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<font size="4"><font face="monospace">Discussion here seems to have
settled down. Here's what seem like the possibly-open parts: <br>
<br>
- Most of the concerns centered around the imports part, not
the main part. We should think some more about alternatives for
Console IO other than auto-importing a static println method;
this might mean that their invocation is something more verbose
than just "println", but better than "System.out.println", and
possibly more discoverable. In the meantime, we'll separate off
the auto-import for STR as part of string templates to decouple
these trains. <br>
<br>
- The notion of "unnamed class" seems to fill a hole alongside
"unnamed package" and "unnamed module", but the analogy is
imperfect, because these classes _will_ have a name, just not
one visible from _inside_ the class. The alternative is to view
these as _implicitly named_ classes, where they have a name,
just derived from the file system rather than the source code.
The distinction might influence whether such simplified class
declarations can have constructors (if you can't name it, you
can't name the constructor), whether static methods or fields
can be accessed from within the class, etc. (However, these
influences are weak; we could still reasonably choose to
disallow constructors and instance initializers even if the name
is denotable.) <br>
<br>
We could, of course, prevent class access from outside the class
too, effectively making such classes visible only to the
launcher. But this would come at no small cost to prevent such
accesses, and for limited benefit. The status-quo proposal
currently says: the name of the class is not visible from within
the class, you can't have constructors or instance initializers,
you get an implicit no-arg constructor like every other
constructor-less class. If you want a more complex construction
protocol, or supertypes, or class declaration annotations, or to
live in a package, declare a class. This still seems pretty
OK. <br>
<br>
<br>
<br>
</font></font><br>
<div class="moz-cite-prefix">On 10/12/2022 7:56 PM, Brian Goetz
wrote:<br>
</div>
<blockquote type="cite" cite="mid:d8e77325-b6a2-f9ea-92a2-7a4c27573b51@oracle.com">
<font size="4"><font face="monospace">I've gotten some good
feedback from this writeup, much of it illuminating how
different people (myself included) make different assumptions
about how certain things work / what things are for. <br>
<br>
#### Role of main<br>
<br>
A main() method can be viewed from two perspectives: it is an
ordinary method with modifiers, parameters, etc, but it is
also an externally significant _entry point_ into a program.
(We've seen a similar double-duty with method parameters in
the context of the underscore discussion: a method parameter
is both a "local" that serves the implementation of the
method, but its also part of the specification of the method
(e.g., the name and type bleed into the Javadoc.)) <br>
<br>
Java made the choice in 1995 that an entry point is "just" a
suitably named / structured / accessible method. This was
somewhat "un-Java-ish", as identification of main-ness is a
structural property, and Java (certainly back then) mostly
avoided such structural techniques where it could. I think
its fair to question whether this decision is still serving us
(Stephen's proposal does exactly that), but I think this was a
pragmatic choice and I'm inclined to continue to run with it,
rather than look at it as something "to be fixed". <br>
<br>
So the proposal broadens the structure parameters around "what
is a main", but ultimately leaves the "some methods are
special" decision in place. (Aside: if we'd had annotations
in Java 1.0, we might well have used an @EntryPoint capture
the metadata of "this method is externally launchable", and
let the launcher reflect over the methods of a class to find a
suitable entry point. But we didn't, and trying to rewrite
history in this way is likely a losing game.) <br>
<br>
#### Role of launcher<br>
<br>
This raises a secondary question about the role of the
launcher. My mental model was one of "the launcher is just
some out-of-sight code that invokes the main method." That
`main` (currently) should be public plays into this, because
if this out-of-sight code is going to call this method, surely
accessibility will have a chance to say "can this code call
this method". I was surprised, though, to find out that
others have a different view of this. Kevin asked whether it
was a "bug" in the model that, in order for the launcher to be
able to call a `main` method, then so must other code. This
is a form of coupling that I had assumed is pure good (because
it means there's a unified model of invocation), but is
reasonable to call it out as a coupling.<br>
<br>
In my view, treating the launcher as "just another caller"
simplifies the story, because we don't have to have a separate
story for invoking entry points. If we're doubling down on
"main is just a method with some special metadata that says
'start here'", then doubling down on accessibility here as
well makes sense to me also. Hence my suggestion to frame the
launcher as "ordinary code in the unnamed package". But as
natural as this seems to me, I recognize that this may seem
weird to others. <br>
<br>
#### Role of unnamed packages<br>
<br>
Another place where there were differing viewpoints is on the
role of unnamed packages. Some felt that using the unnamed
package at all should be reserved for the most transient of
"run it once and delete it" programs; others felt that it had
a role beyond mere experiments. <br>
<br>
In part because of the difficulties described in the On Ramp
document, Java has not found extensive use in scripts, but we
are hoping that this will become more common over time. (The
"Launch Single File Java Programs" JEP was a step towards
being more script-friendly.) I would think that the unnamed
package is the right place for scripts as well, since users
will want to identify the program by its path in the file
system, not by setting up a class path and referring to a
relative position within that. <br>
<br>
Overall, I think the result of this project will be that
*more* code lives in the unnamed package -- and that's OK,
because people will write more simple, self-contained programs
in Java, and the unnamed package is an OK place for these
things to live. <br>
<br>
#### Instance main and interfaces<br>
<br>
Several folks have pointed out that perhaps an instance main
should be specified by an interface (say, Program), and that
unnamed classes implicitly implement Program. This has some
attraction, since this is generally how we prevent names from
being magic. <br>
<br>
There are some downsides, though. We already have two ways to
spell "main"; we might want more in the future, and having to
spell them all out in an interface (even with defaults) could
impede evolution. More importantly, this is using a different
mechanism that we use for the main we currently have, which
means there are two different ways to identify the two kinds
of main. This is starting to veer into the sort of "fix
mistakes of the past" siren song which leaves you with two
different ways of doing the same thing, which appears to new
users as more complexity, not less. <br>
<br>
#### Moral hazards<br>
<br>
A significant proportion of the negative feedback to the
proposal amounted to some sort of "moral hazard". The biggest
component here was that shortening `System.out.println()` to
`println()` would encourage people to use console IO more
often, in places where it was not appropriate, or that it
would be an impediment to testing. I didn't find this
argument very compelling, though; I don't think the number of
characters required to access console IO is the determining
factor here in choosing an IO strategy (or even being aware
that there is such a thing as an IO strategy.)<br>
<br>
#### Console IO<br>
<br>
There were a variety of opinions on the direction we should go
with simplifying console IO. While nearly everyone agrees
that `System.out.println` is a mouthful, there were a number
of alternate approaches suggested, such as:<br>
<br>
- Expose new classes (e.g., SystemOut) with static methods
(SystemOut.println) instead of new static methods;<br>
- Expose a new API for console IO, but don't auto-import
anything;<br>
- Auto-import console IO methods only in unnamed classes;<br>
- others.<br>
<br>
#### Auto imports<br>
<br>
After some thought, I realize that the auto-import feature is
trying to serve two separate goals, and maybe is getting
distorted because of it. The String Templates JEP defines
some canned processors (STR and FMT), which are effectively
part of the language. At first blush (which is when people
are often most vocal about peripheral features), people
already don't like having to type STR-dot (sorry, not changing
our mind about that), and having to import STR feels like
adding insult to injury. STR is effectively part of the
language, so we don't want to place hoops in the way of using
it. But that is a slightly different use case than rescuing
println, for which there are multiple possible paths. Maybe
its best to separate these. <br>
<br>
<br>
The On Ramp document outlined some (admittedly fuzzy) goals
which may be helpful for evaluating some of these thoughts.
We are not trying to create a "beginner's dialect of Java",
nor am I particularly interested in using this effort to turn
back the clock and redesign features that some people wish had
been designed otherwise. The goal here is, to the extent
possible, to make elements of program declaration recede into
the background until they are actually adding value. Access
control, classes, statics, etc, should come into the picture
when they are needed, and not before. I'm sure that there are
other aspects of the language that people would like to have
recede into the background too, and will attempt to attach to
this project, but I'm interested in focusing primarily on
those that are impediments to learning the language. If we do
our job right, these will have benefits well beyond the first
few days of using Java.<br>
<br>
My thoughts: <br>
<br>
- Java made the choice that "entry points" are specially
structured methods, and can be called both programmatically
and via the launcher. I see no reason to tinker with this
concept, or to materially change how entry points are declared
-- just to widen the range of `main` methods that are
considered entry points. <br>
<br>
- While the JLS doesn't really say what the launcher is (just
that there is something that launches programs), positioning
the launcher as some code in the unnamed package seems
helpful, because familiar concepts of access control apply
both to programmatic and command line invocation. <br>
<br>
- While I get that some people find the unnamed package to be
a "smell", I think it is actually fine for a range of
programs, and I expect that range to broaden over time as
people start using Java for scripts. <br>
<br>
- I am sympathetic to the argument that people don't want
"println" and friends to effectively become global; this is
separate from simplifying the locution. Where there may be an
attractive middle ground is:<br>
<br>
- Take the new methods for console IO and put them
somewhere in java.lang, say ConsoleIO;<br>
- Auto-import ConsoleIO only in unnamed classes.<br>
<br>
While this creates a new coupling, it may align well, in that
the programs who most want access to console IO are those that
are most likely to be able to be unnamed classes -- and the
rest can import all the console IO with a single `import
static ConsoleIO.*`. Refactoring to a named class would then
involve adding both the class wrapper and this import. <br>
<br>
- I am open to other approaches for improving the console IO
story. Maybe this is Console.println(), which avoids the need
for auto-static-import at all at the cost of some more words
(but at least doesn't require understanding static fields, and
has the advantage that all the console stuff can be in one
place), or maybe something else. (A big part of the problem
with the status quo now is that not only is System.out.println
undiscoverable, but having found that, you still have no idea
where "readln" is.) <br>
<br>
- While I get why people are attracted to putting instance
main in an interface, I don't think I'd like the result. As
mentioned above, this implicitly places an impediment in
future expansion of the launcher protocol, but more
importantly, means we have two mechanisms for finding main --
an "old" and "new" one. I try hard to avoid creating this
situation, because the benefit is rarely worth the increased
cognitive load. <br>
<br>
<br>
<br>
<br>
</font></font><br>
<div class="moz-cite-prefix">On 9/28/2022 1:57 PM, Brian Goetz
wrote:<br>
</div>
<blockquote type="cite" cite="mid:1b6200d3-a7a6-6479-e8ab-d932eedbceb1@oracle.com"> <font size="4"><font face="monospace">At various points, we've
explored the question of which program elements are most and
least helpful for students first learning Java. After
considering a number of alternatives over the years, I have
a simple proposal for smoothing the "on ramp" to Java
programming, while not creating new things to unlearn. <br>
<br>
Markdown source is below, HTML will appear soon at: <br>
<br>
<a class="moz-txt-link-freetext" href="https://openjdk.org/projects/amber/design-notes/on-ramp" moz-do-not-send="true">https://openjdk.org/projects/amber/design-notes/on-ramp</a><br>
<br>
<br>
# Paving the on-ramp<br>
<br>
Java is one of the most widely taught programming languages
in the world. Tens<br>
of thousands of educators find that the imperative core of
the language combined<br>
with a straightforward standard library is a foundation that
students can<br>
comfortably learn on. Choosing Java gives educators many
degrees of freedom:<br>
they can situate students in `jshell` or Notepad or a
full-fledged IDE; they can<br>
teach imperative, object-oriented, functional, or hybrid
programming styles; and<br>
they can easily find libraries to interact with external
data and services. <br>
<br>
No language is perfect, and one of the most common
complaints about Java is that<br>
it is "too verbose" or has "too much ceremony." And
unfortunately, Java imposes<br>
its heaviest ceremony on those first learning the language,
who need and<br>
appreciate it the least. The declaration of a class and the
incantation of<br>
`public static void main` is pure mystery to a beginning
programmer. While<br>
these incantations have principled origins and serve a
useful organizing purpose<br>
in larger programs, they have the effect of placing
obstacles in the path of<br>
_becoming_ Java programmers. Educators constantly remind us
of the litany of<br>
complexity that students have to confront on Day 1 of class
-- when they really<br>
just want to write their first program. <br>
<br>
As an amusing demonstration of this, in her JavaOne keynote
appearance in 2019,<br>
[Aimee Lucido](<a class="moz-txt-link-freetext" href="https://www.youtube.com/watch?v=BkPPFiXUwYk" moz-do-not-send="true">https://www.youtube.com/watch?v=BkPPFiXUwYk</a>)
talked about when<br>
she learned to program in Java, and how her teacher
performed a rap song<br>
to help students memorize `"public static void main"`. Our
hats are off to<br>
creative educators everywhere for this kind of dedication,
but teachers<br>
shouldn't have to do this.<br>
<br>
Of course, advanced programmers complain about ceremony
too. We will never be<br>
able to satisfy programmers' insatiable appetite for typing
fewer keystrokes,<br>
and we shouldn't try, because the goal of programming is to
write programs that<br>
are easy to read and are clearly correct, not programs that
were easy to type.<br>
But we can try to better align the ceremony commensurate
with the value it<br>
brings to a program -- and let simple programs be expressed
more simply. <br>
<br>
## Concept overload<br>
<br>
The classic "Hello World" program looks like this in Java:<br>
<br>
```<br>
public class HelloWorld { <br>
public static void main(String[] args) { <br>
System.out.println("Hello World");<br>
}<br>
}<br>
```<br>
<br>
It may only be five lines, but those lines are packed with
concepts that are<br>
challenging to absorb without already having some
programming experience and<br>
familiarity with object orientation. Let's break down the
concepts a student<br>
confronts when writing their first Java program:<br>
<br>
- **public** (on the class). The `public` accessibility
level is relevant<br>
only when there is going to be cross-package access; in
a simple "Hello<br>
World" program, there is only one class, which lives in
the unnamed package.<br>
They haven't even written a one-line program yet; the
notion of access<br>
control -- keeping parts of a program from accessing
other parts of it -- is<br>
still way in their future.<br>
<br>
- **class**. Our student hasn't set out to write a
_class_, or model a<br>
complex system with objects; they want to write a
_program_. In Java, a<br>
program is just a `main` method in some class, but at
this point our student<br>
still has no idea what a class is or why they want one.<br>
<br>
- **Methods**. Methods are of course a key concept in
Java, but the mechanics<br>
of methods -- parameters, return types, and invocation
-- are still<br>
unfamiliar, and the `main` method is invoked magically
from the `java`<br>
launcher rather than from explicit code. <br>
<br>
- **public** (again). Like the class, the `main` method
has to be public, but<br>
again this is only relevant when programs are large
enough to require<br>
packages to organize them. <br>
<br>
- **static**. The `main` method has to be static, and at
this point, students<br>
have no context for understanding what a static method
is or why they want<br>
one. Worse, the early exposure to `static` methods will
turn out to be a<br>
bad habit that must be later unlearned. Worse still,
the fact that the<br>
`main` method is `static` creates a seam between `main`
and other methods;<br>
either they must become `static` too, or the `main`
method must trampoline<br>
to some sort of "instance main" (more ceremony!) And if
we get this wrong,<br>
we get the dreaded and mystifying `"cannot be referenced
from a static<br>
context"` error.<br>
<br>
- **main**. The name `main` has special meaning in a Java
program, indicating<br>
the starting point of a program, but this specialness
hides behind being an<br>
ordinary method name. This may contribute to the sense
of "so many magic<br>
incantations."<br>
<br>
- **String[]**. The parameter to `main` is an array of
strings, which are the<br>
arguments that the `java` launcher collected from the
command line. But our<br>
first program -- likely our first dozen -- will not use
command-line<br>
parameters. Requiring the `String[]` parameter is, at
this point, a mistake<br>
waiting to happen, and it will be a long time until this
parameter makes<br>
sense. Worse, educators may be tempted to explain
arrays at this point,<br>
which further increases the time-to-first-program.<br>
<br>
- **System.out.println**. If you look closely at this
incantation, each<br>
element in the chain is a different thing -- `System` is
a class (what's a<br>
class again?), `out` is a static field (what's a
field?), and `println` is<br>
an instance method. The only part the student cares
about right now is<br>
`println`; the rest of it is an incantation that they do
not yet understand<br>
in order to get at the behavior they want.<br>
<br>
That's a lot to explain to a student on the first day of
class. There's a good<br>
chance that by now, class is over and we haven't written any
programs yet, or<br>
the teacher has said "don't worry what this means, you'll
understand it later"<br>
six or eight times. Not only is this a lot of _syntactic_
things to absorb, but<br>
each of those things appeals to a different concept (class,
method, package,<br>
return value, parameter, array, static, public, etc) that
the student doesn't<br>
have a framework for understanding yet. Each of these will
have an important<br>
role to play in larger programs, but so far, they only
contribute to "wow,<br>
programming is complicated." <br>
<br>
It won't be practical (or even desirable) to get _all_ of
these concepts out of<br>
the student's face on day 1, but we can do a lot -- and
focus on the ones that<br>
do the most to help beginners understand how programs are
constructed.<br>
<br>
## Goal: a smooth on-ramp<br>
<br>
As much as programmers like to rant about ceremony, the real
goal here is not<br>
mere ceremony reduction, but providing a graceful _on ramp_
to Java programming.<br>
This on-ramp should be helpful to beginning programmers by
requiring only those<br>
concepts that a simple program needs. <br>
<br>
Not only should an on-ramp have a gradual slope and offer
enough acceleration<br>
distance to get onto the highway at the right speed, but its
direction must<br>
align with that of the highway. When a programmer is ready
to learn about more<br>
advanced concepts, they should not have to discard what
they've already learned,<br>
but instead easily see how the simple programs they've
already written<br>
generalize to more complicated ones, and both the syntatic
and conceptual<br>
transformation from "simple" to "full blown" program should
be straightforward<br>
and unintrusive. It is a definite non-goal to create a
"simplified dialect of<br>
Java for students".<br>
<br>
We identify three simplifications that should aid both
educators and students in<br>
navigating the on-ramp to Java, as well as being generally
useful to simple<br>
programs beyond the classroom as well:<br>
<br>
- A more tolerant launch protocol<br>
- Unnamed classes<br>
- Predefined static imports for the most critical methods
and fields<br>
<br>
## A more tolerant launch protocol<br>
<br>
The Java Language Specification has relatively little to say
about how Java<br>
"programs" get launched, other than saying that there is
some way to indicate<br>
which class is the initial class of a program (JLS 12.1.1)
and that a public<br>
static method called `main` whose sole argument is of type
`String[]` and whose<br>
return is `void` constitutes the entry point of the
indicated class. <br>
<br>
We can eliminate much of the concept overload simply by
relaxing the<br>
interactions between a Java program and the `java` launcher:<br>
<br>
- Relax the requirement that the class, and `main` method,
be public. Public<br>
accessibility is only relevant when access crosses
packages; simple programs<br>
live in the unnamed package, so cannot be accessed from
any other package<br>
anyway. For a program whose main class is in the unnamed
package, we can<br>
drop the requirement that the class or its `main` method
be public,<br>
effectively treating the `java` launcher as if it too
resided in the unnamed<br>
package.<br>
<br>
- Make the "args" parameter to `main` optional, by allowing
the `java` launcher to<br>
first look for a main method with the traditional
`main(String[])`<br>
signature, and then (if not found) for a main method with
no arguments.<br>
<br>
- Make the `static` modifier on `main` optional, by
allowing the `java` launcher to<br>
invoke an instance `main` method (of either signature) by
instantiating an<br>
instance using an accessible no-arg constructor and then
invoking the `main`<br>
method on it.<br>
<br>
This small set of changes to the launch protocol strikes out
five of the bullet<br>
points in the above list of concepts: public (twice),
static, method parameters,<br>
and `String[]`. <br>
<br>
At this point, our Hello World program is now:<br>
<br>
```<br>
class HelloWorld { <br>
void main() { <br>
System.out.println("Hello World");<br>
}<br>
}<br>
```<br>
<br>
It's not any shorter by line count, but we've removed a lot
of "horizontal<br>
noise" along with a number of concepts. Students and
educators will appreciate<br>
it, but advanced programmers are unlikely to be in any hurry
to make these<br>
implicit elements explicit either. <br>
<br>
Additionally, the notion of an "instance main" has value
well beyond the first<br>
day. Because excessive use of `static` is considered a code
smell, many<br>
educators encourage the pattern of "all the static `main`
method does is<br>
instantiate an instance and call an instance `main` method"
anyway. Formalizing<br>
the "instance main" protocol reduces a layer of boilerplate
in these cases, and<br>
defers the point at which we have to explain what instance
creation is -- and<br>
what `static` is. (Further, allowing the `main` method to
be an instance method<br>
means that it could be inherited from a superclass, which is
useful for simple<br>
frameworks such as test runners or service frameworks.)<br>
<br>
## Unnamed classes<br>
<br>
In a simple program, the `class` declaration often doesn't
help either, because<br>
other classes (if there are any) are not going to reference
it by name, and we<br>
don't extend a superclass or implement any interfaces. If
we say an "unnamed<br>
class" consists of member declarations without a class
header, then our Hello<br>
World program becomes:<br>
<br>
```<br>
void main() { <br>
System.out.println("Hello World");<br>
}<br>
```<br>
<br>
Such source files can still have fields, methods, and even
nested classes, so<br>
that as a program evolves from a few statements to needing
some ancillary state<br>
or helper methods, these can be factored out of the `main`
method while still<br>
not yet requiring a full class declaration:<br>
<br>
```<br>
String greeting() { return "Hello World"; }<br>
<br>
void main() {<br>
System.out.println(greeting());<br>
}<br>
```<br>
<br>
This is where treating `main` as an instance method really
shines; the user has<br>
just declared two methods, and they can freely call each
other. Students need<br>
not confront the confusing distinction between instance and
static methods yet;<br>
indeed, if not forced to confront static members on day 1,
it might be a while<br>
before they do have to learn this distinction. The fact
that there is a<br>
receiver lurking in the background will come in handy later,
but right now is<br>
not bothering anybody.<br>
<br>
[JEP 330](<a class="moz-txt-link-freetext" href="https://openjdk.org/jeps/330" moz-do-not-send="true">https://openjdk.org/jeps/330</a>)
allows single-file programs to be<br>
launched directly without compilation; this streamlined
launcher pairs well with<br>
unnamed classes. <br>
<br>
## Predefined static imports<br>
<br>
The most important classes, such as `String` and `Integer`,
live in the<br>
`java.lang` package, which is automatically on-demand
imported into all<br>
compilation units; this is why we do not have to `import
java.lang.String` in<br>
every class. Static imports were not added until Java 5,
but no corresponding<br>
facility for automatic on-demand import of common behavior
was added at that<br>
time. Most programs, however, will want to do console IO,
and Java forces us to<br>
do this in a roundabout way -- through the static
`System.out` and `System.in`<br>
fields. Basic console input and output is a reasonable
candidate for<br>
auto-static import, as one or both are needed by most simple
programs. While<br>
these are currently instance methods accessed through static
fields, we can<br>
easily create static methods for `println` and `readln`
which are suitable for<br>
static import, and automatically import them. At which
point our first program<br>
is now down to:<br>
<br>
```<br>
void main() {<br>
println("Hello World");<br>
}<br>
```<br>
<br>
## Putting this all together<br>
<br>
We've discussed several simplifications:<br>
<br>
- Update the launcher protocol to make public, static, and
arguments optional<br>
for main methods, and for main methods to be instance
methods (when a<br>
no-argument constructor is available); <br>
- Make the class wrapper for "main classes" optional
(unnamed classes);<br>
- Automatically static import methods like `println`<br>
<br>
which together whittle our long list of day-1 concepts down
considerably. While<br>
this is still not as minimal as the minimal Python or Ruby
program -- statements<br>
must still live in a method -- the goal here is not to win
at "code golf". The<br>
goal is to ensure that concepts not needed by simple
programs need not appear in<br>
those programs, while at the same time not encouraging
habits that have to be<br>
unlearned as programs scale up. <br>
<br>
Each of these simplifications is individually small and
unintrusive, and each is<br>
independent of the others. And each embodies a simple
transformation that the<br>
author can easily manually reverse when it makes sense to do
so: elided<br>
modifiers and `main` arguments can be added back, the class
wrapper can be added<br>
back when the affordances of classes are needed (supertypes,
constructors), and<br>
the full qualifier of static-import can be added back. And
these reversals are<br>
independent of one another; they can done in any combination
or any order.<br>
<br>
This seems to meet the requirements of our on-ramp; we've
eliminated most of the<br>
day-1 ceremony elements without introducing new concepts
that need to be<br>
unlearned. The remaining concepts -- a method is a container
for statements, and<br>
a program is a Java source file with a `main` method -- are
easily understood in<br>
relation to their fully specified counterparts. <br>
<br>
## Alternatives<br>
<br>
Obviously, we've lived with the status quo for 25+ years, so
we could continue<br>
to do so. There were other alternatives explored as well;
ultimately, each of<br>
these fell afoul of one of our goals.<br>
<br>
### Can't we go further?<br>
<br>
Fans of "code golf" -- of which there are many -- are surely
right now trying to<br>
figure out how to eliminate the last little bit, the `main`
method, and allow<br>
statements to exist at the top-level of a program. We
deliberately stopped<br>
short of this because it offers little value beyond the
first few minutes, and<br>
even that small value quickly becomes something that needs
to be unlearned. <br>
<br>
The fundamental problem behind allowing such "loose"
statements is that<br>
variables can be declared inside both classes (fields) and
methods (local<br>
variables), and they share the same syntactic production but
not the same<br>
semantics. So it is unclear (to both compilers and humans)
whether a "loose"<br>
variable would be a local or a field. If we tried to adopt
some sort of simple<br>
heuristic to collapse this ambiguity (e.g., whether it
precedes or follows the<br>
first statement), that may satisfy the compiler, but now
simple refactorings<br>
might subtly change the meaning of the program, and we'd be
replacing the<br>
explicit syntactic overhead of `void main()` with an
invisible "line" in the<br>
program that subtly affects semantics, and a new subtle rule
about the meaning<br>
of variable declarations that applies only to unnamed
classes. This doesn't<br>
help students, nor is this particularly helpful for all but
the most trivial<br>
programs. It quickly becomes a crutch to be discarded and
unlearned, which<br>
falls afoul of our "on ramp" goals. Of all the concepts on
our list, "methods"<br>
and "a program is specified by a main method" seem the ones
that are most worth<br>
asking students to learn early.<br>
<br>
### Why not "just" use `jshell`? <br>
<br>
While JShell is a great interactive tool, leaning too
heavily on it as an onramp<br>
would fall afoul of our goals. A JShell session is not a
program, but a<br>
sequence of code snippets. When we type declarations into
`jshell`, they are<br>
viewed as implicitly static members of some unspecified
class, with<br>
accessibility is ignored completely, and statements execute
in a context where<br>
all previous declarations are in scope. This is convenient
for experimentation<br>
-- the primary goal of `jshell` -- but not such a great
mental model for<br>
learning to write Java programs. Transforming a batch of
working declarations<br>
in `jshell` to a real Java program would not be sufficiently
simple or<br>
unintrusive, and would lead to a non-idiomatic style of
code, because the<br>
straightforward translation would have us redeclaring each
method, class, and<br>
variable declaration as `static`. Further, this is probably
not the direction<br>
we want to go when we scale up from a handful of statements
and declarations to<br>
a simple class -- we probably want to start using classes as
classes, not just<br>
as containers for static members. JShell is a great tool for
exploration and<br>
debugging, and we expect many educators will continue to
incorporate it into<br>
their curriculum, but is not the on-ramp programming model
we are looking for. <br>
<br>
### What about "always local"?<br>
<br>
One of the main tensions that `main` introduces is that most
class members are<br>
not `static`, but the `main` method is -- and that forces
programmers to<br>
confront the seam between static and non-static members.
JShell answers this<br>
with "make everything static". <br>
<br>
Another approach would be to "make everything local" --
treat a simple program<br>
as being the "unwrapped" body of an implicit main method.
We already allow<br>
variables and classes to be declared local to a method. We
could add local<br>
methods (a useful feature in its own right) and relax some
of the asymmetries<br>
around nesting (again, an attractive cleanup), and then
treat a mix of<br>
declarations and statements without a class wrapper as the
body of an invisible<br>
`main` method. This seems an attractive model as well -- at
first.<br>
<br>
While the syntactic overhead of converting back to
full-blown classes -- wrap<br>
the whole thing in a `main` method and a `class` declaration
-- is far less<br>
intrusive than the transformation inherent in `jshell`, this
is still not an<br>
ideal on-ramp. Local variables interact with local classes
(and methods, when<br>
we have them) in a very different way than instance fields
do with instance<br>
methods and inner classes: their scopes are different (no
forward references),<br>
their initialization rules are different, and captured local
variables must be<br>
effectively final. This is a subtly different programming
model that would then<br>
have to be unlearned when scaling up to full classes.
Further, the result of<br>
this wrapping -- where everything is local to the main
method -- is also not<br>
"idiomatic Java". So while local methods may be an
attractive feature, they are<br>
similarly not the on-ramp we are looking for.<br>
<br>
<br>
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