// Add implements View.OnClickListener
  public class MainActivity extends AppCompatActivity implements View.OnClickListener {...}

  private void initInstances() {
        btn_calculate = (Button) findViewById(R.id.btn_calculate);
        btn_calculate.setOnClickListener(this); //Add this and Alt Enter choose Make '..Activity' implement '....'
    }

    @Override
    public void onClick(View v) {
        if (v == btn_calculate) {
            // Do something
        }
    }