CurrentUserProvider.java

package pl.brightinventions.samples;

import android.content.Context;
import android.content.SharedPreferences;

import javax.inject.Singleton;

public interface CurrentUserProvider {
    boolean isLoggedIn();
    CurrentUser getCurrentUser();
}

class PreferencesSaver {
    private static final String USER = "user";
    private final Context _context;

    public PreferencesSaver(Context context) {
        _context = context;
    }

    public void savePreferences(CurrentUserProvider userProvider, AppConfig appConfig){
        String keyPrefix =  "appConfig.defaultUser";
        if(userProvider.isLoggedIn()){
            keyPrefix = "appConfig." + userProvider.getCurrentUser().getId();
        }
        
        SharedPreferences preferences = _context.getSharedPreferences(keyPrefix, Context.MODE_PRIVATE);
        preferences.edit()
                .putBoolean("useFeatureA", appConfig.useFeatureA)
                .putBoolean("useFeatureB", appConfig.useFeatureB)
                .apply();
    }
}

CurrentUserProvider2.java

interface UserReference {
    int getId();
}

@Singleton
class InMemoryUserSession implements CurrentUserProvider {
    UserReference _currentUser = new AnonymousUserReference();
    @NonNull
    @Override
    public UserReference getCurrentUserReference() {
        return _currentUser;
    }

    public void setNewUser(User user){
        _currentUser = new RegularUserReference(user);
    }
    //NullObject
    private static class AnonymousUserReference implements UserReference {
        @Override
        public int getId() {
            return -1;
        }
    }

    private static class RegularUserReference implements UserReference {
        private final User _user;

        public RegularUserReference(User user) {
            _user = user;
        }
        @Override
        public int getId() {
            return _user.getId();
        }
    }
}

class PreferencesSaver {
    ...
    public void savePreferences(CurrentUserProvider userProvider, AppConfig appConfig){
        String keyPrefix =  "appConfig." + userProvider.getCurrentUserReference().getId();
        SharedPreferences preferences = _context.getSharedPreferences(keyPrefix, Context.MODE_PRIVATE);
        ...
    }
}