App crashes when I include setContentView and setText

Question

This is my first android app, pretty confused and not sure if I'm doing it right. My app is intended to be a countdown app for a specific date

Here is what my MainActivity class looks like. I replaced ... where the code I wrote relating to the actual countdown variable was - I have tested it in the console and that part of the program works fine.

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        counting();
    }

    public void counting() {
        ...
        String reading = days + " days " + hours + " hours " + minutes + " minutes remaining";
        setContentView(R.layout.activity_main);
        TextView textView = (TextView) findViewById(R.id.textview_first);
        textView.setText(reading);
    }
}

It runs and shows me the 'hello' text string specified in strings.xml

 <string name="hello_first_fragment">hello</string>

as long as I get rid of both the

        setContentView(R.layout.activity_main);

as well as the

        textView.setText(reading);

lines under my counting function, but I don't want it to show that string, I would rather it display the value of the variable I want to choose. If I include either or both of them, it crashes the app.

I'm wondering if maybe I have the textview ID wrong - I got that from my fragment_first.xml file which has

"android:id="@+id/textview_first"" 

under TextView.

Another thing I have tested is with just making the textView.setText field have a non-variable string in it, like

            textView.setText("reading");

but it does the same thing

I'm not sure if I might have to somehow remove the string line in strings.xml somehow or edit it, I tried just deleting it and it wouldn't compile.

Anyone have any advice on what I'm doing wrong?