How to implement the update method for Observer Pattern

I'm trying to adjust the observer pattern to my code, so in my case MSG0100 it's my Subject class that it updates the variable msg100PreselectionAplication, and the MainActivity class it's an concrete observer, and usually you have to create the Observer class, but my problem is that the MainActivity has to extend from that class and that's a problem because Java does not support multiple inheritance. Here is what i have until now, can some one give me an idea of what i need next? Please

MSG0100 class:

    public class MSG0100  implements PreselectionAplicationUseCases {
    private OnMsg100PreselectionChanged listener = null;


    public void setOnMsgPreselectionChanged(OnMsg100PreselectionChanged listener) {
        this.listener = listener;
    }

    public void setMsg100PreselectionAplication(boolean msg100PreselectionAplication) {
        if(listener != null) {
            listener.onPreselectionChanged(msg100PreselectionAplication);
        }
    }
}

    interface OnMsg100PreselectionChanged {
    void onPreselectionChanged(boolean isChanged);
}

Main activity class:

public class MainActivity extends AppCompatActivity {

    private ExpandableListViewAdapter mExpandableListViewAdapter;
    PreselectionAplicationUseCases preselectionAplicationUseCases;

    public void Preselection(){
        preselectionApplicationUseCases.setOnMsgPreselectionChanged(new OnMsg100PreselectionChanged {

            @Override
            void onPreselectionChanged(boolean isChanged) {
                //do something with changed boolean
            }
        });
    }
}

You should create Observer as an Interface which implemented by MainActivity like this:

interface Observer {
   void update(Data data)
}

public class MainActivity extends AppCompatActivity implements Observer {
    
    //…

    @Override
    public void update(Data data) {
        // update your data to your UI here
    }
    //…
}

and modify your subject to receive observer

interface Subject {
    void registerObserver(Observer o);
    void removeObserver(Observer o);
    void notifyObservers();
}

public class MSG0100  implements PreselectionAplicationUseCases, Subject {
    private Data data = new Data();
    private Observer o;

    @Override
    public void registerObserver(Observer o) {
        this.o = o;
    }

    @Override
    public void removeObserver(Observer o) {
        o = null;
    }

    @Override
    public void notifyObservers() {
        o.update(data);
    }
    
    // ... other method that can update the data class
}

that is an Observer Pattern, but the way I see, you simply want to implement OnMsg100PreselectionChanged on Activity like:

public class MainActivity extends AppCompatActivity, OnMsg100PreselectionChanged {
    void someMethod() {
        MSG0100 msg0100 = new MSG0100();
        msg0100.setOnMsgPreselectionChanged(this);
    }
    @Override
    void onPreselectionChanged(boolean isChanged) {
        
    }
}