Modern Java in Practice

enum Drug { PARACETAMOL, IBUPROFEN, CAFFEINE }

sealed interface Dosage permits Tablet, Infusion { }
record Tablet(int morning, int midday, int evening)
		implements Dosage { }
record Infusion(int speed, int duration)
		implements Dosage { }

record Medication(Drug drug, Dosage dosage) { }
record MedicationPlan(
		String patientName,
		List<Medication> medications) { }

String category(Drug drug) {
	return switch (drug) {
		case PARACETAMOL, IBUPROFEN -> "painkiller";
		case CAFFEINE -> "stimulant";
	};
}

record MedicationPlan(
		String patientName,
		List<Medication> medications) {

	MedicationPlan {
		Objects.requireNonNull(patientName);
		medications = List.copyOf(medications);
	}

}

sealed interface Dosage permits Tablet, Infusion { }
record Tablet(int morning, int midday, int evening)
		implements Dosage { }
record Infusion(int speed, int duration)
		implements Dosage { }

int morningPillsOld(Dosage dosage) {
	if (dosage instanceof Tablet) {
		Tablet tablet = (Tablet) dosage;
		return tablet.morning();
	}
	return 0;
}

int morningPills(Dosage dosage) {
	if (dosage instanceof Tablet tablet) {
		return tablet.morning();
	}
	return 0;
}

String describeOrNone(Dosage dosage) {
	return switch (dosage) {
		case null -> "no dosage";
		case Tablet tablet -> "tablets";
		case Infusion infusion -> "infusion";
	};
}

int morningPillsUnnamed(Dosage dosage) {
	return switch (dosage) {
		case Tablet(var morning, _, _) -> morning;
		case Infusion(_, _) -> 0;
	};
}

95-64 .........................HHHHHHH
63-32 HHHHHHHHHHHHHHHHHHHHHHHH.AAAA.TT
31- 0 CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC

63-32 CCCCCCCCCCCCCCCCCCCCCCHHHHHHHHHH
31- 0 HHHHHHHHHHHHHHHHHHHHHVVVVAAAASTT

# training run (⇝ AOTCache)
$ java -XX:AOTCacheOutput=app.aot
       -cp app.jar com.example.App ...
# production run (AOTCache ⇝ performance)
$ java -XX:AOTCache=app.aot
       -cp app.jar com.example.App ...

public class HelloWorld {
	public static void main(String[] args) {
		System.out.println("Hello, World!");
	}
}

// LunchRoulette.java
void main() {
	var lunch = new Restaurant("Sushi");
	IO.println(lunch.name());
}

// Restaurant.java
record Restaurant(String name) { }

java LunchRoulette.java

void main() {
	IO.println("Hello, World!");
}

java HelloWorld.java

void main() {
	var name = IO.readln("Your name: ");
	IO.println("Hello, " + name + "!");
}

var restaurants = List.of(
		"Pizza", "Sushi", "Falafel");
var random = new Random();

IO.println(restaurants.get(
		random.nextInt(restaurants.size())));

$ cat `which hello`
> #!/usr/bin/env -S java --source 25
> void main() {
>	IO.println("Hello, scripts!");
> }
$ hello
> Hello, scripts!

void main() {
	var name = IO.readln("Please enter your name: ");
	IO.println("Nice to meet you, " + name);
}

# on JDK 25
$ java Main.java
> Please enter your name: Nicolai
> Nice to meet you, Nicolai

var transactions = Path.of("transactions.csv");
for (var line : Files.readAllLines(transactions)) {
	// ...
}

var analysis = "...";
var file = Path.of("analysis.txt");
Files.writeString(file, analysis);

var client = HttpClient.newHttpClient();
var request = HttpRequest
		.newBuilder(URI.create("http://example.org/"))
		.build();
var body = client
		.send(request, BodyHandlers.ofString())
		.body();

var jsonDoc = """
	{
		"users": [
			{ "name": "John Doe" },
			{ "name": "Jane Doe" }
		]
	}
	""";

JsonValue json = Json.parse(jsonDoc);

JsonValue json = Json.parse(jsonDoc);
// if structure is known
var name = json
	.get("users")
	.get(0)
	.get("name");
var names = json
	.get("users")
	.asList().stream()
	.map((JsonValue user) -> user.get("name"))
	.toList();

JsonValue json = Json.parse(jsonDoc);
// if structure is unknown
switch (json) {
	case JsonString string -> // ...
	case JsonNumber number -> // ...
	// ...
}

$ jpm install com.github.lalyos:jfiglet:0.0.9
Artifacts new: 1, updated: 0, deleted: 0

java-scripts
 ├─ Hello.java
 ├─ Helper.java
 ├─ Goodbye.java
 └─ lib
     └─ library.jar

java -cp 'lib/*' Hello.java

$ cat `which hello`
> #!/usr/bin/bash
> java /absolute/path/to/Hello.java -cp 'lib/*' "$@"
$ hello
> Hello, scripts!

var socket = (SSLSocket) SSLContext
	.getDefault().getSocketFactory().createSocket();
var params = socket.getSSLParameters();

params.setNamedGroups(new String[] {
	"SecP256r1MLKEM768", "X25519MLKEM768",
	"secp256r1", "x25519"
});
socket.setSSLParameters(params);

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj
0DAQcDQgAEi/kRGOL7wCPTN4KJ
2ppeSt5UYB6ucPjjuKDtFTXbgu
OIFDdZ65O/8HTUqS/sVzRF+dg7
H3/tkQ/36KdtuADbwQ==
-----END PUBLIC KEY-----

X509Certificate cert = // ...

// encode
PEMEncoder encoder = PEMEncoder.of();
String pem = encoder.encodeToString(cert);

// decode
PEMDecoder decoder = PEMDecoder.of();
BinaryEncodable cert2 = decoder.decode(pem);

assert cert.equals(cert2);

private static final Semaphore DB_SEMAPHORE =
	new Semaphore(16);

public <T> T queryDatabase(Callable<T> query)
		throws Exception {
	DB_SEMAPHORE.acquire();
	try {
		return query.call();
	} finally {
		DB_SEMAPHORE.release();
	}
}

static final ThreadLocal<Integer> ANSWER =
	new ThreadLocal<>();

void main() throws Exception {
	ANSWER.set(42);
	IO.println(ANSWER.get());     // "42"
	Thread.ofVirtual().start(() -> {
		IO.println(ANSWER.get()); // "null"
	});
	Thread.sleep(100);
}

static final ThreadLocal<Integer> ANSWER =
	new InheritableThreadLocal<>();

void main() throws Exception {
	ANSWER.set(42);
	IO.println(ANSWER.get());     // "42"
	Thread.ofVirtual().start(() -> {
		IO.println(ANSWER.get()); // "42"
	});
	Thread.sleep(100);
}

static final ScopedValue<Integer> ANSWER =
	ScopedValue.newInstance();

void main() {
	ScopedValue //      ⬐ VALUE
		.where(ANSWER, 42)
		//  |<---------- SCOPE ----------->|
		.run(() -> IO.println(ANSWER.get())); // "42"

	// OUT OF SCOPE
	ANSWER.get(); // ⚡️ NoSuchElementException
}

// implicitly short-circuits on error
try (var scope = StructuredTaskScope.open()) {
	// spawns new (virtual) threads
	Subtask<String> a = scope.fork(this::taskA);
	Subtask<String> b = scope.fork(this::taskB);

	// waits explicitly for success
	// (throws error if there was one)
	scope.join();

	return a.get() + b.get();
} catch (ExecutionException ex) {
	// TODO: handle task errors
} // waits until all tasks/threads complete

try (var scope = StructuredTaskScope.open(
		Joiner.<String> anySuccessfulOrThrow())) {
	// no need to grab the `Subtask` instances
	scope.fork(this::taskA);
	scope.fork(this::taskB);

	// returns the first successful result
	return scope.join();
} catch (ExecutionException ex) {
	// TODO: handle task errors
}

try (var scope = StructuredTaskScope.open(
		Joiner.<String> anySuccessfulOrThrow(
			IOException::new))) {

	scope.fork(this::taskA);
	scope.fork(this::taskB);

	return scope.join();
} catch (IOException ex) {
	// TODO: handle task errors
}

interface Joiner<T, R> {

	boolean onFork(Subtask<? extends T> subtask);

	boolean onComplete(Subtask<? extends T> subtask);

	void onTimeout();

	R result() throws Throwable;

}

var timeout = Duration.ofSeconds(3);
try (var scope = StructuredTaskScope.open(
		Joiner.<String> anySuccessfulOrThrow(),
		cfg -> cfg.withTimeout(timeout))) {
	// [... forks ...]

	// throws when timeout expires
	return scope.join();
} catch (ExecutionException ex) {
	if (ex.getCause() instanceof
		CancelledByTimeoutException tex) {
			// TODO: handle timeout
		}
	// TODO: handle task errors
}

ScopedValue.where(ANSWER, 42).run(() -> {// scope ──┐
	try (var scope = StructuredTaskScope    //      │
			.open() {                       //      │
		               // child threads' scope ──┐  │
		var subA = scope.fork(ANSWER::get); //   │  │
		var subB = scope.fork(ANSWER::get); //   │  │
		                                    //   │  │
		scope.join();                       //   │  │
		                                    //   │  │
		var result = subA.get()+subB.get(); //   │  │
		IO.println(result);  // "84"        //   │  │
	}           // all child threads completed ──┘  │
	IO.println(ANSWER.get()) // "42"        //      │
}                                           // ─────┘

@Override
public Thread newThread(Runnable runnable) {
	var contextMap = MDC.getCopyOfContextMap();
	return Thread.ofVirtual().unstarted(() -> {
		MDC.setContextMap(contextMap);
		runnable.run();
	});
}

void announce(SequencedCollection<String> sessions) {
	System.out.println(
			"Opening: " + sessions.getFirst());
	System.out.println(
			"Closing: " + sessions.getLast());
}

List<String> removeLastViaView() {
	var letters = new ArrayList<>(
			List.of("A", "B", "C"));
	letters.reversed().removeFirst();
	return letters; // [A, B]
}

SortedSet<String> letters = new TreeSet<>(
		List.of("B", "A", "C"));

letters.addLast("D");

List<String> onePerLength() {
	return Stream.of("foo", "bar", "baz", "quux")
			.gather(distinctBy(String::length))
			.toList();
	// [foo, quux]
}

stream
	.gather(distinctBy(String::length))
	.gather(Gatherers.windowFixed(3))
	.gather(Gatherers.mapConcurrent(
			8, this::enrich))
	.toList();

private final LazyConstant<Model> model =
		LazyConstant.of(Model::load);

String recommend(String attendee) {
	return model.get().recommend(attendee);
}

var rooms = List.ofLazy(3, this::loadRoom);

var client = HttpClient.newHttpClient();

var request = HttpRequest.newBuilder(
		URI.create("https://dev.java"))
	.build();

var response = client.send(
		request,
		BodyHandlers.ofString());

var client = HttpClient.newBuilder()
		.version(HttpClient.Version.HTTP_3)
		.build();

var request = HttpRequest.newBuilder(
				URI.create("https://dev.java"))
		.version(HttpClient.Version.HTTP_3)
		.build();

$ jwebserver -p 8080 -d dist
URL http://127.0.0.1:8080/

$ java -XX:StartFlightRecording:
	jdk.MethodTrace#filter=java.util.HashMap::resize,
	filename=recording.jfr
	...
$ jfr print
	--events jdk.MethodTrace --stack-depth 20
	recording.jfr
> jdk.MethodTrace {
>     startTime = 00:39:26.379 (2025-03-05)
>     duration = 0.00113 ms
>     method = java.util.HashMap.resize()
>     eventThread = "main" (javaThreadId = 3)
>     stackTrace = [
>       ...
>     ]
> }

$ java -XX:StartFlightRecording:
	method-timing=::<clinit>,
	filename=clinit.jfr
	...
$ jfr view method-timing clinit.jfr
>
>                                  Method Timing
>
> Timed Method                 Invocations Avg Time
> ---------------------------- ----------- --------
> HBShaper.<clinit>()                    1 32.50 ms
> GraphicsEnvironment$LocalGE.<clinit>() 1 32.40 ms
> DemoFonts.<clinit>()                   1 21.20 ms
> TempFileHelper.<clinit>()              1 17.10 ms
> SecurityProviderConstants.<clinit>()   1  9.86 ms

$ java -XX:StartFlightRecording=
	jdk.CPUTimeSample#enabled=true,
	filename=profile.jfr
	...

class SnippetDocsDemo {

	@Test
	void constructorDemo() {
		// @start region="constructor"
		// How to call the parameterless constructor:
		SnippetDocs docs = new SnippetDocs();
		// @end

		// assert something meaningful
	}

}

/**
 * This class has a constructor. Here's how you call it:
 * {@snippet class="SnippetDocsDemo" region="constructor"}
 */
public class SnippetDocs { ... }