activity_my.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/linearLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    tools:context=".MyActivity">

    <include layout="@layout/my_linear_layout" />

</LinearLayout>

my_linear_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="200dp"
    android:layout_height="200dp"
    android:gravity="center">

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Second Activity"
        android:textAppearance="@style/TextAppearance.AppCompat.Large" />
</LinearLayout>

MyActivity.java

public class MyActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my);

        LinearLayout parent = findViewById(R.id.linearLayout);

        //by doing this we lose all our theme customisation
        LayoutInflater layoutInflater = LayoutInflater.from(getApplicationContext());

        View myLinearLayout = layoutInflater.inflate(R.layout.my_linear_layout, parent, false);

        parent.addView(myLinearLayout);
    }
}