedOperationException (List.of / Arrays.asList is immutable)
Code tried to modify an immutable or fixed-size collection.
Seen on:
Java
Meaning
List.of(), Map.of(), Collections.unmodifiableList() and Stream.toList() are immutable; Arrays.asList() is fixed-size (no add/remove).
Common causes
- add/remove on List.of() or Stream.toList() results
- add on Arrays.asList()
- Modifying an unmodifiable view returned by a library
⚡ Quick fix
- Copy into a mutable collection: new ArrayList<>(list)
- Collect with Collectors.toList() / toCollection(ArrayList::new)
Detailed fix by platform
Java
- java
List<String> names = new ArrayList<>(List.of("a", "b")); names.add("c");
How to diagnose
- Collection — How was it created?
- Recent change — Did it start after a deploy, upgrade, or configuration change? Compare with the last working version.
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Last updated 13 Oct 2026