在方法中声明时可运行的作品； 在方法外声明时崩溃

Question

I'm new to Android.

I want to modify the TextView of my activity after a few seconds (it says "hey hey!" at first; I want it to say "hello!" after a few seconds), so I have:

protected void onResume() {
    super.onResume();

    final TextView t = (TextView)findViewById(R.id.hello);
    Runnable changeTextTask = new Runnable() {
        public void run() {
            t.setText("hello!");
        }
    };

    Handler h = new Handler();
    h.postDelayed(changeTextTask, 3000);
}

Which works. However, when I declare the Runnable at the beginning of the class, like so:

public class MainActivity extends ActionBarActivity {
    final TextView t = (TextView)findViewById(R.id.hello);
    Runnable changeTextTask = new Runnable() {
        public void run() {
            t.setText("hello!");
        }
    };

    .
    .
    .

    protected void onResume() {
        super.onResume();

        Handler h = new Handler();
        h.postDelayed(changeTextTask, 3000);
    }

the app crashes upon starting. Can anyone explain why this happens/what I'm doing wrong?

Answer 1

what I'm doing wrong?

First, use LogCat to examine the Java stack trace of your crash.

Second, do not call inherited methods on your Activity , like findViewById() , until inside of onCreate() , and usually after the super.onCreate() call. onResume() is called after onCreate() completes, which is why your first edition survives better.

Third, specifically with findViewById() , you need to call that after the widget already exists. That will not occur until setContentView() or equivalent means of setting up your UI.

Answer 2

Mixing Sotirios Delimanolis' and CommonsWare's responses here:

LogCat reveals findViewByID() caused a NullPointerException because it was called before onCreate() (where the resource with the TextView is loaded) as soon as the activity starts.