Code style guide¶

1. General style¶

We use the standard Java coding style from Sun, adapted for Kotlin in ways that should be fairly intuitive.

Files no longer have copyright notices at the top, and license is now specified in the global README.md file. We do not mark classes with @author Javadoc annotations.

In Kotlin code, KDoc is used rather than JavaDoc. It’s very similar except it uses Markdown for formatting instead of HTML tags.

We target Java 8 and use the latest Java APIs whenever convenient. We use java.time.Instant to represent timestamps and java.nio.file.Path to represent file paths.

Never apply any design pattern religiously. There are no silver bullets in programming and if something is fashionable, that doesn’t mean it’s always better. In particular:

• Use functional programming patterns like map, filter, fold only where it’s genuinely more convenient. Never be afraid to use a simple imperative construct like a for loop or a mutable counter if that results in more direct, English-like code.
• Use immutability when you don’t anticipate very rapid or complex changes to the content. Immutability can help avoid bugs, but over-used it can make code that has to adjust fields of an immutable object (in a clone) hard to read and stress the garbage collector. When such code becomes a widespread pattern it can lead to code that is just generically slow but without hotspots.
• The tradeoffs between various thread safety techniques are complex, subtle, and no technique is always superior to the others. Our code uses a mix of locks, worker threads and messaging depending on the situation.

2. Comments¶

We like them as long as they add detail that is missing from the code. Comments that simply repeat the story already told by the code are best deleted. Comments should:

• Explain what the code is doing at a higher level than is obtainable from just examining the statement and surrounding code.
• Explain why certain choices were made and the tradeoffs considered.
• Explain how things can go wrong, which is a detail often not easily seen just by reading the code.
• Use good grammar with capital letters and full stops. This gets us in the right frame of mind for writing real explanations of things.

When writing code, imagine that you have an intelligent colleague looking over your shoulder asking you questions as you go. Think about what they might ask, and then put your answers in the code.

Don’t be afraid of redundancy, many people will start reading your code in the middle with little or no idea of what it’s about (e.g. due to a bug or a need to introduce a new feature). It’s OK to repeat basic facts or descriptions in different places if that increases the chance developers will see something important.

API docs: all public methods, constants and classes should have doc comments in either JavaDoc or KDoc. API docs should:

• Explain what the method does in words different to how the code describes it.
• Always have some text, annotation-only JavaDocs don’t render well. Write “Returns a blah blah blah” rather than “@returns blah blah blah” if that’s the only content (or leave it out if you have nothing more to say than the code already says).
• Illustrate with examples when you might want to use the method or class. Point the user at alternatives if this code is not always right.
• Make good use of {@link} annotations.

Bad JavaDocs look like this:

/** @return the size of the Bloom filter. */
public int getBloomFilterSize() {
    return block;
}

Good JavaDocs look like this:

/**
 * Returns the size of the current {@link BloomFilter} in bytes. Larger filters have
 * lower false positive rates for the same number of inserted keys and thus lower privacy,
 * but bandwidth usage is also correspondingly reduced.
 */
public int getBloomFilterSize() { ... }

We use C-style (/** */) comments for API docs and we use C++ style comments (//) for explanations that are only intended to be viewed by people who read the code. When writing multi-line TODO comments, indent the body text past the TODO line, for example:

// TODO: Something something
//       More stuff to do
//       Etc. etc.

3. Threading¶

Classes that are thread safe should be annotated with the @ThreadSafe annotation. The class or method comments should describe how threads are expected to interact with your code, unless it’s obvious because the class is (for example) a simple immutable data holder.

Code that supports callbacks or event listeners should always accept an Executor argument that defaults to MoreExecutors.directThreadExecutor() (i.e. the calling thread) when registering the callback. This makes it easy to integrate the callbacks with whatever threading environment the calling code expects, e.g. serialised onto a single worker thread if necessary, or run directly on the background threads used by the class if the callback is thread safe and doesn’t care in what context it’s invoked.

In the prototyping code it’s OK to use synchronised methods i.e. with an exposed lock when the use of locking is quite trivial. If the synchronisation in your code is getting more complex, consider the following:

1. Is the complexity necessary? At this early stage, don’t worry too much about performance or scalability, as we’re exploring the design space rather than making an optimal implementation of a design that’s already nailed down.
2. Could you simplify it by making the data be owned by a dedicated, encapsulated worker thread? If so, remember to think about flow control and what happens if a work queue fills up: the actor model can often be useful but be aware of the downsides and try to avoid explicitly defining messages, prefer to send closures onto the worker thread instead.
3. If you use an explicit lock and the locking gets complex, and always if the class supports callbacks, use the cycle detecting locks from the Guava library.
4. Can you simplify some things by using thread-safe collections like CopyOnWriteArrayList or ConcurrentHashMap? These data structures are more expensive than their non-thread-safe equivalents but can be worth it if it lets us simplify the code.

Immutable data structures can be very useful for making it easier to reason about multi-threaded code. Kotlin makes it easy to define these via the “data” attribute, which auto-generates a copy() method. That lets you create clones of an immutable object with arbitrary fields adjusted in the clone. But if you can’t use the data attribute for some reason, for instance, you are working in Java or because you need an inheritance hierarchy, then consider that making a class fully immutable may result in very awkward code if there’s ever a need to make complex changes to it. If in doubt, ask. Remember, never apply any design pattern religiously.

We have an extension to the Executor interface called AffinityExecutor. It is useful when the thread safety of a piece of code is based on expecting to be called from a single thread only (or potentially, a single thread pool). AffinityExecutor has additional methods that allow for thread assertions. These can be useful to ensure code is not accidentally being used in a multi-threaded way when it didn’t expect that.

4. Assertions and errors¶

We use them liberally and we use them at runtime, in production. That means we avoid the “assert” keyword in Java, and instead prefer to use the check() or require() functions in Kotlin (for an IllegalStateException or IllegalArgumentException respectively), or the Guava Preconditions.check method from Java.

We define new exception types liberally. We prefer not to provide English language error messages in exceptions at the throw site, instead we define new types with any useful information as fields, with a toString() method if really necessary. In other words, don’t do this:

throw new Exception("The foo broke")

instead do this

class FooBrokenException extends Exception {}
throw new FooBrokenException()

The latter is easier to catch and handle if later necessary, and the type name should explain what went wrong.

Note that Kotlin does not require exception types to be declared in method prototypes like Java does.

5. Properties¶

Where we want a public property to have one super-type in public and another sub-type in private (or internal), perhaps to expose additional methods with a greater level of access to the code within the enclosing class, the style should be:

class PrivateFoo : PublicFoo

private val _foo = PrivateFoo()
val foo: PublicFoo get() = _foo

Notably:

• The public property should have an explicit and more restrictive type, most likely a super class or interface.
• The private, backed property should begin with underscore but otherwise have the same name as the public property. The underscore resolves a potential property name clash, and avoids naming such as “privateFoo”. If the type or use of the private property is different enough that there is no naming collision, prefer the distinct names without an underscore.
• The underscore prefix is not a general pattern for private properties.
• The public property should not have an additional backing field but use “get()” to return an appropriate copy of the private field.
• The public property should optionally wrap the returned value in an immutable wrapper, such as Guava’s immutable collection wrappers, if that is appropriate.
• If the code following “get()” is succinct, prefer a one-liner formatting of the public property as above, otherwise put the “get()” on the line below, indented.

6. Compiler warnings¶

We do not allow compiler warnings, except in the experimental module where the usual standards do not apply and warnings are suppressed. If a warning exists it should be either fixed or suppressed using @SuppressWarnings and if suppressed there must be an accompanying explanation in the code for why the warning is a false positive.