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 AppCompatActivity
{

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

public void doLoginOnClick(View v)
{
    String s1=username.getText().toString();
    inputdata.setText(s1);
}

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

    FloatingActionButton fab = (FloatingActionButton)findViewById(R.id.fab); 
    fab.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View view)
        {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });
} 

}

---

I am trying to capture id of component using

findViewById(R.id.***)

as you can see, I put this code in comment at the very beginning of the code.

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

If I remove the comment above and run it (both in emulator and real device), the program crashes immediately, I am surprised what's wrong here?

even I have tried to initialize the same thing using constructor.

But if I put it inside onCreate() there's no crash? Why?

I was trying to fetch info of username and password and display it to the textview in textView_Inputdata using

EditText username = (EditText)findViewById(R.id.editText_Username);
EditText password = (EditText)findViewById(R.id.editText_Password);
TextView inputdata = (TextView)findViewById(R.id.textView_InputData);
inputdata.setText(username.getText.toString()+" "+password.getText.toString());

Is there better or easier way to do that?

Answer 1

Instance member variables are initialized when the instance itself is initialized. It's too early for findViewById()

Before onCreate() your activity does not yet have a Window that findViewById() needs internally. Before setContentView() (that you should be calling in onCreate()) there are no views to be found either.

Therefore init your view references in onCreate() and after setContentView().