Key New Features in Java 9–16: Private Interface Methods, Var, Switch Expressions, Records and More

Top‑level architect shares a rapid overview of Java 9‑16 new language features, helping developers stay current with the fast‑paced JDK releases.

Java 9 : Interfaces can now contain private methods that default methods may call.

public interface TestInterface {
    default void wrapMethod() {
        innerMethod();
    }
    private void innerMethod() {
        System.out.println("");
    }
}

Anonymous inner classes gain diamond operator support.

List<Integer> numbers = new ArrayList<>() {
    // ...
};

Try‑with‑resources is enhanced to allow resource variables declared outside the try block.

BufferedReader br0 = new BufferedReader(...);
BufferedReader br1 = new BufferedReader(...);
try (br0; br1) {
    System.out.println(br0.readLine() + br1.readLine());
}

Java 10 : Local‑variable type inference with var. var message = "Hello, Java 10"; Java 11 : var can be used in lambda parameters, allowing annotations.

List<String> languages = Arrays.asList("Java", "Groovy");
String language = sampleList.stream()
    .map((@Nonnull var x) -> x.toUpperCase())
    .collect(Collectors.joining(", "));

Java 12 : Switch expressions with arrow syntax and yield for returning values.

typeOfDay = switch (dayOfWeek) {
    case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY -> "Working Day";
    case SATURDAY, SUNDAY -> "Day Off";
};

Java 13 : Text blocks simplify multi‑line string literals.

String json = """
    {
        "id":"1697301681936888",
        "nickname":"空无",
        "homepage":"https://juejin.cn/user/1697301681936888"
    }""";

Java 14 : Introduction of record types to reduce boilerplate POJOs.

public record UserDTO(String id, String nickname, String homepage) {};
var user = new UserDTO("1697301681936888", "空无", "https://juejin.cn/user/1697301681936888");
System.out.println(user.id);
System.out.println(user.nickname);
System.out.println(user.id);

NullPointerException messages become more descriptive, pinpointing the exact null expression.

Map<String,Map<String,Boolean>> wrapMap = new HashMap<>();
wrapMap.put("innerMap", new HashMap<>());
boolean effected = wrapMap.get("innerMap").get("effected");
// StackTrace now shows which map entry was null

Foreign‑memory access API replaces unsafe operations with safe handles.

MemorySegment segment = MemorySegment.allocateNative(200);
MemoryAddress address = segment.baseAddress();
VarHandle vh = MemoryHandles.varHandle(long.class, ByteOrder.nativeOrder());
vh.set(address, 10L);
segment.close();

Java 15 : ZGC and Shenandoah become production‑ready GC algorithms, and sealed classes restrict which classes may extend a type.

public sealed interface Service permits Car, Truck {
    int getMaxServiceIntervalInMonths();
    default int getMaxDistanceBetweenServicesInKilometers() { return 100000; }
}

Java 16 : Mostly consolidates experimental features from earlier releases; no major language changes.

In summary, the rapid JDK evolution introduces many syntactic and runtime improvements that developers should adopt early to write cleaner, safer, and more performant Java code.