medium_dependency_injection

main_fully_injected.java

public static void main(String[] args) {
  // Creating the injection module configured above.
  Injector injector = Guice.createInjector(new WeatherModule());

  // We ask for the injection of a WeatherContract, 
  // which will create an instance of ThermometerContract
  // with the named TemperatureUnit under the hood.
  WeatherContract weather = injector.getInstance(WeatherContract.class);
}

main_not_injected.java

public static void main(String[] args) {
  // Not using dependency injection
  WeatherContract weather = new WeatherService(true, true, null);
}

main_semi_injected.java

public static void main(String[] args) {
  // Using dependency injection
  TemperatureUnit celsius = TemperatureUnit.CELSIUS;
  ThermometerContract thermometer = new Thermometer(celsius);
  WeatherContract weather = new WeatherService(thermometer);
}

overall_service_injected.java

public class WeatherService implements WeatherContract {
  private final ThermometerContract thermometer;

  @Inject
  public WeatherService(ThermometerContract thermometer) {
    this.thermometer = thermometer;
  }
}

public class Thermometer implements ThermometerContract {
  private final TemperatureUnit unit;
  
  @Inject
  public Thermometer(@Named(WeatherModule.TEMPERATURE_UNIT) 
                     TemperatureUnit unit) {
    this.unit = unit;
  }
}

overall_service_not_injected.java

public class Thermometer {
  private final TemperatureUnit unit;
  
  public Thermometer() {
    this.unit = TemperatureUnit.CELSIUS;
  }
}

public class WeatherService implements WeatherContract {

  private final Thermometer thermometer;
 
  // This constructor is not using dependency injection
  public WeatherService() {
    this.thermometer = new Thermometer();
  }
}

tests_example.java

public void testTemperatureStatus() {
  ThermometerContract thermometer = Mockito.mock(ThermometerContract.class);
  Mockito.doReturn(TemperatureUnit.CELSIUS).when(thermometer).getUnit();
  WeatherContract weather = new WeatherService(thermometer);
  
  Mockito.doReturn(-50f).when(thermometer).getTemperature();
  assertEquals(
    TemperatureStatus.COLD,
    weather.getTemperatureStatus()
  );
  
  Mockito.doReturn(10f).when(thermometer).getTemperature();
  assertEquals(
    TemperatureStatus.MODERATE,
    weather.getTemperatureStatus()
  );
}

thermometer_constructor_switch.java

public Thermometer(boolean useCelsius) {
  if (useCelsius) {
    this.unit = TemperatureUnit.CELSIUS;
  } else {
    this.unit = TemperatureUnit.FAHRENHEIT;
  }
}

weather_injection_module.java

public class WeatherModule extends AbstractModule {
  public static final String TEMPERATURE_UNIT = "temp_unit";

  @Override
  protected void configure() {
    // Named input configuration bindings
    bind(TemperatureUnit.class)
      .annotatedWith(Names.named(TEMPERATURE_UNIT))
      .toInstance(TemperatureUnit.CELSIUS);
      
    // Interface - Implementation bindings
    bind(ThermometerContract.class).to(Thermometer.class);
    bind(WeatherContract.class).to(WeatherService.class);
  }
}

weather_service_constructor_not_injected.java

public WeatherService(boolean useRealDevice, 
                      boolean useCelsius,
                      String apiKey) {
  if (useRealDevice) {
    this.thermometer = new Thermometer(useCelsius);
  } else {
    this.thermometer = new ThermometerWebService(useCelsius, apiKey);
  }
}

weather_service_injected.java

public class WeatherService implements WeatherContract {
  // We now use the Interface   
  private final ThermometerContract thermometer;

  // New constructor using dependency injection    
  public WeatherService(ThermometerContract thermometer) {
    this.thermometer = thermometer;
  }
}