Java-21-To-Java-25-Upgrade

// Old approach (Java 21)
switch (x.getStatus()) {
    case 0 -> "okay";
    case 1 -> "warning"; 
    case 2 -> "error";
    default -> "unknown status: " + x.getStatus();
}

// New approach (Java 25 Preview)
switch (x.getStatus()) {
    case 0 -> "okay";
    case 1 -> "warning";
    case 2 -> "error"; 
    case int i -> "unknown status: " + i;
}

switch (x.getYearlyFlights()) {
    case 0 -> ...;
    case int i when i >= 100 -> issueGoldCard();
    case int i -> ... // handle 1-99 range
}

// Old ASM approach
ClassReader reader = new ClassReader(classBytes);
ClassWriter writer = new ClassWriter(reader, 0);
// ... ASM manipulation

// New Class-File API approach
ClassModel classModel = ClassFile.of().parse(classBytes);
byte[] newBytes = ClassFile.of().transform(classModel, 
    ClassTransform.transformingMethods(methodTransform));

// Old HTML JavaDoc
/**
 * Returns the <b>absolute</b> value of an {@code int} value.
 * <p>
 * If the argument is not negative, return the argument.
 * If the argument is negative, return the negation of the argument.
 * 
 * @param a the argument whose absolute value is to be determined
 * @return the absolute value of the argument
 */

// New Markdown JavaDoc  
/// Returns the **absolute** value of an `int` value.
///
/// If the argument is not negative, return the argument.
/// If the argument is negative, return the negation of the argument.
/// 
/// @param a the argument whose absolute value is to be determined
/// @return the absolute value of the argument

// Instead of manual record copying
public record Person(String name, int age, String email) {
    public Person withAge(int newAge) {
        return new Person(name, newAge, email);
    }
}

// Use derived record creation (Preview)
Person updated = person with { age = 30; };

// Custom windowing operations
List<List<String>> windows = stream
    .gather(Gatherers.windowSliding(3))
    .toList();

// Custom filtering with state
List<Integer> filtered = numbers.stream()
    .gather(Gatherers.fold(0, (state, element) -> {
        // Custom stateful logic
        return state + element > threshold ? element : null;