Local-variable type inference is aligned with that goal. Project Amber, under which var was developed, aims "to explore and incubate smaller, productivity-oriented Java language features" and has the goal to generally reduce the ceremony involved in writing and reading Java code. Java's overall tendency to be pretty verbose, particularly compared to younger languages, is one of the biggest pain points for developers and a common critique of the language by novice and experienced Java devs alike. If you're interested in even more detail have a look at the JEP 286 discussions, the var FAQ, or the Project Amber mailing list. Let's have a look behind the scenes and find out why var was introduced, how its impact on readability was envisioned and why there is no val (or let) accompanying it. The basic idea is that local variables are implementation details and can not be referenced from "far away" code, which reduces the need to strictly, explicitly, and verbosely define their type. Sure, an exception could have been made for private fields or methods, but that would make the feature rather weirdly scoped. So because non-private fields and methods become part of a type's contract and because that shouldn't be changed accidentally, these types are not inferred. That's a rather extreme consequence of having changed some implementation detail. In the worst case, changing a method parameter's type can lead to binary incompatibilities and thus runtime errors. ▚Why Can't Field And Method Types Be Inferred?įields and methods have a far larger scope than local variables and as such the distance between changes and errors increases considerably. That error is at quite a distance from the change that caused it and on top of that it's a surely unforeseen consequence of simply assigning a value to a variable.įrom that perspective the decision to limit type inference to the immediate declaration makes sense. The answer is that the if-condition throws an error because id will no longer be an int and can thus not be compared with <. This is not a rhetorical question, think about it. This does not change Java's commitment to static typingĬase in point, here's the result of IntelliJ's (actually Fernflower's) decompilation of the class file with the URL example: The compiler infers all involved types and puts them into the class files as if you typed them yourself. I want to start by stressing that var does not change Java's commitment to static typing by one iota. Local-variable type inference looks like a straight-forward feature, but that's deceptive. That means that only classes called var will no longer work, but that shouldn't happen particularly often. Technically, var is not a keyword, but a reserved type name, meaning it can only be used in places where the compiler expects a type name, but everywhere else it's a valid identifier. IDEs can of course show them on demand, but that doesn't help in any other environment (think code reviews).īy the way, in case you're worried about clashes with methods and variables named var: don't be. The cost is obvious: Some variables' types, of connection for example, are not immediately obvious. When processing var, the compiler looks at the right hand side of the declaration, the so-called initializer, and uses its type for the variable.Īnd not just for internal bookkeeping, it writes that type into the resulting bytecode.Īs you can see, this saves a few characters when typing, but more importantly it deduplicates redundant information and neatly aligns the variable's names, which eases reading them. openConnection ( ) var reader = new BufferedReader ( new InputStreamReader (connection. Var codefx = new URL ( "" ) var connection = codefx.
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