UnsupportedOperationException ☕ Java

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

  1. Copy into a mutable collection: new ArrayList<>(list)
  2. Collect with Collectors.toList() / toCollection(ArrayList::new)

Detailed fix by platform

Java

  1. java
    List<String> names = new ArrayList<>(List.of("a", "b"));
names.add("c");

How to diagnose

  1. Collection — How was it created?
  2. Recent change — Did it start after a deploy, upgrade, or configuration change? Compare with the last working version.

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