在封闭的 switch 表达式之外返回

Question

我在 Java 12 中使用开关表达式¹将字符串转换为HTTP 方法：

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;

              }

          });

}

我想警告默认分支中不受支持的方法并返回 null（然后将其包装在 Optional 中）。

但是上面的代码会导致编译器错误：

在封闭的 switch 表达式之外返回

我如何让它编译？

为了完整起见，这里是枚举的定义RequestMethod：

enum RequestMethod {GET, PUT, POST, HEAD}

¹ _{switch 表达式作为预览功能在 Java 12 中引入。}

Answer 1

yield在 Java 13 中使用在 Java 13 中，switch 表达式使用新的受限标识符¹yield从块中返回值：return Optional.ofNullable(&nbsp; &nbsp; &nbsp; &nbsp; switch (methodStr.strip().toUpperCase(Locale.ROOT)) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; case "GET" -> RequestMethod.GET;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // ... rest omitted&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; default -> {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; log.warn("Unsupported request method: '{}'", methodStr);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // yield instead of return&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; yield null;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; });break在 Java 12 中使用在 Java 12 中，switch 表达式用于break从块中返回值：return Optional.ofNullable(&nbsp; &nbsp; &nbsp; &nbsp; switch (methodStr.strip().toUpperCase(Locale.ROOT)) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; case "GET" -> RequestMethod.GET;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // ... rest omitted&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; default -> {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; log.warn("Unsupported request method: '{}'", methodStr);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // break instead of return&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; break null;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; });¹yield不是关键字，正如用户 skomisa 所指出的那样。