UnsupportedOperationException

Why This Happens

Collections returned by List.of(), Collections.unmodifiableList(), and Arrays.asList() do not support structural modification. Calling add(), remove(), or set() on them throws this exception. You need to copy the elements into a mutable collection first.

The Problem

List<String> list = List.of("a", "b", "c");
list.add("d"); // UnsupportedOperationException

The Fix

List<String> list = new ArrayList<>(List.of("a", "b", "c"));
list.add("d"); // Works on mutable ArrayList

Step-by-Step Fix

1. 1

   Identify the unmodifiable collection

   Check how the collection was created. List.of(), Map.of(), Collections.unmodifiable*(), and Arrays.asList() return unmodifiable views.

2. 2

   Determine if mutation is needed

   Decide whether you actually need to modify the collection or can work with it as-is.

3. 3

   Copy to a mutable collection

   Wrap the unmodifiable collection in new ArrayList<>(), new HashMap<>(), etc. to get a mutable copy.