Incorrect Activity when running App Android Studio

I have starting building an app in Android studio. I have established the MainPage as the launcher activity in the manifest.xml.

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/Theme.AppCompat.NoActionBar">

    <activity android:name=".MainPage">
        android:screenOrientation="portrait"
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity android:name=".courseSelect" />
    <activity android:name=".profile1" />
    <activity android:name=".stats1" />
    <activity android:name=".ReviewRounds" />
    <activity android:name=".ReferFriends" />
    <activity android:name=".RangeMode" />
</application>

I have double checked that the run configuration is set to 'Default' and yet the app is running a different activity, entitled courseSelect. It is also not running some code correctly on the NumberPicker. Even though I've set the picker to have a min, max, and default, the picker only shows 0 and will not scroll. The two issues seem to be related somehow, in terms of what activity is being run.

this is the courseSelectCode:

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.NumberPicker;
import co.ceryle.segmentedbutton.SegmentedButtonGroup;

public class courseSelect extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_course_select);

        //Hole Picker
        NumberPicker holePicker = (NumberPicker)findViewById(R.id.holePicker);
        holePicker.setMaxValue(18);
        holePicker.setMinValue(1);
        holePicker.setWrapSelectorWheel(false);
        holePicker.setValue(1);

        SegmentedButtonGroup sbg = (SegmentedButtonGroup) findViewById(R.id.segmentedButtonGroup);
        sbg.setOnClickedButtonPosition(new SegmentedButtonGroup.OnClickedButtonPosition() {
            @Override
            public void onClickedButtonPosition(int position) {
                // if(position == 0)
            }
        });
    }
}

I tried to set the run configuration specifically to the MainPage activity and it still opens in the courseSelect Page.

EDIT: on request, here is my MainPage.java code:

public class MainPage extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_course_select);

        Window g = getWindow();
        g.setFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION, WindowManager.LayoutParams.TYPE_STATUS_BAR);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    }
    ...
}

edit the Mainfest.xml, in order to enforce portrait layout there already:

<activity android:name=".MainPage"
    android:screenOrientation="portrait"
    android:configChanges="orientation|keyboardHidden">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

...which makes this code merely useless (styles.xml can also be used for window styles):

Window g = getWindow();
g.setFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION, WindowManager.LayoutParams.TYPE_STATUS_BAR);
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

also update setContentView( R.layout.activity_course_select ); to the proper resource.

because it starts the MainPage Activity, but then inflates the wrong XML file.

---

one "suggested edit" before was to swap the order of setContentView() and the paragraph below ...which I've rejected, because setting it in Manifest.xml appeared more organized (less code).