在TextView中设置文本会引发空指针异常

Question

I am just starting out android programming, and I ran into this error while trying to set the text in a TextView. I have researched other people's problems, but I have seemed to take care of the trouble issues that were pointed out by their answers.

Here is my XML Code:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.everythingmath.MainActivity$PlaceholderFragment" >

<GridLayout
    android:id="@+id/problemLayout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerInParent="true"
    android:columnCount="5"
    android:rowCount="1" >

    <TextView 
        android:id="@+id/firstNumber"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="First Number"
        android:textSize="20sp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text=" + "
        android:textSize="30sp" />

    <TextView 
        android:id="@+id/secondNumber"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Second Number"
        android:textSize="20sp" />

    <EditText
        android:id="@+id/numTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="number" 
        android:hint="Text Goes Here"
        android:textSize="20sp"/>

</GridLayout>
</RelativeLayout>

Here is my pertinent Java:

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }

    int firstNumber = createRandomNumber(10);
    int secondNumber = createRandomNumber(10);
    TextView firstNumText = (TextView)findViewById(R.id.firstNumber);
    TextView secondNumText = (TextView)findViewById(R.id.secondNumber);
    firstNumText.setText(Integer.toString(firstNumber));
    secondNumText.setText(Integer.toString(secondNumber));
}

It is the lines in which I try to set the text that throw the error. The random number generator works fine also.

Answer 1

Localize PlaceholderFragment class inside MainActivity class.

Change public View onCreateView method into

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_main, container, false);

    int firstNumber = createRandomNumber(10);
    int secondNumber = createRandomNumber(10);
    TextView firstNumText = (TextView)findViewById(R.id.firstNumber);
    TextView secondNumText = (TextView)findViewById(R.id.secondNumber);
    firstNumText.setText(Integer.toString(firstNumber));
    secondNumText.setText(Integer.toString(secondNumber));

    return rootView;
}

You will probably need to move createRandomNumber method to PlaceholderFragment class. And in case you have changed Fragment's name from fragment_main to something else, change it also in this line

View rootView = inflater.inflate(R.layout.fragment_main, container, false);

Your current code doesn't work because inside

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    [..]
}

you can only find by ID view ( findViewById ) which was declared inside activity_main layout. And your TextView views are probably declared inside fragment_main layout.