Why Android app crashes for initializing variable with findViewById(R.id.******) at the beginning of the class?

Question

I was just testing input/output, there was no special purpose, this is the last code that had run successfully:

public class MainActivity extends AppCompatA         


        

Answer 1

Add this code

 EditText username =  findViewById(R.id.editText_Username);
 EditText password = findViewById(R.id.editText_Password);
 TextView inputdata = findViewById(R.id.textView_InputData);
 TextView welcome = findViewById(R.id.textView_Welcome);
 Button login = findViewById(R.id.button_Login);
 Button anotherLogin = findViewById(R.id.button_Login_Another);

in

onCreate(); 

method after

setContentView(R.layout.activity_main);