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.
Markdown source is below, HTML will appear soon at:
https://openjdk.org/projects/amber/design-notes/on-ramp
# Paving the on-ramp
Java is one of the most widely taught programming languages in the
world. Tens
of thousands of educators find that the imperative core of the
language combined
with a straightforward standard library is a foundation that students can
comfortably learn on. Choosing Java gives educators many degrees of
freedom:
they can situate students in `jshell` or Notepad or a full-fledged
IDE; they can
teach imperative, object-oriented, functional, or hybrid programming
styles; and
they can easily find libraries to interact with external data and
services.
No language is perfect, and one of the most common complaints about
Java is that
it is "too verbose" or has "too much ceremony." And unfortunately,
Java imposes
its heaviest ceremony on those first learning the language, who need and
appreciate it the least. The declaration of a class and the
incantation of
`public static void main` is pure mystery to a beginning programmer.
While
these incantations have principled origins and serve a useful
organizing purpose
in larger programs, they have the effect of placing obstacles in the
path of
_becoming_ Java programmers. Educators constantly remind us of the
litany of
complexity that students have to confront on Day 1 of class -- when
they really
just want to write their first program.
As an amusing demonstration of this, in her JavaOne keynote appearance
in 2019,
[Aimee Lucido](https://www.youtube.com/watch?v=BkPPFiXUwYk) talked
about when
she learned to program in Java, and how her teacher performed a rap song
to help students memorize `"public static void main"`. Our hats are
off to
creative educators everywhere for this kind of dedication, but teachers
shouldn't have to do this.
Of course, advanced programmers complain about ceremony too. We will
never be
able to satisfy programmers' insatiable appetite for typing fewer
keystrokes,
and we shouldn't try, because the goal of programming is to write
programs that
are easy to read and are clearly correct, not programs that were easy
to type.
But we can try to better align the ceremony commensurate with the value it
brings to a program -- and let simple programs be expressed more simply.
## Concept overload
The classic "Hello World" program looks like this in Java:
```
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
```
It may only be five lines, but those lines are packed with concepts
that are
challenging to absorb without already having some programming
experience and
familiarity with object orientation. Let's break down the concepts a
student
confronts when writing their first Java program:
- **public** (on the class). The `public` accessibility level is
relevant
only when there is going to be cross-package access; in a simple
"Hello
World" program, there is only one class, which lives in the
unnamed package.
They haven't even written a one-line program yet; the notion of access
control -- keeping parts of a program from accessing other parts
of it -- is
still way in their future.
- **class**. Our student hasn't set out to write a _class_, or model a
complex system with objects; they want to write a _program_. In
Java, a
program is just a `main` method in some class, but at this point
our student
still has no idea what a class is or why they want one.
- **Methods**. Methods are of course a key concept in Java, but the
mechanics
of methods -- parameters, return types, and invocation -- are still
unfamiliar, and the `main` method is invoked magically from the `java`
launcher rather than from explicit code.
- **public** (again). Like the class, the `main` method has to be
public, but
again this is only relevant when programs are large enough to require
packages to organize them.
- **static**. The `main` method has to be static, and at this
point, students
have no context for understanding what a static method is or why
they want
one. Worse, the early exposure to `static` methods will turn out
to be a
bad habit that must be later unlearned. Worse still, the fact
that the
`main` method is `static` creates a seam between `main` and other
methods;
either they must become `static` too, or the `main` method must
trampoline
to some sort of "instance main" (more ceremony!) And if we get
this wrong,
we get the dreaded and mystifying `"cannot be referenced from a static
context"` error.
- **main**. The name `main` has special meaning in a Java program,
indicating
the starting point of a program, but this specialness hides behind
being an
ordinary method name. This may contribute to the sense of "so
many magic
incantations."
- **String[]**. The parameter to `main` is an array of strings,
which are the
arguments that the `java` launcher collected from the command
line. But our
first program -- likely our first dozen -- will not use command-line
parameters. Requiring the `String[]` parameter is, at this point,
a mistake
waiting to happen, and it will be a long time until this parameter
makes
sense. Worse, educators may be tempted to explain arrays at this
point,
which further increases the time-to-first-program.
- **System.out.println**. If you look closely at this incantation, each
element in the chain is a different thing -- `System` is a class
(what's a
class again?), `out` is a static field (what's a field?), and
`println` is
an instance method. The only part the student cares about right
now is
`println`; the rest of it is an incantation that they do not yet
understand
in order to get at the behavior they want.
That's a lot to explain to a student on the first day of class.
There's a good
chance that by now, class is over and we haven't written any programs
yet, or
the teacher has said "don't worry what this means, you'll understand
it later"
six or eight times. Not only is this a lot of _syntactic_ things to
absorb, but
each of those things appeals to a different concept (class, method,
package,
return value, parameter, array, static, public, etc) that the student
doesn't
have a framework for understanding yet. Each of these will have an
important
role to play in larger programs, but so far, they only contribute to "wow,
programming is complicated."
It won't be practical (or even desirable) to get _all_ of these
concepts out of
the student's face on day 1, but we can do a lot -- and focus on the
ones that
do the most to help beginners understand how programs are constructed.
## Goal: a smooth on-ramp
As much as programmers like to rant about ceremony, the real goal here
is not
mere ceremony reduction, but providing a graceful _on ramp_ to Java
programming.
This on-ramp should be helpful to beginning programmers by requiring
only those
concepts that a simple program needs.
Not only should an on-ramp have a gradual slope and offer enough
acceleration
distance to get onto the highway at the right speed, but its direction
must
align with that of the highway. When a programmer is ready to learn
about more
advanced concepts, they should not have to discard what they've
already learned,
but instead easily see how the simple programs they've already written
generalize to more complicated ones, and both the syntatic and conceptual
transformation from "simple" to "full blown" program should be
straightforward
and unintrusive. It is a definite non-goal to create a "simplified
dialect of
Java for students".
We identify three simplifications that should aid both educators and
students in
navigating the on-ramp to Java, as well as being generally useful to
simple
programs beyond the classroom as well:
- A more tolerant launch protocol
- Unnamed classes
- Predefined static imports for the most critical methods and fields
## A more tolerant launch protocol
The Java Language Specification has relatively little to say about how
Java
"programs" get launched, other than saying that there is some way to
indicate
which class is the initial class of a program (JLS 12.1.1) and that a
public
static method called `main` whose sole argument is of type `String[]`
and whose
return is `void` constitutes the entry point of the indicated class.
We can eliminate much of the concept overload simply by relaxing the
interactions between a Java program and the `java` launcher:
- Relax the requirement that the class, and `main` method, be
public. Public
accessibility is only relevant when access crosses packages; simple
programs
live in the unnamed package, so cannot be accessed from any other
package
anyway. For a program whose main class is in the unnamed package,
we can
drop the requirement that the class or its `main` method be public,
effectively treating the `java` launcher as if it too resided in
the unnamed
package.
- Make the "args" parameter to `main` optional, by allowing the
`java` launcher to
first look for a main method with the traditional `main(String[])`
signature, and then (if not found) for a main method with no arguments.
- Make the `static` modifier on `main` optional, by allowing the
`java` launcher to
invoke an instance `main` method (of either signature) by
instantiating an
instance using an accessible no-arg constructor and then invoking
the `main`
method on it.
This small set of changes to the launch protocol strikes out five of
the bullet
points in the above list of concepts: public (twice), static, method
parameters,
and `String[]`.
At this point, our Hello World program is now:
```
class HelloWorld {
void main() {
System.out.println("Hello World");
}
}
```
It's not any shorter by line count, but we've removed a lot of "horizontal
noise" along with a number of concepts. Students and educators will
appreciate
it, but advanced programmers are unlikely to be in any hurry to make these
implicit elements explicit either.
Additionally, the notion of an "instance main" has value well beyond
the first
day. Because excessive use of `static` is considered a code smell, many
educators encourage the pattern of "all the static `main` method does is
instantiate an instance and call an instance `main` method" anyway.
Formalizing
the "instance main" protocol reduces a layer of boilerplate in these
cases, and
defers the point at which we have to explain what instance creation is
-- and
what `static` is. (Further, allowing the `main` method to be an
instance method
means that it could be inherited from a superclass, which is useful
for simple
frameworks such as test runners or service frameworks.)
## Unnamed classes
In a simple program, the `class` declaration often doesn't help
either, because
other classes (if there are any) are not going to reference it by
name, and we
don't extend a superclass or implement any interfaces. If we say an
"unnamed
class" consists of member declarations without a class header, then
our Hello
World program becomes:
```
void main() {
System.out.println("Hello World");
}
```
Such source files can still have fields, methods, and even nested
classes, so
that as a program evolves from a few statements to needing some
ancillary state
or helper methods, these can be factored out of the `main` method
while still
not yet requiring a full class declaration:
```
String greeting() { return "Hello World"; }
void main() {
System.out.println(greeting());
}
```
This is where treating `main` as an instance method really shines; the
user has
just declared two methods, and they can freely call each other.
Students need
not confront the confusing distinction between instance and static
methods yet;
indeed, if not forced to confront static members on day 1, it might be
a while
before they do have to learn this distinction. The fact that there is a
receiver lurking in the background will come in handy later, but right
now is
not bothering anybody.
[JEP 330](https://openjdk.org/jeps/330) allows single-file programs to be
launched directly without compilation; this streamlined launcher pairs
well with
unnamed classes.
## Predefined static imports
The most important classes, such as `String` and `Integer`, live in the
`java.lang` package, which is automatically on-demand imported into all
compilation units; this is why we do not have to `import
java.lang.String` in
every class. Static imports were not added until Java 5, but no
corresponding
facility for automatic on-demand import of common behavior was added
at that
time. Most programs, however, will want to do console IO, and Java
forces us to
do this in a roundabout way -- through the static `System.out` and
`System.in`
fields. Basic console input and output is a reasonable candidate for
auto-static import, as one or both are needed by most simple
programs. While
these are currently instance methods accessed through static fields,
we can
easily create static methods for `println` and `readln` which are
suitable for
static import, and automatically import them. At which point our
first program
is now down to:
```
void main() {
println("Hello World");
}
```
## Putting this all together
We've discussed several simplifications:
- Update the launcher protocol to make public, static, and arguments
optional
for main methods, and for main methods to be instance methods (when a
no-argument constructor is available);
- Make the class wrapper for "main classes" optional (unnamed classes);
- Automatically static import methods like `println`
which together whittle our long list of day-1 concepts down
considerably. While
this is still not as minimal as the minimal Python or Ruby program --
statements
must still live in a method -- the goal here is not to win at "code
golf". The
goal is to ensure that concepts not needed by simple programs need not
appear in
those programs, while at the same time not encouraging habits that
have to be
unlearned as programs scale up.
Each of these simplifications is individually small and unintrusive,
and each is
independent of the others. And each embodies a simple transformation
that the
author can easily manually reverse when it makes sense to do so: elided
modifiers and `main` arguments can be added back, the class wrapper
can be added
back when the affordances of classes are needed (supertypes,
constructors), and
the full qualifier of static-import can be added back. And these
reversals are
independent of one another; they can done in any combination or any order.
This seems to meet the requirements of our on-ramp; we've eliminated
most of the
day-1 ceremony elements without introducing new concepts that need to be
unlearned. The remaining concepts -- a method is a container for
statements, and
a program is a Java source file with a `main` method -- are easily
understood in
relation to their fully specified counterparts.
## Alternatives
Obviously, we've lived with the status quo for 25+ years, so we could
continue
to do so. There were other alternatives explored as well; ultimately,
each of
these fell afoul of one of our goals.
### Can't we go further?
Fans of "code golf" -- of which there are many -- are surely right now
trying to
figure out how to eliminate the last little bit, the `main` method,
and allow
statements to exist at the top-level of a program. We deliberately
stopped
short of this because it offers little value beyond the first few
minutes, and
even that small value quickly becomes something that needs to be
unlearned.
The fundamental problem behind allowing such "loose" statements is that
variables can be declared inside both classes (fields) and methods (local
variables), and they share the same syntactic production but not the same
semantics. So it is unclear (to both compilers and humans) whether a
"loose"
variable would be a local or a field. If we tried to adopt some sort
of simple
heuristic to collapse this ambiguity (e.g., whether it precedes or
follows the
first statement), that may satisfy the compiler, but now simple
refactorings
might subtly change the meaning of the program, and we'd be replacing the
explicit syntactic overhead of `void main()` with an invisible "line"
in the
program that subtly affects semantics, and a new subtle rule about the
meaning
of variable declarations that applies only to unnamed classes. This
doesn't
help students, nor is this particularly helpful for all but the most
trivial
programs. It quickly becomes a crutch to be discarded and unlearned,
which
falls afoul of our "on ramp" goals. Of all the concepts on our list,
"methods"
and "a program is specified by a main method" seem the ones that are
most worth
asking students to learn early.
### Why not "just" use `jshell`?
While JShell is a great interactive tool, leaning too heavily on it as
an onramp
would fall afoul of our goals. A JShell session is not a program, but a
sequence of code snippets. When we type declarations into `jshell`,
they are
viewed as implicitly static members of some unspecified class, with
accessibility is ignored completely, and statements execute in a
context where
all previous declarations are in scope. This is convenient for
experimentation
-- the primary goal of `jshell` -- but not such a great mental model for
learning to write Java programs. Transforming a batch of working
declarations
in `jshell` to a real Java program would not be sufficiently simple or
unintrusive, and would lead to a non-idiomatic style of code, because the
straightforward translation would have us redeclaring each method,
class, and
variable declaration as `static`. Further, this is probably not the
direction
we want to go when we scale up from a handful of statements and
declarations to
a simple class -- we probably want to start using classes as classes,
not just
as containers for static members. JShell is a great tool for
exploration and
debugging, and we expect many educators will continue to incorporate
it into
their curriculum, but is not the on-ramp programming model we are
looking for.
### What about "always local"?
One of the main tensions that `main` introduces is that most class
members are
not `static`, but the `main` method is -- and that forces programmers to
confront the seam between static and non-static members. JShell
answers this
with "make everything static".
Another approach would be to "make everything local" -- treat a simple
program
as being the "unwrapped" body of an implicit main method. We already
allow
variables and classes to be declared local to a method. We could add
local
methods (a useful feature in its own right) and relax some of the
asymmetries
around nesting (again, an attractive cleanup), and then treat a mix of
declarations and statements without a class wrapper as the body of an
invisible
`main` method. This seems an attractive model as well -- at first.
While the syntactic overhead of converting back to full-blown classes
-- wrap
the whole thing in a `main` method and a `class` declaration -- is far
less
intrusive than the transformation inherent in `jshell`, this is still
not an
ideal on-ramp. Local variables interact with local classes (and
methods, when
we have them) in a very different way than instance fields do with
instance
methods and inner classes: their scopes are different (no forward
references),
their initialization rules are different, and captured local variables
must be
effectively final. This is a subtly different programming model that
would then
have to be unlearned when scaling up to full classes. Further, the
result of
this wrapping -- where everything is local to the main method -- is
also not
"idiomatic Java". So while local methods may be an attractive
feature, they are
similarly not the on-ramp we are looking for.
