Android SetText空指针异常

Question

I've just begin Android development and I'm sure you can help with this (I'm sorry about my bad English)

I have a main activity, and at a certain moment i want to call another activity, in wich a want to change a textview with some message. At this moment I get a Null pointer Exception if I dont put setContentView(R.layout.register);

But when I put that line, I see for a milisecond correctly the Register activiy with my new text "Android2" and then jump again to a register activity with no text. I mean I draw it twice. I hope I have explained enough.

The question is Where do I have to put setcontentview and with what layaout.

Thank you very much, Daniel

I show you some code:

My main activity has this method:

  protected void onActivityResult(int requestCode, int resultCode, Intent data)
    {

        if (requestCode == REQUEST_CODE && resultCode == RESULT_OK)
        {
            Intent i = new Intent(this, register.class);
            startActivity(i);       

            setContentView(R.layout.register);
            TextView text = (TextView) findViewById(R.id.texto);

            try {
                text.setText("Android2");
                }catch(Exception e) {
                    Log.i("Log", e.getMessage()+"Error!"); // LogCat message
                }
        }
        //super.onActivityResult(requestCode, resultCode, data);
    }

My second activity class called register:

public class register extends Activity {
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.register);
   }
}

The register Interface is register.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/about_content"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="@string/register" />

    <TextView
        android:id="@+id/texto"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <Button
            android:id="@+id/continue_button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/save" />

        <Button
            android:id="@+id/new_button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/repeat" />
    </LinearLayout>

</LinearLayout>

Answer 1

What you basically do is the following:

1. you prepare an intent to start another activity
2. you start the activity you prepared the intent for
3. you set the current activity's content to R.layout.register
4. you get the textView (on the current activity)
5. you set the text of the textView to Android2

And, at this moment the new activity appears on the screen. Please note that your code is not correct, since you manipulate UI elements in the current activity and you expect changes on the newly started activity.

Move this code

TextView text = (TextView) findViewById(R.id.texto);

try {
    text.setText("Android2");
}catch(Exception e) {
    Log.i("Log", e.getMessage()+"Error!"); // LogCat message
}

to the register activity's onCreate() method.

BTW, usually, when you create a class, it's name should begin with a capital letter according to the standards.

Answer 2

You have two different activities. One of them is using the register.xml view, and the second one is trying to access the register view. The view only exists in your "register" activity. The other activity seems to have no view? That's probably why you're getting NULL .

You should merge those two classes together, since it looks like you're trying to access texto from the same view.

So, to summarise, findViewById should be called from within the activity that calls setContentView .

Answer 3

Remove this line and it should work

   startActivity(i); 

Not sure why you are calling that as an external activity.

Otherwise move below code to your register class

setContentView(R.layout.register);
        TextView text = (TextView) findViewById(R.id.texto);

        try {
            text.setText("Android2");
            }catch(Exception e) {
                Log.i("Log", e.getMessage()+"Error!"); // LogCat message
            }
    }