java10-local-type-inference

Gentle introduction to Java 10 local variable type inference.

• references
  • https://blog.codefx.org/java/java-10-var-type-inference/
  • https://www.journaldev.com/19871/java-10-local-variable-type-inference
  • Var with Style: Local Variable Type Inference in Java 10 by Stuart Marks

preface

• type inference - automatic detection of the data type of an expression

overview

• when processing var

  • compiler looks at the right hand side of the declaration
    • so-called initializer
  • and uses its type for the variable
  • it writes that type into the resulting bytecode

• types are not inferred at runtime but at compile time

  • resulting bytecode is the same as with explicit type declaration
  • no extra processing at runtime
    • compiler infers all involved types
    • puts them into the class files
    • as if you typed them yourself

• var is not a keyword, but a reserved type name

  • it can only be used in places where the compiler expects a type name, but everywhere else it’s a valid identifier
    • only classes called var will no longer work

• can be used only for "local variable declarations with initializers"

  // none of this works
  var ints = {0, 1, 2};
  var appendSpace = a -> a + " ";
  var x = 1, y = 2;
  

• only other eligible spots besides local variables are for loops and try-with-resources blocks.

  for (var number : numbers) {
      System.out.println(number);
  }
  

  try (var linesStream = Files.lines(Paths.get("src/test/java/LocalTypeInferenceTest.java"))) {
      linesStream.takeWhile(e -> !e.contains("public")).forEachOrdered(System.out::println);
  }
  

• where can var NOT be used?

  • method parameters, method return types, fields
  • why?
    • these types are part of a class's API - best to be explicit
    • they are written into the class file
    • they affect binary compatibility
    • too easy to introduce errors by inadvertently changing an inferred type

• can be reassigned to values of a compatible type

  var name = "Terry";
  name = "Diana";
  

  • if you want a variable that cannot be reassigned: final var name = ...

• pros

  • enables developers to write better code
  • reduces verbosity and clutter
    • from unimportant or redundant explicit declarations
    • helps with verbose generic types
  • lets more important things like the variable's name stand out
  • reduces the cost of extracting an expression into a local variable
  • the issue is one of readability
    • not the amount of keyboarding one needs to do