From Java 8 to 17

// Immutable collection factories — Java 9
List<String> list = List.of("a", "b", "c");
Set<String> set = Set.of("a", "b");
Map<String, Integer> map = Map.of("a", 1, "b", 2);
Map<String, Integer> bigger = Map.ofEntries(Map.entry("a", 1), Map.entry("b", 2));

// Defensive copies — Java 10
List<String> snapshot = List.copyOf(mutableList);

// var for local variables — Java 10
var orders = new ArrayList<Order>();                 // type is inferred, still static
var entry = Map.entry("key", 1);                      // genuinely useful for verbose generics
for (var order : orders) { }

// Not allowed: fields, method parameters, return types, or without an initialiser

// String methods — Java 11
"  text  ".strip();                  // Unicode-aware, unlike trim()
"".isBlank();                         // true for whitespace-only
"ab".repeat(3);                       // "ababab"
"a\nb".lines().toList();              // a Stream<String> of lines

// Files convenience — Java 11
String content = Files.readString(path);
Files.writeString(path, content);

// Optional and Stream additions — Java 9 to 11
optional.or(() -> otherOptional);
optional.ifPresentOrElse(this::handle, this::missing);
optional.stream();
stream.takeWhile(p);  stream.dropWhile(p);
Stream.iterate(1, n -> n < 100, n -> n * 2);
Stream.ofNullable(maybeNull);
Collectors.flatMapping(...);  Collectors.filtering(...);

// A standard HTTP client — Java 11, replacing HttpURLConnection
HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder(URI.create(url)).GET().build();
HttpResponse<String> response = client.send(request, BodyHandlers.ofString());

// Single-file source execution — Java 11
// $ java Hello.java        (no separate javac step)

// Old message
Exception in thread "main" java.lang.NullPointerException
    at Example.main(Example.java:12)

order.customer().address().city().length();

Exception in thread "main" java.lang.NullPointerException:
    Cannot invoke "Address.city()" because the return value of "Customer.address()" is null
        at Example.main(Example.java:12)

// module-info.java at the source root
module com.example.orders {
    requires com.example.core;             // dependencies, checked at compile and run time
    requires transitive java.sql;           // consumers get java.sql too

    exports com.example.orders.api;         // public to everyone
    exports com.example.orders.spi to com.example.plugin;   // public to one module

    opens com.example.orders.model;         // reflection allowed — for frameworks

    provides OrderHandler with DefaultOrderHandler;    // a service implementation
    uses PaymentGateway;                                // a service this module consumes
}

# The migration escape hatch, when a library reflects into JDK internals
--add-opens java.base/java.lang=ALL-UNNAMED
--add-exports java.base/sun.nio.ch=ALL-UNNAMED

# Find the problems before you hit them at runtime
jdeps --jdk-internals --multi-release 17 app.jar

// Switch expressions, records, sealed classes, text blocks and pattern matching
// are covered in their own articles — they are the headline language features of 17.

// G1 is the default collector since Java 9; ZGC and Shenandoah are production-ready in 15
// Compact strings (Java 9) store Latin-1 text in one byte per char — ~25% less heap for typical apps
// Application class-data sharing (Java 12+) cuts JVM startup time with no code change
// Unified JVM logging: -Xlog:gc*:file=gc.log replaces the old ad-hoc GC flags
// jshell (Java 9) gives you a REPL for trying code without a class or main method
// Deprecated for removal now: the primitive wrapper constructors — use Integer.valueOf