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What you'll learn

• List.of() / List.copyOf() return immutable, null-hostile lists; List.copyOf() takes a snapshot that is unaffected by later mutation of the source
• Collections.unmodifiableList() is a live view: it blocks direct mutation through itself, but still reflects changes made to the backing list
• Stream.toList() guarantees an unmodifiable result, while Collectors.toList() makes no mutability guarantee (it happens to return a mutable ArrayList today)
• SequencedCollection / SequencedMap add getFirst() / getLast() / reversed() uniformly across List, LinkedHashSet and LinkedHashMap

Why it matters

Picking the wrong "immutable-looking" collection factory is a common source of bugs — an UnsupportedOperationException at runtime, or a list that mutates under you because it was only ever a view. Knowing which guarantee each API actually makes avoids both.

Read more

JEP 431: Sequenced Collections

Quiz

var backing = new ArrayList<>(List.of("Ann", "Bob"));
var view = Collections.unmodifiableList(backing);
backing.add("Cid");
System.out.println(view.size());

What does this print?

Answer

3 — `unmodifiableList()` wraps the same backing list rather than copying it, so changes made directly to `backing` are visible through `view`. Only mutation attempted *through* `view` itself (e.g. `view.add(...)`) throws `UnsupportedOperationException`.

Note on local verification: this environment only has JDK 21 installed, and the Gradle build requires JDK 25 via toolchain; JDK 25 auto-provisioning (foojay) and all alternative JDK download hosts tried were blocked by this session's network policy, so I couldn't run ./gradlew here. All APIs used (List.of/copyOf, Stream.toList(), SequencedCollection/SequencedMap) are available since JDK 21, and I compiled and ran the test with the JUnit Console launcher against JDK 21 to confirm all 7 assertions pass. Opening as a draft until CI (JDK 25) confirms.

Closes #95

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Test Results

294 tests  +7   291 ✅ +7   4s ⏱️ -1s
 82 suites +1     3 💤 ±0 
 82 files   +1     0 ❌ ±0 

Results for commit 7fc0041. ± Comparison against base commit ea35b4f.

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