Java 21 — Key Features

// Virtual threads — millions of concurrent I/O tasks
try (ExecutorService exec = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 100_000).forEach(i ->
        exec.submit(() -> {
            Thread.sleep(Duration.ofMillis(100)); // blocks, but no OS thread wasted
            return processRequest(i);
        }));
} // all 100,000 tasks complete, ~100ms total

// Sequenced Collections (Java 21 — JEP 431)
// SequencedCollection: List, Deque, LinkedHashSet
List<String> list = new ArrayList<>(List.of("a", "b", "c"));
list.getFirst();       // "a"  (was: list.get(0))
list.getLast();        // "c"  (was: list.get(list.size()-1))
list.addFirst("z");    // ["z","a","b","c"]
list.addLast("w");     // ["z","a","b","c","w"]
list.reversed();       // ["w","c","b","a","z"] view

// SequencedMap: LinkedHashMap
LinkedHashMap<String, Integer> map = new LinkedHashMap<>();
map.put("one", 1); map.put("two", 2); map.put("three", 3);
map.firstEntry();  // one=1
map.lastEntry();   // three=3
map.reversed();    // reversed order view

// Record patterns — destructure in instanceof
record Point(int x, int y) {}
record Circle(Point centre, double radius) {}

Object shape = new Circle(new Point(0, 0), 5.0);

// Destructure nested records in one pattern
if (shape instanceof Circle(Point(int x, int y), double r)) {
    System.out.printf("Circle at (%d,%d) r=%.1f%n", x, y, r);
}

// Record patterns in switch
String describe(Object obj) {
    return switch (obj) {
        case Integer i when i < 0          -> "negative int: " + i;
        case Integer i                     -> "positive int: " + i;
        case String s when s.isEmpty()     -> "empty string";
        case String s                      -> "string: " + s;
        case Point(int x, int y)           -> "point (%d,%d)".formatted(x, y);
        case Circle(Point(int x, int y), double r)
                                           -> "circle at (%d,%d) r=%.1f".formatted(x,y,r);
        case null                          -> "null";
        default                            -> "unknown: " + obj.getClass().getSimpleName();
    };
}

// String Templates (preview — enable with --enable-preview)
String name  = "Alice";
double price = 99.99;

// STR processor — simple interpolation
String msg = STR."Hello, \{name}! Your total is \{price}.";
// "Hello, Alice! Your total is 99.99."

// FMT processor — formatted interpolation
String formatted = FMT."Price: %-10s\{name} $%,.2f\{price}";
// "Price: Alice      $99.99"

// Custom processor — validates SQL (prevents injection)
// (Illustrative — real implementation would parameterise queries)
PreparedStatement ps = SQL."SELECT * FROM users WHERE id = \{userId}";

// Unnamed patterns (preview) — _ discards unwanted bindings
if (obj instanceof Point(int x, _)) {
    System.out.println("x-coord: " + x); // don't care about y
}

// Switch discard
switch (shape) {
    case Circle(_, double r) -> System.out.println("radius: " + r);
    case _                   -> System.out.println("other shape");
}

// Foreign Function & Memory API (finalised JEP 454)
// Call C libraries without JNI
try (Arena arena = Arena.ofConfined()) {
    MemorySegment str = arena.allocateUtf8String("Hello");
    // pass to native function...
}