onCreate 方法之外的 findViewById 方法？

Question

lately, I was working on a very small training project. But there is one error in my code:

public class MainActivity extends AppCompatActivity {
    TextView textView = (TextView) findViewById(R.id.textview);

    @override
    protected void onCreate (Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textView.setText("Welcome");
    }
}

but when I tried to cut the textview global variable and paste it inside the onCreate method, the error has gone. Why is this error occur although I already have inflated the textView in a global variable?!

Answer 1

You need to call findViewById(...) after calling setContentView(...) .

Fetching the view before inflating the layout isn't possible since there is no view.

Answer 2

findViewById() should only be used after setContentView() in onCreate.

That's where your entire layout get inflated. Only then you can access views using findViewById() .

Answer 3

This is what you want:

public class MainActivity extends AppCompatActivity {
TextView textView;

@override
protected void onCreate (Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main); 

    textView = (TextView) findViewById(R.id.textview);

    textView.setText("Welcome");

    }
}