Finalising the on-ramp feature

[Gavin Bierman]

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` 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