Language 3 of 10 · Topic 0.5

Control Flow: Switch Expressions & Pattern Matching (Java 21)

1Concept

Java 21 supports Pattern Matching for switch and Switch Expressions with arrow syntax (->), yielding values directly without break statements, and exhaustive type matching on Sealed Classes and Records.

2Architecture Diagram

String status = switch (statusCode) {
    case 200, 201 -> "OK / Created";
    case 400, 404 -> "Client Error";
    case 500 -> "Server Fault";
    default -> "Unknown";
};

3Code Example

Stage 0 Language Foundations
public class SwitchPatternDemo {
    sealed interface ApiResponse permits SuccessResponse, ErrorResponse {}
    record SuccessResponse(String data, int code) implements ApiResponse {}
    record ErrorResponse(String errorMessage, int code) implements ApiResponse {}

    public static String handleResponse(ApiResponse response) {
        // Java 21 Pattern Matching for switch
        return switch (response) {
            case SuccessResponse s when s.code() == 200 -> "Success: " + s.data();
            case SuccessResponse s -> "Success (Status " + s.code() + "): " + s.data();
            case ErrorResponse e when e.code() >= 500 -> "Critical Server Failure: " + e.errorMessage();
            case ErrorResponse e -> "Client Error (" + e.code() + "): " + e.errorMessage();
        };
    }

    public static void main(String[] args) {
        ApiResponse r1 = new SuccessResponse("Payload loaded", 200);
        ApiResponse r2 = new ErrorResponse("Internal DB Timeout", 503);

        System.out.println(handleResponse(r1));
        System.out.println(handleResponse(r2));
    }
}

4Expected Output

Success: Payload loaded
Critical Server Failure: Internal DB Timeout

5Key Takeaways

  • ✓Switch expressions with -> return values and eliminate forgotten break statement bugs.
  • ✓Guarded patterns (when clause) allow fine-grained boolean conditions inside switch cases.
  • ✓Switching over sealed hierarchies guarantees compile-time exhaustiveness checks.