NullPointerException when starting ListActivity

问题

I'm working for hours on an error I get, when I want to start a ListActivity. Short workaround about what I want to do: I have a main application with a normal menu, from which I want to start a setting list where the user can add specific settings.

I'm trying to start the ListView like this:

startActivity(new Intent(this, FileManagerSettings.class));

Then in the on Create method I got the error:

        userSettings = deserializeObject();

    if(userSettings.isEmpty())
    {
        userSettings.add(new SettingItem("Camera", android.os.Environment.DIRECTORY_DCIM, false));
    }
    super.onCreate(icicle);
    Context mContext = this.getApplicationContext(); 
    settingsList = (ListView)findViewById(R.id.list_settings);
    settingsList.setAdapter(new CustomSettingsAdapter(mContext, userSettings));

The last line causes the error. For better understanding here the Constructor of the adapter class:

     public CustomSettingsAdapter(Context context, ArrayList<SettingItem> sitems)
 {
     settingsArrayList = sitems;
     mInflater = LayoutInflater.from(context);
 }

And here is the error from the logcat: (Sorry, don't know how to format it right)

04-01 11:07:26.756: ERROR/AndroidRuntime(375): FATAL EXCEPTION: main 04-01 11:07:26.756: ERROR/AndroidRuntime(375): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.openintents.filemanager/org.openintents.filemanager.FileManagerSettings}: java.lang.NullPointerException 04-01 11:07:26.756: ERROR/AndroidRuntime(375): Caused by: java.lang.NullPointerException 04-01 11:07:26.756: ERROR/AndroidRuntime(375): at org.openintents.filemanager.FileManagerSettings.onCreate(FileManagerSettings.java:43)

In my thinking the error is caused because the LayoutInflater of the context is null... But I don't know how to get the LayoutInflater then and even don't know if this is really the error...

edit: error log with using setContentView():

04-01 12:06:56.315: ERROR/AndroidRuntime(669): FATAL EXCEPTION: main 04-01 12:06:56.315: ERROR/AndroidRuntime(669): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.openintents.filemanager/org.openintents.filemanager.FileManagerSettings}: java.lang.RuntimeException: Your content must have a ListView whose id attribute is 'android.R.id.list'

this is the xml-File:

<LinearLayout android:layout_width="fill_parent"
    android:id="@+id/list"
    android:layout_height="fill_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">
    <TextView android:id="@+id/settings_list_title"
        android:layout_height="wrap_content"
        android:layout_width="fill_parent"
        android:text="@string/settings_header"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:gravity="center" />
    <ListView android:id="@+id/list"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    </LinearLayout>

回答1:

findViewById() only returns a valid reference to a View if the layout of the Activty has been set. Looking at the code you have pasted in to your question, it does not appear that you are making a call to

setContentView(R.layout.whatever_your_layout_file_id);

Your variable settingsList, would then be a null reference

来源：https://stackoverflow.com/questions/5512710/nullpointerexception-when-starting-listactivity

标签

android

android-context

listactivity