how to add the Navigation View using Java Code

You are almost in the right path, you need to change the mNavView gravity to the Gravity.START, and use DrawerLayout.LayoutParams for the NavigationView,

Also you need to convert the NavigationView from dp to pixel before setting it as a Layout parameter.

Below is a full functional example for creating DrawerLayout and NavigationView programmatically

Note: I didn't create the NavigationView layout header & menu programmatically to keep things simpler.

Behavior

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Building Main layout
        LinearLayout mainLayout = new LinearLayout(this);
        mainLayout.setGravity(Gravity.CENTER);
        mainLayout.setOrientation(LinearLayout.HORIZONTAL);
        TextView textView = new TextView(this);
        textView.setText("Some text In Main Layout");
        textView.setTextSize(30);

        mainLayout.addView(textView);
        mainLayout.setLayoutParams(new LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));

        // Building NavigationView layout
        NavigationView navView = new NavigationView(this);
        DrawerLayout.LayoutParams navParams = new DrawerLayout.LayoutParams(
                convertDpToPx(250), LinearLayout.LayoutParams.MATCH_PARENT);
        navParams.gravity = Gravity.START;
        navView.setLayoutParams(navParams);

        // Navigation View header
        View child = getLayoutInflater().inflate(R.layout.nav_header_layout, null);
        navView.addHeaderView(child);

        // Navigation View menu
        navView.inflateMenu(R.menu.nav_menu);

        // Building DrawerLayout
        DrawerLayout drawerLayout = new DrawerLayout(this);

        // adding main layout to the DrawerLayout
        drawerLayout.addView(mainLayout);
        // adding NavigationView to the DrawerLayout
        drawerLayout.addView(navView);

        // Set activity layout to the DrawerLayout
        setContentView(drawerLayout);
    }

    /**
     * Convert from dp to Pixels
     */
    private int convertDpToPx(int dp) {
        return Math.round(dp * (getResources().getDisplayMetrics().xdpi / DisplayMetrics.DENSITY_DEFAULT));
    }

}

Layout

NavigationView Header Layout (nav_header_layout.xml)

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="180dp"
    android:background="@color/colorPrimary"
    android:gravity="bottom"
    android:orientation="vertical"
    android:paddingLeft="16dp"
    android:paddingTop="16dp"
    android:paddingRight="16dp"
    android:paddingBottom="16dp"
    android:theme="@style/ThemeOverlay.AppCompat.Dark">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingTop="8dp"
        app:srcCompat="@mipmap/ic_launcher_round" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="8dp"
        android:text="Header Title"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Header Subtitle" />

</LinearLayout>

NavigationView menu (nav_menu.xml)

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:showIn="navigation_view">

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_home"
            android:icon="@drawable/ic_menu_camera"
            android:title="Home" />
        <item
            android:id="@+id/nav_gallery"
            android:icon="@drawable/ic_menu_gallery"
            android:title="Gallery" />
        <item
            android:id="@+id/nav_slideshow"
            android:icon="@drawable/ic_menu_slideshow"
            android:title="Slideshow" />
    </group>
</menu>

Here is a demo

Wish that help you out