Finalising the on-ramp feature

[Brian Goetz]

Consistency, and smoothing the entry to the highway.

If it's in java.io, then the java.base module import picks up IO for 
implicit compilation units, but not for named ones, so the locution 
"IO.println" stops working without an explicit module import, as well as 
all the existing Java code out there.  By putting IO in java.lang, then 
`IO.x` means the same thing for all compilation units.

Essentially, it is a combination of uniformity and strength-reduction 
from "import-static a whole bunch of things in implicit classes only" to 
"import one new class symbol, IO, everywhere."


On 1/21/2025 12:17 PM, Ethan McCue wrote:
> One question I have: If the implicit import module java.base; remains, 
> what was the impetus for moving IO to java.lang?
>
> On Tue, Jan 21, 2025 at 9:55 AM Gavin Bierman 
> <gavin.bierman at oracle.com> wrote:
>
>     Dear Experts,
>
>     With JDK 24 preparations complete, we are now planning our work
>     for JDK 25 and
>     in particular the on-ramp JEP. It is our intention to *finalise*
>     the on-ramp
>     feature in JDK 25. We are grateful for the feedback that we have
>     received from
>     our expert and developer communities - thank you! - and we have
>     been performing
>     our own extensive in-house experiments with the preview feature.
>     Given this, we
>     propose that we make the following changes/simplifications to the
>     feature when
>     it finalizes in JDK 25.
>
>     ## 1. Remove the auto-static-import feature and move the `IO` class
>
>     We had proposed that the static members of a new class `IO` would
>     automatically
>     be imported by simple compilation units. This allows the use of
>     the simple names
>     `println`, `print`, `readln`, and `read`. Whilst this is very
>     convenient, it
>     creates a bump in the on-ramp experience when migrating a simple
>     compilation
>     unit to an ordinary compilation unit, which does not implicitly
>     import these static
>     methods. This means that when migrating we either add an explicit
>     static import,
>     or rewrite all the `println`, `print`, `readln` and `read` method
>     calls to
>     qualified calls.
>
>     We have come to the conclusion that the graceful on-ramp
>     experience is the more
>     important goal. So, we propose in JDK 25 that (1) we drop the
>     automatic static
>     import of the `IO` class by simple compilation units, and (2) We
>     move the `IO`
>     class from package `java.io <http://java.io>` to `java.lang`.
>
>     This means that in simple compilation units calls to the `IO`
>     methods should be
>     qualified, i.e. `IO.println`, `IO.print`, `IO.readln` and
>     `IO.read`. However,
>     when migrating from a simple compilation unit to an ordinary
>     compilation unit,
>     no static import declaration will need to be added, nor will any
>     of these calls
>     need to be rewritten; the on-ramp experience is simpler. For example:
>
>     ```
>     // Simple.java
>     void main() {
>         IO.println("Hello, world.");
>     }
>     ```
>
>     is migrated to:
>
>     ```
>     // Ordinary.java
>     class Ordinary {
>         void main() {
>             IO.println("Hello, world.”);
>         }
>     }
>     ```
>
>
>     ## 2. Changes to the `IO` class
>
>     The new `IO` class is intended to contain the most basic
>     line-oriented I/O
>     methods for beginners.
>
>     Currently the implementation of the methods of this class are thin
>     wrappers
>     around their equivalents in the `Console` class. We propose in JDK
>     25 to
>     decouple `IO` completely from `Console` which we think will better
>     reflect the
>     expectation of developers (see, for example,
>     https://mail.openjdk.org/pipermail/amber-dev/2024-September/008933.html).
>
>     Thus we propose that `IO` printing and reading are based on
>     `System.out` and
>     `System.in`, respectively.
>
>     We do not plan to add other functionality to the `IO` class, e.g.
>     a `readInt`
>     method, in JDK 25. We observe:
>
>     1. This JEP is not the final word on improving the on-ramp
>     experience in Java,
>     but merely the first step. We can continue to work on this area in
>     future
>     JEPs.
>
>     2. I/O and data conversion are, we believe, separate concerns and,
>     as such, data
>     conversion doesn't belong in a basic I/O class. Conversion, and in
>     particular
>     the related issues around error handling, can be considered
>     separately,
>     and given the auto-import of the `java.base` module, we can easily add
>     additional utility classes in support of this in future releases.
>
>     ## 3. Revise some terminology
>
>     We're going to replace the term "simple compilation unit" with
>     "**compact**
>     compilation unit" in the spec (and replace "simple source file"
>     with "compact
>     source file" in the JEP text). Hopefully, "compact" is more
>     concrete, evocative
>     terminology (and we have used it elsewhere with records).
>
>
>     Comments welcome!
>
>     Thanks,
>     Gavin 
>
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