Return outside of enclosing switch expression

Question

I'm using a switch expression¹ in Java 12 to convert a string to a HTTP method:

static Optional<RequestMethod> parseRequestMethod(String methodStr) {
    return Optional.ofNullable(
          switch (methodStr.strip().toUpperCase(Locale.ROOT)) {
              case "GET" -> RequestMethod.GET;
              case "PUT" -> RequestMethod.PUT;
              case "POST" -> RequestMethod.POST;
              case "HEAD" -> RequestMethod.HEAD;

              default -> {
                  log.warn("Unsupported request method: '{}'", methodStr);
                  return null;
              }
          });
}

I'd like to warn about the unsupported method in the default branch and return null (which is then wrapped in an Optional).

But the code above causes a compiler error:

Return outside of enclosing switch expression

How do I get this to compile?

---

For completeness, here's the definition of the RequestMethod enum:

enum RequestMethod {GET, PUT, POST, HEAD}

¹ _{switch expressions were introduced in Java 12 as a preview feature.}

Skillful

Why not use RequestMethod.valueOf(methodStr.strip().toUpperCase(Locale.ROOT))? – Indoeuropean

@VGR: Because that can throw an IllegalArgumentException. But the code in the question is only an example showing how the compiler error "Return outside of enclosing switch expression" can occur. – Skillful

Expressions (including switch expressions) must either yield a value, or throw. You cannot break, continue, or return to other contexts, other than completing normally (with a value; break value in 12, changed to yield value in 13) or throwing. – Dunnite

Answer 1

use yield in Java 13

In Java 13, switch expressions use the new restricted identifier¹ yield to return a value from a block:

return Optional.ofNullable(
        switch (methodStr.strip().toUpperCase(Locale.ROOT)) {
            case "GET" -> RequestMethod.GET;
            // ... rest omitted

            default -> {
                log.warn("Unsupported request method: '{}'", methodStr);
                // yield instead of return
                yield null;
            }
        });

use break in Java 12

In Java 12, switch expressions use break to return a value from a block:

return Optional.ofNullable(
        switch (methodStr.strip().toUpperCase(Locale.ROOT)) {
            case "GET" -> RequestMethod.GET;
            // ... rest omitted

            default -> {
                log.warn("Unsupported request method: '{}'", methodStr);
                // break instead of return
                break null;
            }
        });

---

_{¹ yield is not a keyword, as helpfully pointed out by user skomisa.}

Skillful

Thank for teaching me: it is time to closely closely observe these new Java releases. They yield unexpected content, which breaks out of my mind. – Phosphorism

Being pedantic, regarding "...the new keyword yield...", the proposed JLS for Java 13 which incorporates changes for JEP 354 ("Switch Expressions") explicitly states that yield is not a keyword. From section 3.9 Keywords: "The restricted identifiers var and yield are not keywords....yield has special meaning in a yield statement...". It's also worth noting that in contrast, break is a keyword. Adding new keywords to Java at this stage (and yield in particular) might be highly problematic... – Incorporation

... So presumably yield should be described as a "restricted identifier" rather than a "keyword". You can easily verify that yield is not a keyword because int yield = 0; compiles just fine on Java 13 in Eclipse 2019-09. In contrast, int break = 0; gives the error Syntax error on token "break", invalid VariableDeclarator – Incorporation

Yet another inconsistency in Java (JEP/JLS) documentation: while it's true, that break is used to yield a value from switch expressions, corresponding JEP's document says, that switch expressions can complete abruptly, which means - return must also be working here.. – Krone

@GiorgiTsiklauri this line might explain that inconsistency: The only reason an expression can complete abruptly is that an exception is thrown, because of either a throw with a given value (§14.18) or a run-time exception or error (§11 (Exceptions), §15.6). – Discovery