Question

// AvgTemp.java

public abstract class  AvgTemp {

// This function receives nottification from other Temperature Sensors

public AvgTemp() {

}

public void notifyReceived(String  eventName, Object arg) {
if (eventName.equals("temperatureMeasurement"))  
             { 
                onNewtemperatureMeasurement((TempStruct) arg); 
             } 
}

public abstract void onNewtemperatureMeasurement(TempStruct tempStruct);
} 

For receiving notifications, AvgTemp.java file has to subscribe to a temperature sensor. It means I have to call subscribetemperatureMeasurement().

Now, my question is “Where should I call subscribetemperatureMeasurement() in AvgTemp.java file, so I can get notification from Sensor?”

Should I call subscribetemperatureMeasurement() function in the constructor of the AvgTemp class or in somewhere else?

Answer 1

Looks like your question is missing Sensor skeleton, I guess it looks like this:

public class Sensor {
    public void subscribeTemperatureMeasurement(AvgTemp avgTemp) {
        //keep avgTemp reference for later use
    }
}

and you have a choice between:

public AvgTemp(Sensor sensor) {
    sensor.subscribeTemperatureMeasurement(this);
}

or (somewhere outside):

AvtGemp avgTemp = SomeAvgTemp();
sensor.subscribeTemperatureMeasurement(avgTemp);

The former approach has several drawbacks:

• introduces unnecessary coupling from AvgTemp to Sensor

• what if you want to subscribe to several sensors, you provide first one as a constructor argument and the remaining using the latter approach?

• this reference escapes from the constructor, very bad, your notifyReceived might get called before the object is fully initialized (especially because this is an abstract class)

• the AvgTemp cannot live without a Sensor which seems to strict and makes testing harder (mocking/stubbing required)