{"id":1143,"date":"2018-03-03T11:44:27","date_gmt":"2018-03-03T10:44:27","guid":{"rendered":"http:\/\/benjiweber.co.uk\/blog\/?p=1143"},"modified":"2018-09-04T20:05:10","modified_gmt":"2018-09-04T19:05:10","slug":"representing-the-impractical-and-impossible-with-jdk-10-var","status":"publish","type":"post","link":"https:\/\/benjiweber.co.uk\/blog\/2018\/03\/03\/representing-the-impractical-and-impossible-with-jdk-10-var\/","title":{"rendered":"Representing the Impractical and Impossible with JDK 10 “var”"},"content":{"rendered":"

Having benefited from “var” for many years when writing c#, I’m delighted that Java is at last getting support for local variable type inference<\/a> in JDK 10.<\/p>\n

From JDK 10 instead of saying <\/p>\n

ArrayList<String> foo = new ArrayList<String>();\r\n<\/pre>\n
we can say<\/p>\n
var foo = new ArrayList<String>();\r\n<\/pre>\n
and the type of “foo” is inferred as ArrayList<String><\/p>\n
While this is nice in that it removes repetition and reduces boilerplate slightly, the real benefits come from the ability to have variables with types that are impractical or impossible to represent<\/strong>.<\/p>\n
Impractical Types<\/h2>\n
When transforming data it’s easy to be left with intermediary representations of the data that have deeply nested generic types. <\/p>\n
Let’s steal an example from a c# linq query<\/a>, that groups a customer’s orders by year and then by month.<\/p>\n
While Java doesn’t have LINQ, we can get fairly close thanks to lambdas<\/a>. <\/p>\n
from(customerList)\r\n    .select(c -> tuple(\r\n        c.companyName(),\r\n        from(c.orders())\r\n            .groupBy(o -> o.orderDate().year())\r\n            .select(into((year, orders) -> tuple(\r\n                year,\r\n                from(orders)\r\n                    .groupBy(o -> o.orderDate().month())\r\n            )))\r\n       ));\r\n<\/pre>\n
While not quite as clean as the c# version, it’s relatively similar. But what happens when we try to assign our customer order groupings to a local variable<\/a>?<\/p>\n
CollectionLinq<Tuple<String, CollectionLinq<Tuple<Integer, Group<Integer, Order>>>>> customerOrderGroups =\r\n   from(customerList)\r\n   .select(c -> tuple(\r\n       c.companyName(),\r\n       from(c.orders())\r\n           .groupBy(o -> o.orderDate().year())\r\n           .select(into((year, orders) -> tuple(\r\n               year,\r\n               from(orders)\r\n                   .groupBy(o -> o.orderDate().month())\r\n           )))\r\n   ));\r\n<\/pre>\n
Oh dear, that type description is rather awkward. The Java solutions to this have tended to be one of <\/p>\n