Zoom on java type inference
by snonov
Nice coverage of the subject in “The Well-Grounded Java Developer, Second Edition” by Jason Clark, Benjamin Evans, Martijn Verburg. This zoom on is a synthesis extended by some external references.
Enhanced type inference in java is done with var keyword that is not a reserved keyword (you can name a variable var). This feature of the source code compiler enables the compiler to work out some of the type information in programs automatically. As a result, it doesn’t need to be told everything explicitly.
But here are some steps before :
- Java 5, when generic methods were introduced with JSR 14 inspired by GJ
- Java 7, significant enhancement to type inference in Java came with diamond syntax, part of JSR 334 with Improved Type Inference for Generic Instance Creation (diamond). Those enhancement were called also named Coin project.
- Java 8, more type inference to support usage with lambda expressions with JEP 101, example
Function<String, Integer> lengthFn = s -> s.length(); - Java 10, major change with arrival of Local Variable Type Inference (LVTI), otherwise known as var. This come with JEP 286
- Java 11, linked to previous JEP 286, JEP 323 LVTI for lambda parameters delivered
Take care :
- does not introduce dynamic typing, and all Java variables continue to have static types at all times
- compiler applies a form of constraint solving
- var is implemented solely in the source code compiler (javac) and has no runtime or performance effect whatsoever
LVTI style guide with best practises : https://openjdk.org/projects/amber/guides/lvti-style-guide
Giving Principles :
- Reading code is more important than writing code
- Code should be clear from local reasoning
- Code readability shouldn’t depend on IDEs
- Explicit types are a tradeoff
and Guidelines :
- Choose variable names that provide useful information
- Minimize the scope of local variables
- Consider var when the initializer provides sufficient information to the reader
- Use var to break up chained or nested expressions with local variables
- Don’t worry too much about “programming to the interface” with local variables
- Take care when using var with diamond or generic methods
- Take care when using var with literals