Java 21 💣💥

public void categorize(Page page) {
	if (page instanceof GitHubIssuePage) {
		GitHubIssuePage issue = (GitHubIssuePage) page;
		categorizeIssuePage(issue);
	} else if (page instanceof GitHubPrPage pr) {
		// ... etc. for all types
	}
}

public void categorize(Page page) {
	if (page instanceof GitHubIssuePage issue)
		categorizeIssuePage(issue);
	else if (page instanceof GitHubPrPage pr)
		// ... etc. for all types
}

record ExternalPage(URI url, String content) { }

if (page instanceof
		ExternalPage(var url, var content)) {
	// use `url` and `content`
}

public void categorize(Page page) {
	switch (page) {
		case GitHubIssuePage issue
			-> categorizeIssuePage(issue);
		case ExternalPage(var url, var content)
			-> categorizeExternalUrl(url);
		// ... etc. for all types
	}
}

error: the switch expression does not cover
       all possible input values

public void categorize(Page page) {
	switch (page) {
		case GitHubIssuePage issue ->
			categorizeIssuePage(issue);
		// ... etc. for all types
		default ->
			throw new IllegalArgumentException();
	}
}

public sealed interface Page
	permits GitHubIssuePage, GitHubPrPage,
			ExternalPage, ErrorPage {
	// ...
}

public void categorize(Page page) {
	switch (page) {
		case GitHubIssuePage issue -> // ...
		case GitHubPrPage pr -> // ...
		case ExternalPage external -> // ...
		case ErrorPage error -> // ...
	}
}

public sealed interface Page
	permits GitHubIssuePage, GitHubPrPage,
			GitHubCommitPage,
			ExternalPage, ErrorPage {
	// ...
}

public void categorize(Page page) {
	switch (page) {
		case GitHubIssuePage issue -> // ...
		case GitHubPrPage pr -> // ...
		case ExternalPage external -> // ...
		case ErrorPage error -> // ...
		// missing case
	}
}

public static String createPageName(Page page) {
	return switch (page) {
		case ErrorPage(var url, _)
			-> "💥 ERROR: " + url.getHost();
		case GitHubIssuePage(_, _, _, int issueNumber)
			-> "🐈 ISSUE #" + issueNumber;
		// ...
	};
}

public static String createPageEmoji(Page page) {
	return switch (page) {
		case GitHubIssuePage issue -> "🐈";
		case GitHubPrPage pr -> "🐙";
		case ErrorPage _, ExternalPage _ -> "n.a.";
	};
}

try (var executor = Executors
		.newVirtualThreadPerTaskExecutor()) {
	IntStream
		.range(0, 1_000_000)
		.forEach(number -> {
			executor.submit(() -> {
				Thread.sleep(Duration.ofSeconds(1));
				return number;
			});
		});
} // executor.close() is called implicitly, and waits

@Bean
public TomcatProtocolHandlerCustomizer<?>
		createExecutorForSyncCalls() {
	return handler -> handler.setExecutor(
			Executors.newVirtualThreadPerTaskExecutor());
}

@Bean
public AsyncTaskExecutor
		createExecutorForAsyncCalls() {
	return new TaskExecutorAdapter(
			Executors.newVirtualThreadPerTaskExecutor());
}

@GET
@Path("api")
@RunOnVirtualThread
public String handle() {
	// ...
}

// limits concurrent queries but pools 👎🏾
private static final ExecutorService DB_POOL =
	Executors.newFixedThreadPool(16);

public <T> Future<T> queryDatabase(Callable<T> query) {
	return DB_POOL.submit(query);
}

// limits concurrent queries without pool 👍🏾
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();
	}
}

// guarantees sequential access, but pins (for now) 👎🏾
public synchronized String accessResource() {
	return access();
}

// guarantees sequential access without pinning 👍🏾
private static final ReentrantLock LOCK =
	new ReentrantLock();

public String accessResource() {
	// lock guarantees sequential access
	LOCK.lock();
	try {
		return access();
	} finally {
		LOCK.unlock();
	}
}

// copies value for each inheriting thread 👎🏾
static final ThreadLocal<Principal> PRINCIPAL =
	new ThreadLocal<>();

public void serve(Request request, Response response) {
	var level = request.isAdmin() ? ADMIN : GUEST;
	var principal = new Principal(level);
	PRINCIPAL.set(principal);
	Application.handle(request, response);
}

// immutable, so no copies needed 👍🏾
static final ScopedValue<Principal> PRINCIPAL =
	new ScopedValue<>();

public void serve(Request request, Response response) {
	var level = request.isAdmin() ? ADMIN : GUEST;
	var principal = new Principal(level);
	ScopedValue
		.where(PRINCIPAL, principal)
		.run(() -> Application
			.handle(request, response));
}

String property = "last_name";
String value = "Doe";

// concatenation
String query =
	"SELECT * FROM Person p WHERE p."
		+ property + " = '" + value + "'";
// formatting
String query =
	"SELECT * FROM Person p WHERE p.%s = '%s'"
		.formatted(property, value);

// (fictional syntax!)
String query =
	"SELECT * FROM Person p "
		+ "WHERE p.\{property} = '\{value}'";

String query = STR."""
	SELECT * FROM Person p
	WHERE p.\{property} = '\{value}'
	""";

String form = STR."""
	Desc     Unit   Qty   Amount
	\{desc} $\{price} \{qty} $\{price * qty}

	Subtotal  $\{price * qty}
	Tax       $\{price * qty * tax}
	Total     $\{price * qty * (1.0 + tax)}
	""";

Desc     Unit   Qty   Amount
hammer   $7.88  3     $23.64

Subtotal  $23.64
Tax       $3.546
Total     $27.186

String form = FMT."""
	Desc        Unit      Qty   Amount
	%-10s\{desc} $%5.2f\{price} %5d\{qty} $%5.2f\{price * qty}

	Subtotal  $%5.2f\{price * qty}
	Tax       $%5.2f\{price * qty * tax}
	Total     $%5.2f\{price * qty * (1.0 + tax)}
	""";

Desc        Unit      Qty   Amount
hammer      $ 7.88      3   $23.64

Subtotal  $23.64
Tax       $ 3.55
Total     $27.19

public interface Processor<RESULT, EX> {
	RESULT process(StringTemplate s) throws EX;
}

// prevents SQL injections
Statement query = SQL."""
	SELECT * FROM Person p
	WHERE p.\{property} = '\{value}'
	""";

// validates & escapes JSON
JSONObject doc = JSON."""
	{
		"name": "\{name}",
		"year": "\{bday.getYear()}"
	}
	""";

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

java Prog.java

MyFirstJava
 ├─ Prog.java
 ├─ Helper.java
 └─ Lib
     └─ library.jar

java -cp 'Lib/*' Prog.java