Android Button On Click Listener and TextView

gistfile1.txt

package com.example.mycalculator;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    EditText num1, num2;
    TextView textView;
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //1
        setContentView(R.layout.activity_main);
        Log.d("ON_CREATE", "called");

        button = this.findViewById(R.id.button);
        textView = this.findViewById(R.id.textView);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(getApplicationContext(), "Hello world", Toast.LENGTH_SHORT) .show();
                textView.setText("hello world");
            }
        });
    }
}