Let's update MainActivity to look like this:

@BindView(R.id.hello_world_salute)
TextView helloText;

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

    ButterKnife.bind(this);

    Observable.just("Hello! Please use this app responsibly!")
        .subscribe(new Consumer<String>() {
            @Override
            public void accept(String s) {
                helloText.setText(s);
            }
        }); 
}

The import block for the preceding code is as shown:

import butterknife.BindView;
import butterknife.ButterKnife;
import io.reactivex.Observable;
import io.reactivex.functions.Consumer;

The contents of activity_main.xml at the moment are set to the following:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="6dp"
    tools:context="packt.reactivestocks.MainActivity">

    <TextView
        android:id="@+id/hello_world_salute"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="" />
</RelativeLayout>

This will create a very basic screen with a single text element that will say "Hello! Please use this app responsibly!".