Observemos estos tres activities, en un primer momento solo los dos primeros comparten la misma clase padre y cuentan con elementos comunes dentro de su función onCreate()

MainActivity.java

public class MainActivity extends AppCompatActivity implements PlaceListFragment.Callback {

    private boolean mTwoPane = false;
    private String LOG_TAG = MainActivity.class.getSimpleName();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (findViewById(R.id.place_detail_container) != null) {
            mTwoPane = true;
            if (savedInstanceState == null) {
                getSupportFragmentManager().beginTransaction()
                        .add(R.id.place_detail_container, new PlaceDetailFragment())
                        .commit();
            }
        } else {
            mTwoPane = false;
        }

        // Show home icon
        ActionBar actionBar = getSupportActionBar();
        actionBar.setDisplayShowHomeEnabled(true);

        PlaceSyncAdapter.initializeSyncAdapter(this);
    }

PlaceDetailActivity.java

public class PlaceDetailActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_place_detail);
        if (savedInstanceState == null) {

            long placeId = getIntent().getLongExtra(PlaceDetailFragment.PLACE_KEY, -1);

            Bundle arguments = new Bundle();
            arguments.putLong(PlaceDetailFragment.PLACE_KEY, placeId);

            PlaceDetailFragment fragment = new PlaceDetailFragment();
            fragment.setArguments(arguments);

            getSupportFragmentManager().beginTransaction()
                    .add(R.id.place_detail_container, fragment)
                    .commit();
        }
    }

SettingsActivity.java

public class SettingsActivity extends PreferenceActivity implements Preference.OnPreferenceChangeListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        addPreferencesFromResource(R.xml.pref_general);
        //bindPreferenceSummaryToValue(findPreference(getString(R.string.pref_location_default)));
    }