Companion code for the ankurm.com post "Sequenced Collections in Java 21+: getFirst, getLast and reversed()." Module in java-core-examples, the Java-core series.

Versions this was built and tested against

Component	Version	Notes
JDK	25.0.4.1+1 (Temurin, LTS)	JEP 431 shipped in JDK 21 (GA September 2023) and is unchanged since.
JUnit Jupiter	5.11.0
Maven	3.9.11

Quickstart

export JAVA_HOME=/path/to/jdk-21-or-newer
mvn compile
java -cp target/classes com.ankurm.sequenced.AcrossCollectionsDemo
java -cp target/classes com.ankurm.sequenced.FindLastDemo
java -cp target/classes com.ankurm.sequenced.ReversedViewIsLiveDemo

scripts/run-all.sh regenerates every file in output/. scripts/run.sh <ClassName> runs one demo ad hoc.

What's in here

File	What it shows
`src/main/java/.../AcrossCollectionsDemo.java`	`getFirst`/`getLast`/`addFirst`/`addLast`/`reversed()` run against `ArrayList`, `ArrayDeque`, `LinkedHashSet`, `LinkedHashMap` and `TreeMap` - the one shared contract across five previously-unrelated types.
`src/main/java/.../FindLastDemo.java`	The three pre-Java-21 ways people found "the last element" of a `Set` (two stream tricks and a throwaway copy) next to the one-line `getLast()` replacement, with a note on why the old ways cost more than they looked like they did.
`src/main/java/.../ReversedViewIsLiveDemo.java`	`reversed()` is a live view, not a copy - mutating either side is visible through the other, immediately.
`src/test/java/.../SequencedCollectionsTest.java`	Pins all of the above as assertions, 6/6 passing.
`output/00-javap-sequenced-interfaces.txt`	`javap` output for `SequencedCollection`, `SequencedMap`, `SequencedSet` straight from the compiled JDK - the primary-source check this repo's discipline requires before describing any interface's method list.
`output/01-04`	Captured runs of the three demos and the test suite.

Notes worth knowing before reading the post

TreeMap/TreeSet already had most of this. firstEntry()/lastEntry()/pollFirstEntry()/pollLastEntry() existed on NavigableMap since Java 6; what JEP 431 adds for sorted collections is mostly reversed() as a named counterpart to descendingMap()/descendingSet(), plus the shared interface. The real new capability lands on LinkedHashSet and LinkedHashMap, which had an iteration order but no vocabulary for "first" or "last" before this.
reversed() is a view. See output/03 - mutating the original after taking reversed() is visible through the view and vice versa. This is the most common assumption bug the feature produces.
LinkedHashSet.getLast() is O(1) in practice, not by interface contract - it resolves through SequencedCollection's default method path to the reversed view's first element, and LinkedHashSet/LinkedHashMap already maintain a doubly-linked insertion-order list internally, so there's no O(n) walk. That is an implementation detail of the JDK's LinkedHashMap, not a guarantee SequencedCollection itself makes for arbitrary implementors.

License

MIT - see the repo-wide LICENSE.