Google Guava v07 examples

We have something called Weekly Technology Workshops at TouK, that is, every Friday at 16:00 somebody has a presentation for everyone willing to come. We present stuff we learn and work on at home, but we also have a bulletin board with topics that people would like to listen about. Last week Maciej Próchniak had a talk about Clojure, this time a few folks asked for an introduction to Google Guava libraries.Since this was a dead simple task, I was happy to deliver.

WTF is Guava?

It’s a set of very simple, basic classes, that you end up writing yourself anyway. Think in terms of Apache commons, just by Google. Just to make your life a little bit easier.

There is an early (v04) presentation and there was a different one (in Polish) at Javarsowia 2010 by Wiktor Gworek.

At the time of writing this, the latest version is v07, it’s been mavenized and is available at a public maven repo.

Here’s a quick review of a few interesting things. Don’t expect anything fancy though, Guava is very BASIC.


@VisibleForTesting

A simple annotation that tells you why a particular property access restriction has been relaxed.

A common trick to use in testing is to relax access restrictions to default for a particular property, so that you can use it in a unit test, which resides in the same package (though in different catalog). Whether you thing it’s good or bad, remember to give a hint about that to the developer.

Consider:

public class User {
    private Long id;
    private String firstName;
    private String lastName;
    String login;

Why is login package scoped?

public class User {
    private Long id;
    private String firstName;
    private String lastName;
    @VisibleForTesting String login;

Ah, that’s why.


Preconditions

Guava has a few preconditions for defensive programming (Design By Contract), but they are not quite as good as what Apache Commons / Spring framework has. One thing interesting is that Guava solution returns the object, so could be inlined. Consider:

Using hand written preconditions:

public User(Long id, String firstName, String lastName, String login) {
        validateParameters(id, firstName, lastName, login);
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        this.login = login.toLowerCase();
    }

    private void validateParameters(Long id, String firstName, String lastName, String login) {
        if(id == null ) {
            throw new IllegalArgumentException("id cannot be null");
        }

        if(firstName == null || firstName.length() == 0) {
            throw new IllegalArgumentException("firstName cannot be empty");
        }

        if(lastName == null || lastName.length() == 0) {
            throw new IllegalArgumentException("lastName cannot be empty");
        }

        if(login == null || login.length() == 0) {
            throw new IllegalArgumentException("login cannot be empty");
        }
    }

Using guava preconditions:

public void fullyImplementedGuavaConstructorWouldBe(Long id, String firstName, String lastName, String login) {
        this.id = checkNotNull(id);
        this.firstName = checkNotNull(firstName);
        this.lastName = checkNotNull(lastName);
        this.login = checkNotNull(login);

        checkArgument(firstName.length() > 0);
        checkArgument(lastName.length() > 0);
        checkArgument(login.length() > 0);
    }

(Thanks Yom for noticing that checkNotNull must go before checkArgument, though it makes it a bit unintuitive)

Using spring or apache commons preconditions (the use looks exactly the same for both libraries):

public void springConstructorWouldBe(Long id, String firstName, String lastName, String login) {
        notNull(id); hasText(firstName); hasText(lastName); hasText(login);
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        this.login = login;
    }

CharMatcher

For people who hate regexp or just want a simple and good looking object style pattern matching solution.

Examples:

And/or ease of use

        String input = "This invoice has an id of 192/10/10";
        CharMatcher charMatcher = CharMatcher.DIGIT.or(CharMatcher.is('/'));
        String output = charMatcher.retainFrom(input);

output is: 192/10/10

Negation:

        String input = "DO NOT scream at me!";
        CharMatcher charMatcher = CharMatcher.JAVA_LOWER_CASE.or(CharMatcher.WHITESPACE).negate();
        String output = charMatcher.retainFrom(input);

output is: DONOT!

Ranges:

        String input = "DO NOT scream at me!";
        CharMatcher charMatcher = CharMatcher.inRange('m', 's').or(CharMatcher.is('a').or(CharMatcher.WHITESPACE));
        String output = charMatcher.retainFrom(input);

output is: sram a m

Joiner / Splitter

As the names suggest, it’s string joining/splitting done the right way, although I find the inversion of calls a bit… oh well, it’s java.

        String[] fantasyGenres = {"Space Opera", "Horror", "Magic realism", "Religion"};
        String joined = Joiner.on(", ").join(fantasyGenres);

Output: Space Opera, Horror, Magic realism, Religion

You can skip nulls:

        String[] fantasyGenres = {"Space Opera", null, "Horror", "Magic realism", null, "Religion"};
        String joined = Joiner.on(", ").skipNulls().join(fantasyGenres);

Output: Space Opera, Horror, Magic realism, Religion

You can fill nulls:

        String[] fantasyGenres = {"Space Opera", null, "Horror", "Magic realism", null, "Religion"};
        String joined = Joiner.on(", ").useForNull("NULL!!!").join(fantasyGenres);

Output: Space Opera, NULL!!!, Horror, Magic realism, NULL!!!, Religion

You can join maps

        Map map = newHashMap();
        map.put(1, "Space Opera");
        map.put(2, "Horror");
        map.put(3, "Magic realism");
        String joined = Joiner.on(", ").withKeyValueSeparator(" -> ").join(map);

Output: 1 → Space Opera, 2 → Horror, 3 → Magic realism

Split returns Iterable instead of JDK arrays:

        String input = "Some very stupid data with ids of invoces like 121432, 3436534 and 8989898 inside";
        Iterable splitted = Splitter.on(" ").split(input);

Split does fixed length splitting, although you cannot give a different length for each “column” which makes it’s use a bit limited while parsing some badly exported excels.

        String input =
                "A  1  1  1  1\n" +
                "B  1  2  2  2\n" +
                "C  1  2  3  3\n" +
                "D  1  2  5  3\n" +
                "E  3  2  5  4\n" +
                "F  3  3  7  5\n" +
                "G  3  3  7  5\n" +
                "H  3  3  9  7";
        Iterable splitted = Splitter.fixedLength(3).trimResults().split(input);

You can use CharMatcher while splitting

        String input = "Some very stupid data with ids of invoces like 123231/fv/10/2010, 123231/fv/10/2010 and 123231/fv/10/2010";
        Iterable splitted = Splitter.on(CharMatcher.DIGIT.negate())
                                            .trimResults()
                                            .omitEmptyStrings()
                                            .split(input);

Predicates / Functions

Predicates alone are not much, it’s just an interface with a method that returns true, but if you combine predicates with functions and Collections2 (a guava class that simplifies working on collections), you get a nice tool in your toolbox.

But let’s start with basic predicate use. Imagine we want to find whether there are users who have logins with digits inside. The inocation would be (returns boolean):

Predicates.in(users).apply(shouldNotHaveDigitsInLoginPredicate);

And the predicate looks like that

public class ShouldNotHaveDigitsInLoginPredicate implements Predicate {
    @Override
    public boolean apply(User user) {
        checkNotNull(user);
        return CharMatcher.DIGIT.retainFrom(user.login).length() == 0;
    }    
}

Now lets add a function that will transform a user to his full name:

public class FullNameFunction implements Function {
    @Override
    public String apply(User user) {
        checkNotNull(user);
        return user.getFirstName() + " " + user.getLastName();
    }    
}

You can invoke it using static method transform:

List users = newArrayList(new User(1L, "sylwek", "stall", "rambo"),
  new User(2L, "arnold", "schwartz", "commando"));

List fullNames = transform(users, new FullNameFunction());

And now lets combine predicates with functions to print names of users that have logins which do not contain digits:

List users = newArrayList(new User(1L, "sylwek", "stall", "rambo"), 
  new User(2L, "arnold", "schwartz", "commando"), 
  new User(3L, "hans", "kloss", "jw23"));

Collection usersWithoutDigitsInLogin = filter(users, new ShouldNotHaveDigitsInLoginPredicate());
String names = Joiner.on("\n").join( transform(usersWithoutDigitsInLogin, new FullNameFunction()) );

What we do not get: fold (reduce) and tuples. Oh well, you’d probably turn to Java Functional Library anyway, if you wanted functions in Java, right?

CaseFormat

Ever wanted to turn those ugly PHP Pear names into nice java/cpp style with one liner? No? Well, anyway, you can:

String pearPhpName = "Really_Fucked_Up_PHP_PearConvention_That_Looks_UGLY_because_of_no_NAMESPACES";
String javaAndCPPName = CaseFormat.UPPER_UNDERSCORE.to(CaseFormat.UPPER_CAMEL , pearPhpName);

Output: ReallyFuckedUpPhpPearconventionThatLooksUglyBecauseOfNoNamespaces

But since Oracle has taken over Sun, you may actually want to turn those into sql style, right?

        String sqlName = CaseFormat.UPPER_CAMEL.to(CaseFormat.LOWER_UNDERSCORE, javaAndCPPName);

Output: really_fucked_up_php_pearconvention_that_looks_ugly_because_of_no_namespaces

Collections

Guava has a superset of Google collections library 1.0, and this indeed is a very good reason to include this dependency in your poms. I won’t even try to describe all the features, but just to point out a few nice things:

  • you have an Immutable version of pretty much everything
  • you get a few nice static and statically typed methods on common types like Lists, Sets, Maps, ObjectArrays, which include:
    • easy way of creating based on return type: e.g. newArrayList
    • transform (way to apply functions that returns Immutable version)
    • partition (paging)
    • reverse

And now for a few more interesting collections.
Mutlimaps

Mutlimap is basically a map that can have many values for a single key. Ever had to create a Map> in your code? You don’t have to anymore.

Multimap multimap = HashMultimap.create();
        multimap.put(1, "a");
        multimap.put(2, "b");
        multimap.put(3, "c");
        multimap.put(1, "a2");

There are of course immutable implementations as well: ImmutableListMultimap, ImmutableSetMultomap, etc.

You can construct immutables either in line (up to 5 elements) or using a builder:

Multimap multimap = ImmutableSetMultimap.of(1, "a", 2, "b", 3, "c", 1, "a2"); 
Multimap multimap = new ImmutableSetMultimap.Builder()
        .put(1, "a")
        .put(2, "b")
        .put(3, "c")
        .put(1, "a2")
        .build();

BiMap

BiMap is a map that have only unique values. Consider this:

@Test(expected = IllegalArgumentException.class)
public void biMapShouldOnlyHaveUniqueValues() {
 BiMap biMap = HashBiMap.create();
 biMap.put(1, "a");
 biMap.put(2, "b");
 biMap.put(3, "a"); //argh! an exception
}

That allows you to inverse the map, so the values become key and the other way around:

BiMap biMap = HashBiMap.create();
biMap.put(1, "a");
biMap.put(2, "b");
biMap.put(3, "c");

BiMap invertedMap = biMap.inverse();

Not sure what I’d actually want to use it for.

Constraints

This allows you to add constraint checking on a collection, so that only values which pass the constraint may be added.

Imagine we want a collections of users with first letter ‘r’ in their logins.

Constraint loginMustStartWithR = new Constraint() {
    @Override
    public User checkElement(User user) {
        checkNotNull(user);

        if(!user.login.startsWith("r")) {
            throw new IllegalArgumentException("GTFO, you are not Rrrrrrrrr");
        }

        return user;
    }
};

And now for a test:

@Test(expected = IllegalArgumentException.class)
public void shouldConstraintCollection() {
 //given
 Collection users = newArrayList(new User(1L, "john", "rambo", "rambo"));
 Collection usersThatStartWithR = constrainedCollection(users, loginMustStartWithR);

 //when
 usersThatStartWithR.add(new User(2L, "arnold", "schwarz", "commando"));
}

You also get notNull constraint out of the box:

//notice it's not an IllegalArgumentException :( 
@Test(expected = NullPointerException.class)
public void notNullConstraintShouldWork() {
 //given
 Collection users = newArrayList(1);
 Collection notNullCollection = constrainedCollection(users, notNull());

 //when
 notNullCollection.add(null);
}

Thing to remember: constraints are not checking the data already present in a collection.

Tables

Just as expected, a table is a collection with columns, rows and values. No more Map> I guess. The usage is simple and you can transpose:

Table table = HashBasedTable.create();
table.put(1, "a", "1a");
table.put(1, "b", "1b");
table.put(2, "a", "2a");
table.put(2, "b", "2b");

Table transponedTable = Tables.transpose(table);

That’s all, folks. I didn’t present util.concurent, primitives, io and net packages, but you probably already know what to expect.

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Super Confitura Man

How Super Confitura Man came to be :)

Recently at TouK we had a one-day hackathon. There was no main theme for it, you just could post a project idea, gather people around it and hack on that idea for a whole day - drinks and pizza included.

My main idea was to create something that could be fun to build and be useful somehow to others. I’d figured out that since Confitura was just around a corner I could make a game, that would be playable at TouK’s booth at the conference venue. This idea seemed good enough to attract Rafał Nowak @RNowak3 and Marcin Jasion @marcinjasion - two TouK employees, that with me formed a team for the hackathon.

Confitura 01

The initial plan was to develop a simple mario-style game, with preceduraly generated levels, random collectible items and enemies. One of the ideas was to introduce Confitura Man as the main character, but due to time constraints, this fall through. We’ve decided to just choose a random available sprite for a character - hence the onion man :)

Confitura 02

How the game is played?

Since we wanted to have a scoreboard and have unique users, we’ve printed out QR codes. A person that would like to play the game could pick up a QR code, show it against a camera attached to the play booth. The start page scanned the QR code and launched the game with username read from paper code.

The rest of the game was playable with gamepad or keyboard.

Confitura game screen

Technicalities

Writing a game takes a lot of time and effort. We wanted to deliver, so we’ve decided to spend some time in the days before the hackathon just to bootstrap the technology stack of our enterprise.

We’ve decided that the game would be written in some Javascript based engine, with Google Chrome as a web platform. There are a lot of HTML5 game engines - list of html5 game engines and you could easily create a game with each and every of them. We’ve decided to use Phaser IO which handles a lot of difficult, game-related stuff on its own. So, we didn’t have to worry about physics, loading and storing assets, animations, object collisions, controls input/output. Go see for yourself, it is really nice and easy to use.

Scoreboard would be a rip-off from JIRA Survivor with stats being served from some web server app. To make things harder, the backend server was written in Clojure. With no experience in that language in the team, it was a bit risky, but the tasks of the server were trivial, so if all that clojure effort failed, it could be rewritten in something we know.

Statistics

During the whole Confitura day there were 69 unique players (69 QR codes were used), and 1237 games were played. The final score looked like this:

  1. Barister Lingerie 158 - 1450 points
  2. Boilerdang Custardbath 386 - 1060 points
  3. Benadryl Clarytin 306 - 870 points

And the obligatory scoreboard screenshot:

Confitura 03

Obstacles

The game, being created in just one day, had to have problems :) It wasn’t play tested enough, there were some rough edges. During the day we had to make a few fixes:

  • the server did not respect the highest score by specific user, it was just overwritting a user’s score with it’s latest one,
  • there was one feature not supported on keyboard, that was available on gamepad - turbo button
  • server was opening a database connection each time it got a request, so after around 5 minutes it would exhaust open file limit for MongoDB (backend database), this was easily fixed - thou the fix is a bit hackish :)

These were easily identified and fixed. Unfortunately there were issues that we were unable to fix while the event was on:

  • google chrome kept asking for the permission to use webcam - this was very annoying, and all the info found on the web did not work - StackOverflow thread
  • it was hard to start the game with QR code - either the codes were too small, or the lighting around that area was inappropriate - I think this issue could be fixed by printing larger codes,

Technology evaluation

All in all we were pretty happy with the chosen stack. Phaser was easy to use and left us with just the fun parts of the game creation process. Finding the right graphics with appropriate licensing was rather hard. We didn’t have enough time to polish all the visual aspects of the game before Confitura.

Writing a server in clojure was the most challenging part, with all the new syntax and new libraries. There were tasks, trivial in java/scala, but hard in Clojure - at least for a whimpy beginners :) Nevertheless Clojure seems like a really handy tool and I’d like to dive deeper into its ecosystem.

Source code

All of the sources for the game can be found here TouK/confitura-man.

The repository is split into two parts:

  • game - HTML5 game
  • server - clojure based backend server

To run the server you need to have a local MongoDB installation. Than in server’s directory run: $ lein ring server-headless This will start a server on http://localhost:3000

To run the game you need to install dependencies with bower and than run $ grunt from game’s directory.

To launch the QR reading part of the game, you enter http://localhost:9000/start.html. After scanning the code you’ll be redirected to http://localhost:9000/index.html - and the game starts.

Conclusion

Summing up, it was a great experience creating the game. It was fun to watch people playing the game. And even with all those glitches and stupid graphics, there were people vigorously playing it, which was awesome.

Thanks to Rafał and Michał for great coding experience, and thanks to all the players of our stupid little game. If you’d like to ask me about anything - feel free to contact me by mail or twitter @zygm0nt

Recently at TouK we had a one-day hackathon. There was no main theme for it, you just could post a project idea, gather people around it and hack on that idea for a whole day - drinks and pizza included.

My main idea was to create something that could be fun to build and be useful somehow to others. I’d figured out that since Confitura was just around a corner I could make a game, that would be playable at TouK’s booth at the conference venue. This idea seemed good enough to attract >Conclusion