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Code Reviewing My Earliest Surviving Program

Posted 2016-01-08
Hello World BlogStrategic Scala Style: Principle of Least Power

Programmers usually view old, legacy code with a mix of fear and respect. As a professional software engineer, you often have to deal with systems written long before you turned up. You have to dive into them, understand them, review them, and improve them. You often wonder...

Why did they do this


What were they thinking?

Sometimes to realize later that the they is in fact, yourself!

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy Haoyi's book Hands-on Scala Programming

You learn things to get the job done: the code's purpose, it's constraints and abilities. Often though, you learn more than that: you feel the thoughts, feelings and characters of the people who wrote it. You feel their frustration leaking through their comments and workarounds. Their style, their imprint on the code that is hard to quantify but often uniquely identifies the code as "theirs". While figuring out how code works, you often walk the same path walked by someone months or years ago, with all the same triumphs and failures.

Old code is literally the crystallized logic and thoughts of someone long ago. In understanding the code, you are re-constructing that person's thoughts. Even when that someone is yourself, you as an individual change over the months and years, and best practices evolve, so even when you yourself wrote the code it always feels slightly foreign.

With that in mind, I went to take a look at some of my own earliest work, over a decade old, to give it a modern, professional review given what I know now as a software engineer. I will contrast what 2004-me did with what 2016-me would want, and consider the 12 years of progress in experience, tools, and the overall maturity of the field of software engineering that lead to these differences!


The earliest working code I could find dates back from 2004.

I was 15 years old, and had just learned programming in secondary-school. I would eventually pick up a huge number of various programming-in-Java books, write a 3D combat flight simulator, learn C#, pick up Python, PHP & Javascript in MIT, become involved in the Open-Source Scala community, and get a job at Dropbox. However, at the time, there I was with 1 year of Java lessons in school, and probably having worked through a single book, Java in Easy Steps:


It wasn't just myself which was younger in 2004, the entire field was younger. In 2004, Java was on version 1.4. The Scala language barely existed. Facebook had just started writing their world-changing website in PHP, possibly because at the time web frameworks like Ruby on Rails and Django did not exist.

In this case, the 2004 code is a 2D Asteroids clone, written in Java. I had written earlier programs: for class, for my own amusement. Those include some pretty elaborate console applications: I had a complete 2D-grid console-based Dungeons-&-Dragons game implemented at one point. But none of those earlier works survive in an intact state today.

Here's how it begins:

class Asteroids extends JFrame implements KeyListener, ActionListener{

    final int X = GUI.X;
    final int Y = GUI.Y;

    class Projectile{
        int SHOTS = 100;
        double POSY;
        double POSX;

The full ~300 lines of code can be found on Github:

I did not know what git was back then! I more recently uploaded it in case anyone wants to take a look. You can download the contents from github and run it with Java today. This game, and a few others in the same file, can also be played online:

As I had ported them to Scala.js as an exercise a year or two ago to try out the capabilities of the then-very-nascent Scala.js project. As far as I can tell, the games play exactly identically in the browser as they used to on the desktop, except for minor differences in anti-aliasing and the lack of sound.

Running it

The first thing you try to do with legacy code is run it. Whether through automated tests, manual fiddling, running the code and seeing it work should give you a quick sense of what it is the application does. Broad questions like:

When you see all the disjoint snippets of code later during review, you'll be better able to place them in the overall scope of the application.

You should be able to try out this code right now! The compiled class-files are checked into the git repository mixed in with all the sources (best practices? What're those?)

haoyi-mbp:~ haoyi$ git clone https://github.com/lihaoyi/Java-Games
Cloning into 'Java-Games'...
remote: Counting objects: 680, done.
remote: Total 680 (delta 0), reused 0 (delta 0), pack-reused 680
Receiving objects: 100% (680/680), 16.95 MiB | 2.37 MiB/s, done.
Resolving deltas: 100% (228/228), done.
Checking connectivity... done.

haoyi-mbp:~ haoyi$ cd Java-Games/GameLibrary/

haoyi-mbp:GameLibrary haoyi$ java GameLibrary

This should pop up the game picker:


Clicking on Asteroids should open up the game we're looking at:


And you control your ship with Left, Right Up, Down arrows, and spacebar to shoot. Destroy all the asteroids and don't get hit. That's all there is!

Now that we've got the code running - the first step in dealing with any legacy code - let's review it!

Code Review

Overall, it's a relatively simple ~300 lines of code: I have Vectors of objects representing the asteroids and bullets moving around (because there are more than one), each one with some variables representing their speed and size and whatnot. I have some variables representing the player (only one). I listen to key events and update the controls LEFT RIGHT FORWARD BACK when you press or release something. I have a timer that calls the actionPerformed method, which then does all the erasing of images, moving of objects, re-drawing, and collision-detection.

Let's go into individual facets of this codebase.

File Layout

The first thing you've probably already noticed is the fact that the code for this game is in one huge, 12 thousand line file, together with the code for all sorts of other games and utilities. In total there only are a few .java files in the GUI/ folder. Many seem very random:

There are also two folders, one containing more code, on containing some .wav files.

In general, most of these files can be ignored for now: they're only used in later games (*also* in GameLibrary.java!) that make use of 3D graphics, files, etc., and Asteroids doesn't need any of it. The GUI/ folder itself was presumably named because this is where all the graphical interfaced, non-terminal programs I ever wrote lived. The code we're interested in all lives in the first few hundred lines of GameLibrary.java.

Suffice to say, directory/file organization would not pass code review in my workplace today. In 2004, though, I was not competent at working with multiple files at a time! I was performing a lot of work using Notepad and running javac and java from the Windows Command Prompt, or I was using Netbeans as Notepad with syntax highlighting, ignoring the rest of its features. Having everything in one file meant Ctrl-F to find things was easy, as was finding things via their position in the scroll-bar. Given my skill level at the time, this "everything in one file" approach may well have been optimal! With a modern text editor, window manager and easy navigation between files, it makes more sense to quickly start breaking it up into separate files.

Class Structure

We've already gone through the "macro-level" layout of the entire folder, let's take a look at the structure of the Asteroids class itself.

class Asteroids extends JFrame implements KeyListener, ActionListener

To begin with, we see that it extends JFrame. This is done because the program is a Java Swing application, and I wanted the Asteroids game to fit in as a Swing component. In theory, the logic for the game could easily be separated from the the GUI toolkit logic: that would follow the best practice of Composition over Inheritance. However, in 2004, 15-year-old-me did not know anything about best practices, so here we have it.

The Asteroids class also implements KeyListener and ActionListener. As far as I can tell, that was the best practice at the time: if you wanted access to an event-handler in some code, you have the surrounding class implement the event handler and add yourself as a listener, e.g. further down


If I was programming in Python, C#, Javascript, or Scala today, I would not do that. I would pass in a callback function to addKeyListener. However, in Java you did not have callback functions, so this inheritence dancewas how you received the callback. In theory I could pass in a clunky Anonymous Inner Class, but I would not learn I could use those as event handlers for another 4 years (halfway through college). Even in Java today, Java 8 provides inline Function Literals, so I wouldn't be surprised if the convention gradually changed. At the time though, in Java, this was the way you did things.

The insides of the Asteroids class is laid out roughly as follows:

class Asteroids...{
    <constants for X and Y>
    <game variables>
    <class constructor>
    <event handlers>
    <canvas plumbing code>

This isn't a precise description: there are some helper methods scattered around various places such as createAsteroids, or splitAsteroid, but it's pretty close. The rest of the games in GameLibrary.java follow the same layout convention.

It turns out, that even back then I had used Java's Nested Classes without really ever having thought about it, or all the subtleties related to them. To 15-year-old me, a class was where I put all my stuff related to something: whether they were methods, fields, or other classes, it never occurred to me that they would be treated differently. Only later did I realize how you couldn't put a method inside another method, or that inner-classes defined inside methods had odd limitations such as only being able to access final enclosing fields.

But now, in 2016, I'm programming in Scala and am again free to put "anything inside anything", regardless of whether those "anything"s are classes, methods or variables. And that's the way it should be!

Naming Conventions

Almost immediately, anyone looking at this code would notice it is rather odd: All variables are ALLCAPS, except some which are arbitrarily PascalCased! For example:

class Projectile{
    int SHOTS = 100;
    double POSY;
    double POSX;
    double YMOMENTUM;
    double XMOMENTUM;
    double SPEED;
    boolean CONTROLABLE;
    boolean EXITED = false;
    int IMAGE;
    int TYPE;
    int SIZE = 3;
    Shape Image;

That does not fit into any naming convention I've ever seen, and I have no idea where I picked it up. Later on, if you look at the code towards the bottom of this file, I dropped the ALLCAPS naming and kept variables PascalCased.

It wasn't until I was in college, years later in 2010, that I realized that the rest of the world was using camelCase for their variables! I remember flipping through all my old books for validation that I, in fact, picked up the PascalCased-variables convention from somewhere. Alas, all of them had "standard" camelCased variables. It turns out I must have made it all up myself!

Nowadays, professionally, I am extremely conscious of this sort of style choice or convention, trying hard to ensure it's consistent across the team, organization, or better yet with the rest of the broader community. At the time, though, I was programming basically alone, and idiosyncrasies, like this mistaken naming convention, could persist for years.

Don't Repeat Yourself

In general, the code is relatively repetitive. For example, a lot of code is duplicated for X and Y

XMOD = SPEED * Math.cos(Math.toRadians(ANGLE - 30));
YMOD = SPEED * Math.sin(Math.toRadians(ANGLE - 30));

Because I did not have a proper 2D point class with the relevant point/vector operations (this is fixed in the next game, which uses java.awt.Point2D.Float). By this point I was already capable of defining objects to represent common structures I find in my data, and had done so in earlier programs, but it mustn't have occured to me to do so here. In general, I was a lot less averse to duplicate code in 2004 than I am today in 2016.

Some other code needs repetitive casting because I was on Java 1.3 and 1.4 at the time, which did not have generics: in Java 5+ we would defined Shots and Asteroids as

Vector Asteroids = new Vector<Projectile>();
Vector Shots = new Vector<Projectile>();

And not need to do any of those casts.

Nowadays, having this sort of generic container is table-stakes in many programming languages. Google Go gets a lot of flak for not having them only because they have become part of the expectation of software engineers working in statically-typed languages. In 2004, though, generics were still a relatively new and novel concept, and so I just did without.

Lastly, other parts of the code seemed to have repetitive casting for no apparent reason. This would not pass code review today!


The entire program basically works via mutable state: the changing states of the player, bullets and asteroids is modelled via mutable state, which is probably what I would do even today. The initialization of the Swing panel, setting its size and properties, is also done by mutable state:

this.setSize(X, Y);
Container contentArea = getContentPane();

Area = new Background();


As was the convention in Java and the Swing library (which is builtin and not something I wrote). Nowadays, I know that this is sub-optimal: this means you are running lots of code while the window still hasn't finished being initialized, and accessing the wrong field or method during this initialization process could return you garbage data and cause weird bugs.

Ideally you would want the window to be in a "ready" state the moment it is finished constructing: pass in constructor arguments, get working window. There are some problems with this approach thought:

Much of the "configuration" is optional

Java does not have a good way to handle optional arguments. Apart from using mutator set methods to allow optional-ness, Telescoping Constructors are another method which unfortunately doesn't scale as the number of optional arguments grows more than 4 or 5. In 2016 many in the community have switched to the Builder Pattern, and if I was writing Java today that would hopefully be what the library exposes to me.

If I was writing in C# or Python or Scala or some other modern language, I would just use optional constructor arguments and avoid this whole discussion.

Many of these configuration options actually model mutable properties

For example, the size of the window can be changed later after creation, and listeners can be attached and detached. Given that, it makes sense to have the configuration itself simply be mutable properties: you don't want to have to close and re-open the window every time you want to tweak it's size! That's the "obvious" downside if the window's properties are immutable.

In 2016, we actually do have a better way around this. In general, the Virtual Dom concept pioneered by React.js has proven itself in many production environments, and it has shown that you can in fact model GUI components as immutable data structures. With a Virtual Dom, your code would indeed throw away the old immutable virtual window object and create a whole new immutable virtual window object, but the framework would do the grunt work of making sure only the bits of the real window that differ are actually changed! That way your code works largely with immutable copies of the window (which are less prone to bugs involving accidental/hard-to-track-down mutation) while the user would see and interact with a single, long-lived, smoothly-updated entity: the best of both worlds

While some of the mutable state is inherent in modelling the mutable game, much of it isn't. With modern 2016 technology, we now have techniques that help to reduce the amount of this "incidental" mutation: the Builder Pattern and languages with optional method/constructor arguments would remove the need for mutability as optional configuration values, and modern techniques like the Virtual Dom even let us remove the mutability from the code modelling long-lived, mutable GUIs.

But none of this existed in 2004 Java. While Python and C# existed, both of them were relatively niche languages no where near as popular or widespread as they are today. The Scala language only appeared in 2004, and has only become popular in the last few years. React.js and its Virtual Dom technique only appeared in 2013.

In 2004 Java Swing, that's just the way it is.

Modern Comparison

That's the review, so how would it compare to code I would write today? Luckily we can actually do this comparison, because I ported the code to Scala two years ago to make it run with Scala.js in the browser:

The source code for this demo is also available online:

It is comparable in scope to the original Java code linked above: while the Scala version has some helpers e.g. a 2D point class, the Java code also has such things available through the Java standard library.

Overall, the code looks pretty different. Large nested constructs like the original collision detection loop used to walk over the data structures and both mutate them and draw on the canvas in-line:

for(int i = 0; i < Shots.size(); i++){
    for(int j = 0; j < Asteroids.size(); j++){
                if(((Projectile)Asteroids.elementAt(j)).SIZE > 1){
                    Projectile ASTEROID = ((Projectile)Asteroids.elementAt(j));
                    Projectile SHOT = ((Projectile)Shots.elementAt(i));
                    splitAsteroid(ASTEROID, SHOT);


        }catch(Exception n){}

In the Scala collision detection code, this gets replaced by a number of queries to figure out what needs to happen, and then only then doing all the mutation all at one go at the end

val changes = for{
  b <- bullets
  a <- asteroids
  if a.contains(b.position)
} yield {
  val newAsteroids =
    if (a.level == 1) Nil
    else {
      Seq(30, -30).map(d =>
        new Asteroid(a.level - 1, a.position, a.momentum.rotate(d*Math.PI/180))
  (Seq(a, b), newAsteroids)
val (removed, added) = changes.unzip
val flatRemoved = removed.flatten
asteroids = asteroids.filter(!flatRemoved.contains(_)) ++ added.flatten
bullets =
    .filter(_.position.within(Point(0, 0), bounds))

This is hopefully easier to reason about, as the "reading" and the "writing" get split up into two distinct phases: you no longer need to worry about your writing somehow messing with the things you are about to read. One interesting thing is that the old code pain-stakingly blacked out the shapes which were removed. The modern code, on the other hand, simply removes the shapes and relies on the per-frame wipe-canvas-redraw-everything step to draw things back correctly. This is another technique that I had learned by the games later down that file, but did not know when i wrote the 2004 version of Asteroids.

The modern Scala version is overall about 40% the size of the original Java version. To me, that represents progress: that's a significant savings in code that will need to be written, read, and debugged. Arguably, the "overhead" in writing this code has dropped by an even larger ratio, since there is some intrinsic complexity to the logic that will always need to be described! I'd say the modern version is barely above the minimum complexity needed to implement the game, while the original version has tens of times as much unnecessary overhead and complexity.


Clearly the old Java code has issues. Compared to the modern Scala version, it is 2.5x as long, follows no community conventions, uses out of date techniques, and has poor organization. Not entirely surprising, given that it was written by some 14-year-old kid, in 2004 Java, with no community, training or experience! Despite it's flaws, code is never going anywhere. It was written 12 years ago by a younger, less experienced me, and served it's purposes as a stepping stone for me to learn more about programming.

As described in the beginning, the point of this exercise is not to criticise but to learn. To see how the thought process has evolved since the code was written 12 years ago, what has changed and what has stayed the same. Far from just calling it "bad code", we've gone through several concrete issues that exist in that small 300-line codebase, and how they would be resolved by real software engineers with modern programming languages and best practices.

Furthermore, I think that this investigation reveals things about the progress of the field of software engineering between 2004 and 2016. For all the complaints about the plethora of Javascript MVC frameworks, "clever tricks" like the Virtual Dom really are solving fundamental, decades-old problems. Programming languages have improved: features like optional arguments, lambdas, and generics in statically-typed languages are now table stakes, and demonstrably improve the code from the awkward patterns that were necessary back in 2004.

Hopefully you've enjoyed the journey, and are as fascinated as I am by the code and way the field has changed since then. What does the your earliest surviving code look like? Put it on-line and post a link in the comments!

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy Haoyi's book Hands-on Scala Programming

Hello World BlogStrategic Scala Style: Principle of Least Power

Updated 2016-02-14 2016-02-14 2016-02-05 2016-01-08 2016-01-08 2016-01-08 2016-01-08 2016-01-08 2016-01-08 2016-01-08 2016-01-08