Java-17-To-Java-21-Upgrade

// Old approach (Java 17)
public String processObject(Object obj) {
    if (obj instanceof String) {
        String s = (String) obj;
        return s.toUpperCase();
    } else if (obj instanceof Integer) {
        Integer i = (Integer) obj;
        return i.toString();
    }
    return "unknown";
}

// New approach (Java 21)
public String processObject(Object obj) {
    return switch (obj) {
        case String s -> s.toUpperCase();
        case Integer i -> i.toString();
        case null -> "null";
        default -> "unknown";
    };
}

switch (obj) {
    case String s when s.length() > 10 -> "Long string: " + s;
    case String s -> "Short string: " + s;
    case Integer i when i > 100 -> "Large number: " + i;
    case Integer i -> "Small number: " + i;
    default -> "Other";
}

public record Point(int x, int y) {}
public record ColoredPoint(Point point, Color color) {}

// Destructuring in switch
public String describe(Object obj) {
    return switch (obj) {
        case Point(var x, var y) -> "Point at (" + x + ", " + y + ")";
        case ColoredPoint(Point(var x, var y), var color) -> 
            "Colored point at (" + x + ", " + y + ") in " + color;
        default -> "Unknown shape";
    };
}

// Nested record patterns
switch (shape) {
    case Rectangle(ColoredPoint(Point(var x1, var y1), var c1), 
                   ColoredPoint(Point(var x2, var y2), var c2)) 
        when c1 == c2 -> "Monochrome rectangle";
    case Rectangle r -> "Multi-colored rectangle";
}

// Old platform thread approach
ExecutorService executor = Executors.newFixedThreadPool(100);
executor.submit(() -> {
    // blocking I/O operation
    httpClient.send(request);
});

// New virtual thread approach
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> {
        // blocking I/O operation - now scales to millions
        httpClient.send(request);
    });
}

// Structured concurrency (Preview)
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Future<String> user = scope.fork(() -> fetchUser(userId));
    Future<String> order = scope.fork(() -> fetchOrder(orderId));
    
    scope.join();           // Join all subtasks
    scope.throwIfFailed();  // Propagate errors
    
    return processResults(user.resultNow(), order.resultNow());
}

// Traditional concatenation
String message = "Hello, " + name + "! You have " + count + " messages.";

// String Templates (Preview)
String message = STR."Hello,