1. assert

Marks a runtime check that throws AssertionError when the expression is false.

assert value > 0 : "value must be positive";

Enable with -ea on the JVM.

2. default

Two contexts:

• Interface method – supplies a concrete implementation since Java 8.
• Annnotation element – supplies a fallback value.

interface Logger {
    default void info(String msg) {
        System.out.println("[INFO] " + msg);
    }
}

@interface Retryable {
    int maxAttempts() default 3;
}

3. transient

Excludes a field from Java’s built-in serialization mechanism.

class User implements Serializable {
    private String login;
    private transient String password;   // never serialized
}

4. strictfp

Guarantees identical floating-point results across JVMs by enforcing IEEE 754 rules. Redundant from Java 17 onward.

strictfp class Calc {
    strictfp double sum(double a, double b) { return a + b; }
}

5. volatile

Ensures visibility and prevents instruction re-ordering for variables accessed by multiple threads.

class TaskRunner {
    private volatile boolean running = true;

    void stop() { running = false; }

    void loop() {
        while (running) {
            // work
        }
    }
}

6. native

Declares a method implemented in platform-specific code (JNI).

public class Display {
    public native void show();   // implemented in C/C++
    static { System.loadLibrary("display"); }
}

7. Module directives

Introduced in Java 9.

• module – starts a module descriptor.
• exports – exposes a package.
• requires – declares a dependency.
• opens – grants reflective access.
• provides … with – registers service implementations.

module com.example.app {
    requires java.net.http;
    exports com.example.api;
    opens com.example.internal to junit;
}

8. yield

Returns a value from a switch expression (Java 14+).

int days = switch (month) {
    case JAN, MAR, MAY -> 31;
    case APR, JUN, SEP -> 30;
    case FEB -> {
        System.out.println("Leap year check");
        yield 28;
    }
    default -> throw new IllegalArgumentException();
};

9. sealed and permits

Restrict wich classes may extend or implement a type (Java 15+).

public sealed class Shape permits Circle, Rectangle { }

final class Circle extends Shape { }
final class Rectangle extends Shape { }

10. var

Local-variable type inference (Java 10+). Must be initialized and cannot denote null.

var names = new ArrayList<String>();
for (var n : names) System.out.println(n);

11. to and when

Reserved for future language features; no semantic role in Java 23 yet.