Google Collections to the rescue

A few days ago, I was writing a piece of code that turned a line at a time into an Object. And it was using iterators. I had a RecordStream, which wrapped a LineStream (just a thin veneer over LineNumberReader).

Then I discovered that there was a terminating record at the end of each file. And it was in a completely different format to all the other lines. Bother.

Ok, I know, I’ll insert another iterator in the middle, which specifically ignores that record. Well, easier said than done as it turns out. I spent the best part of a day trying to create an Iterator which reads the next value and pretends that it’s not there. It turns out to have an awful lot of state.

Eventually I managed the task, and it worked. But boy, was it ugly. And it was long—about two pages of code.

Then the light bulb went off. I remembered that google collections had some tools for dealing with Iterators. In particular, there’s a function filter(), which takes a Predicate. And look! The Predicates class contains some handy builtins!

After about 5 minutes work, my two pages of code boiled down to three lines of code.

    import static com.google.common.base.Predicates.*;

    private static final String END_RECORD = "END RECORD,END RECORD,END RECORD";

    public Iterator<T> iterator() {
        // Produce an iterator that returns one line at a time.
        Iterator<String> lines = new LineStream(reader).iterator();
        // A predicate to return all records which are not the end record.
        Predicate<String> notEndRecord = not(isEqualTo(END_RECORD));
        // Apply the predicate to the iterator.
        final Iterator<String> it = Iterators.filter(lines, notEndRecord);
        return new Iterator<T>() { … };
    }

Marvellous and powerful stuff. It’s seriously worth checking out in case you haven’t played with it before. My favourite is the static factory methods. e.g.

  // Before
  Map<String, String> myMap = new HashMap<String,String>();

  // After
  Map<String, String> myMap = Maps.newHashMap();

Isn’t it lovely how the compiler just figures it all out for you? Anything that can save space like that has to be a Good Thing™.

There are a whole bunch of other useful things in there.

• Preconditions.checkNotNull() is a compact way of validity checking your arguments.
• Join.join()—I don’t know how many times I’ve written this by hand (usually badly). Much better to have somebody else do it for me.

Do yourself a favour and go check them out. You won’t regret it.