Java 22 & 23: Unnamed Variables and Foreign Memory

// Before Java 22 — you had to name variables you don't use
try {
    riskyOperation();
} catch (IOException e) { // e is never used — but you had to name it
    log.warn("Operation failed, retrying...");
}

// Java 22 — use _ to signal intentional discard
try {
    riskyOperation();
} catch (IOException _) {
    log.warn("Operation failed, retrying...");
}

// In enhanced for loops
for (var _ : list) {
    count++;
}

// In try-with-resources
try (var _ = ScopedContext.acquire()) {
    // use the context, but don't need the reference
    doWork();
}

// Ignoring return values
var _ = queue.offer(item); // explicitly ignoring the boolean return

record Point(int x, int y, int z) {}

Object obj = new Point(1, 2, 3);

// Only care about x — ignore y and z
if (obj instanceof Point(int x, _, _)) {
    System.out.println("X coordinate: " + x);
}

// In switch
String describe(Object o) {
    return switch (o) {
        case Point(int x, int y, _) when x == y -> "Diagonal point";
        case Point(int x, _, _)                 -> "Point with x=" + x;
        default                                  -> "Not a point";
    };
}

import java.lang.foreign.*;
import java.lang.invoke.MethodHandle;

// Call the C standard library's strlen function
try (Arena arena = Arena.ofConfined()) {
    // Look up the function in the C standard library
    Linker linker = Linker.nativeLinker();
    SymbolLookup stdlib = linker.defaultLookup();

    MethodHandle strlen = linker.downcallHandle(
        stdlib.find("strlen").orElseThrow(),
        FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS)
    );

    // Allocate a C string in off-heap memory
    MemorySegment cString = arena.allocateFrom("Hello, native world!");

    // Call strlen
    long length = (long) strlen.invoke(cString);
    System.out.println("Length: " + length); // 20
}
// Memory is automatically freed when the arena closes

// Allocate a large off-heap buffer (not subject to GC pressure)
try (Arena arena = Arena.ofConfined()) {
    // Allocate 1 MB of off-heap memory
    MemorySegment buffer = arena.allocate(1024 * 1024);

    // Write structured data
    MemoryLayout pointLayout = MemoryLayout.structLayout(
        ValueLayout.JAVA_INT.withName("x"),
        ValueLayout.JAVA_INT.withName("y")
    );

    VarHandle xHandle = pointLayout.varHandle(MemoryLayout.PathElement.groupElement("x"));
    VarHandle yHandle = pointLayout.varHandle(MemoryLayout.PathElement.groupElement("y"));

    MemorySegment point = arena.allocate(pointLayout);
    xHandle.set(point, 0L, 10);
    yHandle.set(point, 0L, 20);

    System.out.println("x=" + xHandle.get(point, 0L)); // x=10
    System.out.println("y=" + yHandle.get(point, 0L)); // y=20
} // off-heap memory freed here

// Java 23 preview — primitive types in instanceof
Object obj = 42;

if (obj instanceof int i) {
    System.out.println("Integer value: " + i);
}

// Primitive types in switch
static String describe(Object o) {
    return switch (o) {
        case int i when i < 0  -> "negative int: " + i;
        case int i             -> "non-negative int: " + i;
        case long l            -> "long: " + l;
        case double d          -> "double: " + d;
        case String s          -> "string: " + s;
        default                -> "other";
    };
}

describe(42);    // "non-negative int: 42"
describe(-5);    // "negative int: -5"
describe(3.14);  // "double: 3.14"

// Before — individual imports
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

// Java 23 preview — import the entire java.base module
import module java.base;

// Now all types from java.base are available without individual imports
List<String> names = Stream.of("Alice", "Bob")
    .collect(Collectors.toList());

/// Returns the greeting for the given name.
///
/// ## Example
///
/// ```java
/// String greeting = greet("Alice");
/// assert greeting.equals("Hello, Alice!");
/// ```
///
/// @param name the name to greet
/// @return the greeting string
public String greet(String name) {
    return "Hello, " + name + "!";
}