更改TextView的文本将引发NullPointer异常

Question

I'm just following the basic tutorial on the Android website. However, I'm trying to change a few things just for fun. For instance, instead of creating a runtime TextView (as in the tutorial) within the method onCreate(...) of the DisplayMessageActivty class, I've defined a static TextView in the XML layout file because I'd like to show also other things (eg, a static message as well as the resulting entered message from the main screen). So, in the corresponding XML file of DisplayMessageActivty I've defined the TextView and other stuff as:

[...]

<TextView android:id="@+id/dynamic_msg"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:text="ciao"
    android:textSize="40dp"
    android:layout_below="@id/static_msg"
    android:layout_alignParentLeft="true"
    android:layout_toLeftOf="@+id/times" />

[...]

then, I access to that TextView in the following way ( onCreate(...) method of the DisplayMessageActivty class):

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Get the message from the intent
    Intent intent = getIntent();
    String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

    // HERE I WANT TO CHANGE THE CONTENT OF THE TEXTVIEW
    TextView dynamicText = (TextView) findViewById(R.id.dynamic_msg);
    dynamicText.setText(message);

    setContentView(R.layout.activity_display_message);

    // Up button
    getActionBar().setDisplayHomeAsUpEnabled(true);
}

Unfortunately, when I press the Send button in the main activity my code throws the following exception:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

I've searched on the Internet and the approach adopted in other tutorials looks quite similar, so I cannot understand what I'm doing wrong. Thanks!

Answer 1

The content is not set into the view, the findviewByid will return NULL if you do it BEFORE setContentView .

move setContentView BEFORE findviewByid and it will work.