MainActivity.java

public class MainActivity extends AppCompatActivity {

    EditText input_name, input_email;
    private ListView list_data;
    private ProgressBar circular_progress;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ...

        //Control
        circular_progress = (ProgressBar) findViewById(R.id.circular_progress);
        input_name = (EditText) findViewById(R.id.name);
        input_email = (EditText) findViewById(R.id.email);
        list_data = (ListView) findViewById(R.id.list_data);
        
        ...
    }
    
    ...
}