We often discuss Java limitations on IRC and try to come up with (sometimes silly) workarounds. Unfortunately after time passes it’s often easy to forget the outcome, and lose code snippets. So I thought I’d start blogging some of them so I don’t lose them, and other people might suggest other ways of doing things that we’ve overlooked.<\/p>\n
This particular problem occurs when you want to assign the result of a method that can throw an exception to a final variable. For example:<\/p>\n
\r\nfinal Customer c;\r\ntry\r\n{\r\n\tc = getCustomer(id);\r\n} catch (CustomerNotFoundException e)\r\n{\r\n\tc = createNewCustomer();\r\n}\r\n<\/pre>\nThis will fail to compile with “variable c might already have been assigned”. Of course making c not final would solve the problem, but that’s no fun.<\/p>\n
If we were not using Exceptions, Java provides a useful ternary operator “?:” that lets us do things like:<\/p>\n
\r\nfinal Customer c = customerExists(id) ? getCustomer(id) : createNewCustomer();\r\n<\/pre>\nWhich is nice and clean, but means that getCustomer is going to have to return null or throw a RuntimeException in the case that there is no matching customer, which is undesirable. Also customerExists() may be expensive.<\/p>\n
We could also possibly use something along the lines of<\/p>\n
\r\nfinal Option c = getCustomer();\r\n<\/pre>\nBoth of these alternatives, however, require changing the API you’re consuming, and avoiding Exceptions. It would be nice if there was an equivalent of “?:” for try\/catch so that you could assign the result to a final variable. The best I can manage in Java is below, can anyone do better?<\/p>\n
\r\nimport java.lang.reflect.ParameterizedType;\r\n\r\nabstract class TryCatch\r\n{\r\n\tpublic T value()\r\n\t{\r\n\t\ttry\r\n\t\t{\r\n\t\t\treturn Try();\r\n\t\t} catch (Exception e)\r\n\t\t{\r\n\t\t\tif (getTypeOfU().isAssignableFrom(e.getClass()))\r\n\t\t\t{\r\n\t\t\t\treturn Catch();\r\n\t\t\t} else\r\n\t\t\t{\r\n\t\t\t\tthrow new RuntimeException(e);\r\n\t\t\t}\r\n\t\t}\r\n\t}\r\n\r\n\t@SuppressWarnings(\"unchecked\")\r\n\tprivate Class getTypeOfU()\r\n\t{\r\n\t\treturn (Class) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[1];\r\n\t}\r\n\r\n\tpublic abstract T Try() throws U;\r\n\r\n\tpublic abstract T Catch();\r\n}\r\n\r\n\/\/Example\r\npublic class Main\r\n{\r\n\tprivate static CustomerRepo repo = new CustomerRepo();\r\n\r\n\tpublic static void main(String[] args) \r\n\t{\r\n\t\tfinal Customer c = new TryCatch()\r\n\t\t{\r\n\t\t\tpublic Customer Try() throws CustomerNotFoundException\r\n\t\t\t{\r\n\t\t\t\tSystem.out.println(\"in try\");\r\n\t\t\t\treturn repo.getCustomer(1);\r\n\t\t\t}\r\n\r\n\t\t\tpublic Customer Catch()\r\n\t\t\t{\r\n\t\t\t\tSystem.out.println(\"in catch\");\r\n\t\t\t\treturn repo.createCustomer();\r\n\t\t\t}\r\n\t\t}.value();\r\n\t}\r\n\r\n}\r\n\r\nclass CustomerRepo\r\n{\r\n\r\n\tpublic Customer getCustomer(int id) throws CustomerNotFoundException\r\n\t{\r\n\t\tthrow new CustomerNotFoundException();\r\n\t}\r\n\r\n\tpublic Customer createCustomer()\r\n\t{\r\n\t\treturn new Customer();\r\n\t}\r\n}\r\n\r\nclass Customer\r\n{\r\n}\r\n\r\nclass CustomerNotFoundException extends Exception\r\n{\r\n}\r\n<\/pre>\nIn C# we don’t run into the same problem since readonly<\/a> is much less useful than Java’s final<\/a>. However, if we wanted to try\/catch at the same time we can do a bit better. Here’s an alternative in C#:<\/p>\n \r\nusing System;\r\n\r\npublic class Example\r\n{\r\n\tpublic static void Main()\r\n\t{\r\n\t\tExample t = new Example();\r\n\t\tt.Foo();\r\n\t}\r\n\r\n\tpublic void Foo()\r\n\t{\r\n\t\tString result1 = this.Try(() => GetBar(true)).Catch(() => \"Caught a BarException\");\r\n\t\tString result2 = this.Try(() => GetBar(false)).Catch(() => \"Caught a BarException\");\r\n\t\tConsole.WriteLine(result1);\r\n\t\tConsole.WriteLine(result2);\r\n\t}\r\n\r\n\tpublic String GetBar(bool succeed)\r\n\t{\r\n\t\tif (succeed)\r\n\t\t\treturn \"Success!\";\r\n\t\telse\r\n\t\t\tthrow new BarException();\r\n\t}\r\n}\r\n\r\npublic class BarException : Exception {}\r\n\r\npublic class Tryer\r\n{\r\n\tprivate readonly Func toTry;\r\n\tinternal Tryer(Func toTry)\r\n\t{\r\n\t\tthis.toTry = toTry;\r\n\t}\r\n\r\n\tpublic TResult Catch(Func whenCaught)\r\n\t\twhere TException : Exception\r\n\t{\r\n\t\ttry\r\n\t\t{\r\n\t\t\treturn toTry();\r\n\t\t} catch (TException)\r\n\t\t{\r\n\t\t\treturn whenCaught();\r\n\t\t}\r\n\t}\r\n}\r\n\r\nnamespace System\r\n{\r\n\tpublic static class ProvidesTry\r\n\t{\r\n\t\tpublic static Tryer Try(this T other, Func toTry)\r\n\t\t{\r\n\t\t\treturn new Tryer(toTry);\r\n\t\t} \r\n\t}\r\n}\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"We often discuss Java limitations on IRC and try to come up with (sometimes silly) workarounds. Unfortunately after time passes it’s often easy to forget the outcome, and lose code snippets. So I thought I’d start blogging some of them so I don’t lose them, and other people might suggest other ways of doing things… Read more »<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[8,1],"tags":[23],"yoast_head":"\n\n\n\n\n\n\n\n\n\n\n\n