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

Introduction

Java releases have accelerated dramatically, with new language features arriving almost every six months. Understanding these changes helps developers keep up with the evolving platform and write more concise, safe, and performant code.

Java 9

Private methods in interfaces

Interfaces can now define private methods that default methods can call, allowing code reuse without exposing implementation details.

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

Diamond operator for anonymous classes

The diamond operator (<>), introduced in Java 7 for generic type inference, is now supported in anonymous inner classes.

List
numbers = new ArrayList<>();

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

Java 10

Local‑variable type inference (var)

The var keyword lets the compiler infer the type of a local variable, reducing boilerplate.

var message = "Hello, Java 10";

Java 11

var in lambda parameters

Lambda expressions can now use var for parameters, enabling annotations on inferred types.

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

Single‑file source‑code execution

Java 11 allows running a single source file directly without an explicit compilation step.

$ java HelloWorld.java
Hello Java 11!

Java 12

Enhanced switch expressions

Switch can now be used as an expression that yields a value and supports multiple case labels separated by commas.

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

Java 13

Text blocks

Multi‑line string literals are now written with triple quotes, preserving formatting without escape sequences.

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

Java 14

Records

Records provide a compact syntax for immutable data carriers, automatically generating constructors, accessors, equals , hashCode , and toString .

public record UserDTO(String id, String nickname, String homepage) { }

public static void main(String[] args) {
    UserDTO 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.homepage());
}

Improved NullPointerException messages

The JVM now reports the exact variable that is null in a chain of method calls, making debugging easier.

Map
> wrapMap = new HashMap<>();
wrapMap.put("innerMap", new HashMap<>());
boolean effected = wrapMap.get("innerMap").get("effected"); // NPE shows that "effected" is null

Foreign‑memory access API

A safe API replaces the unsafe sun.misc.Unsafe for allocating and manipulating off‑heap memory.

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

Java 15

New production garbage collectors

ZGC and Shenandoah are now fully supported, offering low‑pause garbage collection.

Sealed classes

Sealed classes restrict which other classes may extend or implement them, providing a middle ground between final and unrestricted inheritance.

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

Java 16

Java 16 consolidates the experimental features introduced in 14 and 15, with no major language changes.

Conclusion

The rapid cadence of Java releases means many of these features quickly move from preview to stable. Early adoption can improve code readability, safety, and performance, while staying current helps avoid falling behind the ecosystem.