动态创建片段时，如何摆脱“空指针异常”？

Question

I want to do to-do list application by using fragments. While creating this fragments dynamically, I take a null pointer exception at this line (in mainactivity.java ):

adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, todoItems);
listToDo.setListAdapter(adapter);

I cannot see this message on logcat: Log.w("6", "altı"); What is wrong with my code? How can I solve this problem?

The activity_main.xml file:

    <LinearLayout 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=".MainActivity" 
    android:orientation="vertical">


   <FrameLayout 
        android:id="@+id/list_view_fragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1.5"
        >    </FrameLayout>
    </LinearLayout>

to_do_fragment.xml

    <?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/add_to_do_fragment" >  
    <Button 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:id="@+id/addButton"
        android:contentDescription="@string/addItemContentDescription"
        android:text="@string/add_button"/>
    <EditText
        android:id="@+id/editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_toLeftOf="@id/addButton"
        android:hint="@string/edit_text" />   
    </RelativeLayout>

list_view.xml

    <?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/list_view" >
    <ListView 
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/listView"></ListView>
    </RelativeLayout>


LogCat

    10-30 19:28:26.355: E/AndroidRuntime(1763): FATAL EXCEPTION: main
    10-30 19:28:26.355: E/AndroidRuntime(1763): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.todolistwithdynamicfragments/com.example.todolistwithdynamicfragments.MainActivity}: java.lang.IllegalArgumentException: No view found for id 0x7f080003 (com.example.todolistwithdynamicfragments:id/add_to_do_fragment) for fragment AddToDoFragment{41770868 #0 id=0x7f080003}
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.os.Handler.dispatchMessage(Handler.java:99)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.os.Looper.loop(Looper.java:137)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.ActivityThread.main(ActivityThread.java:5103)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at java.lang.reflect.Method.invokeNative(Native Method)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at java.lang.reflect.Method.invoke(Method.java:525)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at dalvik.system.NativeStart.main(Native Method)
    10-30 19:28:26.355: E/AndroidRuntime(1763): Caused by: java.lang.IllegalArgumentException: No view found for id 0x7f080003 (com.example.todolistwithdynamicfragments:id/add_to_do_fragment) for fragment AddToDoFragment{41770868 #0 id=0x7f080003}
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:877)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1057)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.BackStackRecord.run(BackStackRecord.java:682)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1435)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.Activity.performStart(Activity.java:5142)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
    10-30 19:28:26.355: E/AndroidRuntime(1763):     ... 11 more

Answer 1

Your toDoItems is not getting initialized in the following line:

todoItems = savedInstanceState.getStringArrayList("todoItemTag");

I would suggest you test the array to ensure that it is not null. Something like:

Log.i("ARRAY",todoItems.length);

Answer 2

Since todoItems is the only one of the three parameters in that line that could be null, that must be the culprit. The only way I see for it to be null is if the savedInstanceState does not include "todoItemTag" which could easily happen under many scenarios. Your code is assuming that the savedInstanceState always has "todoItemTag" if savedInstanceState is not null, but it very well could be non-null and have nothing in it. I would rewrite your code sort of like this, but actually I don't understand why you are only creating the fragment under certain circumstances, so this probably needs more work:

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

if (savedInstanceState!=null)
    todoItems = savedInstanceState.getStringArrayList("todoItemTag");

if (todoItems == null) {
    todoItems = new ArrayList<String>();
}

adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, todoItems);
listToDo.setListAdapter(adapter);

//TODO fragment management

}

Answer 3

Move this line

listToDo = (ToDoListFragment) fm.findFragmentById(R.id.list_view_fragment);

into your else statement.

You are essentially nulling out listToDo on your first run since the fragment manager will not find your fragment that you just committed. commit() schedules your fragments to be added. It does not mean they are added right away. So, calling findFragmentById will return null when you call it immediately after which is why listToDo ends up null.