Step 2: Dart classes

In this step, you create your first classes and run tests on it.

Note: Throughout this code lab, continue to edit the files in s1_basics. You can use the files in the other samples subdirectories to compare to your code or to recover if you get off track.

Keywords: class, test

Create `Country` class

Edit lib/src/map.dart, as follows.

→ Add the following class:

/// Country class.
class Country {
  /// The country id.
  final String id;
  /// The country neighbours.
  final List<String> neighbours;

  Country(this.id, this.neighbours);
}

You've just implemented a class!

Key information:

It defines Country class with two fields: id which is the country Id and neighbours which is the list of country neighbours.
The fields are final, it means that they can only be initialized once, in constructors.
Generic types allow to define a List of String with List<String>.
The constructor Country(this.id, this.neighbours); use syntactic sugar for setting id and neighbours before the constructor body runs.
Lines starting with /// or /** are documentation comments, used to generate documentation.

Instantiate a `Country` class

Edit bin/main.dart, as follows.

→ Instantiate a Country class and run this code:

library risk.main;

import '../lib/risk.dart';

main() {
  Country country = new Country('eastern_australia', ['western_australia', 'new_guinea']);
  var neighbours = country.neighbours;
  print("Hello ${country.id} and $neighbours!");
}

The import is used to import a namespace from one library in the local scope.
The ../lib/risk.dart import is a library defined in the lib directory. lib/src/map.dart is already part of this library.
Either 'string' or "string" can be used to define a String.
['western_australia', 'new_guinea'] allows to easily define List.
var is a way to declare a variable without specifying its type.
$neighbours and ${country.id} are string interpolations. It includes the variable or expression’s string equivalent inside of a string literal.

Create `Continent` class

Continue to edit lib/src/map.dart.

→ Implements the Continent:

The Continent class has three fields id, bonus and countries:
- id (a String) is the continent id.
- bonus (an int) is the reinforcement bonus given if the same player owns all continent countries.
- countries (a List of String) is the list of the ids of the countries of this continent.
Like Country, fields must be immutable.
The Continent class must be instanciable with the following instruction:

new Continent('australia', 2, ["eastern_australia", "indonesia"])

Add `Country` and `Continent` constants

Edit lib/src/map.dart, as follows.

→ Add the following top-level constants:

/// List of all existing countries
final List<Country> COUNTRIES = [//
  // australia
  new Country('eastern_australia', ['western_australia', 'new_guinea', 'eastern_australia']), //
  new Country('indonesia', ['siam', 'new_guinea', 'western_australia']), //
  new Country('new_guinea', ['indonesia', 'eastern_australia', 'western_australia']), //
  new Country('western_australia', ['eastern_australia', 'indonesia', 'new_guinea']), //
  // south_america
  new Country('argentina', ['brazil', 'peru']), //
  new Country('brazil', ['north_africa', 'venezuela', 'argentina', 'peru']), //
  new Country('peru', ['venezuela', 'brazil', 'argentina']), //
  new Country('venezuela', ['central_america', 'brazil', 'peru']), //
  // africa
  new Country('congo', ['east_africa', 'south_africa', 'north_africa']), //
  new Country('egypt', ['southern_europe', 'middle_east', 'east_africa', 'north_africa']), //
  new Country('east_africa', ['middle_east', 'madagascar', 'south_africa', 'congo', 'north_africa', 'egypt']), //
  new Country('madagascar', ['east_africa', 'south_africa']), //
  new Country('north_africa', ['southern_europe', 'western_europe', 'brazil', 'egypt', 'east_africa', 'congo']), //
  new Country('south_africa', ['madagascar', 'east_africa', 'congo']), //
  // north_america
  new Country('alaska', ['kamchatka', 'northwest_territory', 'alberta']), //
  new Country('alberta', ['alaska', 'ontario', 'northwest_territory', 'western_united_states']), //
  new Country('central_america', ['venezuela', 'eastern_united_states', 'western_united_states']), //
  new Country('eastern_united_states', ['quebec', 'ontario', 'western_united_states', 'central_america']), //
  new Country('greenland', ['iceland', 'ontario', 'northwest_territory', 'quebec']), //
  new Country('northwest_territory', ['alaska', 'ontario', 'greenland', 'alberta']), //
  new Country('ontario', ['northwest_territory', 'greenland', 'eastern_united_states', 'western_united_states', 'quebec', 'alberta']), //
  new Country('quebec', ['greenland', 'ontario', 'eastern_united_states']), //
  new Country('western_united_states', ['alberta', 'ontario', 'eastern_united_states', 'central_america']), //
  // europe
  new Country('great_britain', ['iceland', 'western_europe', 'northern_europe', 'scandinavia']), //
  new Country('iceland', ['greenland', 'great_britain', 'scandinavia']), //
  new Country('northern_europe', ['ukraine', 'scandinavia', 'great_britain', 'western_europe', 'southern_europe']), //
  new Country('scandinavia', ['iceland', 'great_britain', 'northern_europe', 'ukraine']), //
  new Country('southern_europe', ['north_africa', 'egypt', 'middle_east', 'ukraine', 'northern_europe', 'western_europe']), //
  new Country('ukraine', ['ural', 'afghanistan', 'middle_east', 'southern_europe', 'northern_europe', 'scandinavia']), //
  new Country('western_europe', ['north_africa', 'southern_europe', 'northern_europe', 'great_britain']), //
  // asia
  new Country('afghanistan', ['ukraine', 'ural', 'china', 'india', 'middle_east']), //
  new Country('china', ['siberia', 'ural', 'afghanistan', 'india', 'siam', 'mongolia']), //
  new Country('india', ['middle_east', 'afghanistan', 'china', 'siam']), //
  new Country('irkutsk', ['siberia', 'kamchatka', 'mongolia', 'yakursk']), //
  new Country('japan', ['kamchatka', 'mongolia']), //
  new Country('kamchatka', ['alaska', 'yakursk', 'irkutsk', 'mongolia', 'japan']), //
  new Country('ural', ['siberia', 'china', 'afghanistan', 'ukraine']), //
  new Country('middle_east', ['southern_europe', 'ukraine', 'afghanistan', 'india', 'egypt', 'east_africa']), //
  new Country('mongolia', ['china', 'siberia', 'irkutsk', 'kamchatka', 'japan']), //
  new Country('siam', ['india', 'china', 'indonesia']), //
  new Country('siberia', ['yakursk', 'irkutsk', 'mongolia', 'china', 'ural']), //
  new Country('yakursk', ['kamchatka', 'siberia', 'irkutsk', 'eastern_australia']),//
  new Country('eastern_australia', ['western_australia', 'new_guinea', 'eastern_australia']), //
];

/// All [Country]s indexed by country id
final Map<String, Country> COUNTRY_BY_ID = new Map.fromIterable(COUNTRIES, key: (country) => country.id);

/// List of all existing continents
final List<Continent> CONTINENTS = [//
  new Continent('australia', 2, ["eastern_australia", "indonesia", "new_guinea"]), //
  new Continent('north_america', 5, ["alaska", "alberta", "central_america", "eastern_united_states", "greenland", "northwest_territory", "ontario", "quebec",
      "western_united_states"]), //
  new Continent('south_america', 2, ["argentina", "brazil", "peru", "venezuela"]), //
  new Continent('africa', 3, ["congo", "egypt", "east_africa", "madagascar", "north_africa", "south_africa"]), //
  new Continent('europe', 5, ["great_britain", "iceland", "northern_europe", "scandinavia", "southern_europe", "ukraine", "western_europe"]), //
  new Continent('asia', 7, ["afghanistan", "china", "india", "irkutsk", "japan", "kamchatka", "ural", "middle_east", "mongolia", "siam", "siberia", "yakursk", "eastern_australia"]),//
];

Key information:

Those constants defines the countries and continents for the Risk game.
COUNTRY_BY_ID is a Map built from COUNTRIES. It helps to find quickly a Country by its id.
Some errors are hidden in those constants, they need to be fixed!

Run tests on the implementations

To check if Country and Continent classes are well implemented and to fix errors in constants, it's a good pratice to run unit tests.

Open test/s2_classes_test.dart.

→ Get familiar with the code:

library risk.map.test;

import 'package:unittest/unittest.dart';
import '../lib/risk.dart';

main() {
  test('Country should be instanciable', () {
    var country = new Country('eastern_australia', ['western_australia', 'new_guinea']);
    expect(country, isNotNull);
    expect(country.id, equals('eastern_australia'));
    expect(country.neighbours, equals(['western_australia', 'new_guinea']));
  });

  group('COUNTRIES', () {
    test('for each country should have at least 1 neighbour', () { ... });
    // ...
  });

  group('COUNTRY_BY_ID', () { ... });
  group('CONTINENTS', () { ... });
}

→ Then run tests: right-click test/s2_classes_test.dart and select Run.
→ You should have test failures in console, fix classes and/or constants.

Key information:

Note how a test is written: we wrap it in a call to test(String testName, functionToTest);.
Within the function we are testing, we write assertions, using expect(actualValue, expectedValueMatcher);.
The unittest package provides a set of built-in Matcher like equals(xxx), isPositive, isNotNull, isTrue...
It can be helpful to group similar tests together, which can be done with group().
Use the solo_ prefix to quickly run just one unit test or one group: solo_test(...) or solo_group(...)
Use the skip_ prefix to exclude unit tests or groups you don't want to run: skip_test(...) or skip_group(...)

Write a test

Open test/s2_classes_test.dart.

→ Test that COUNTRIES and COUNTRY_BY_ID have exactly 42 countries.
→ Fix COUNTRIES removing the duplicated country.

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step-2.md

step-2.md

Step 2: Dart classes

Create `Country` class

Instantiate a `Country` class

Create `Continent` class

Add `Country` and `Continent` constants

Run tests on the implementations

Write a test

Learn more

Problems?

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step-2.md

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Step 2: Dart classes

Create Country class

Instantiate a Country class

Create Continent class

Add Country and Continent constants

Run tests on the implementations

Write a test

Learn more

Problems?

Home | < Previous | Next >

Create `Country` class

Instantiate a `Country` class

Create `Continent` class

Add `Country` and `Continent` constants