PreferenceFragment：NullPointerException

Question

我做了一个PreferenceActivity来控制我的应用程序的设置。 每次单击首选项时，应用程序崩溃并创建NullPointerException错误。

这是PreferenceFragment：

public class SettingsFragment extends PreferenceFragment {

EditText enterPassword, newPassword;
public final String filename = "PasswordFile";
String myPassword;
static SharedPreferences myFolder;
public static String dataReturned = "";
Context context;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    addPreferencesFromResource(R.xml.preferences_new);

Preference password_pref = (Preference) findPreference("password");
password_pref.setOnPreferenceClickListener(new OnPreferenceClickListener() {

    public boolean onPreferenceClick(Preference preference) {
        // TODO Auto-generated method stub
        LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        final View text = inflater.inflate(R.layout.changepassword, null);
        final EditText currentPassword = (EditText)text.findViewById(R.id.currentPassword);
        final EditText newPassword = (EditText)text.findViewById(R.id.newPassword);

        final AlertDialog.Builder alert = new AlertDialog.Builder(context);
        alert.setTitle("Change Your Password");
        alert.setView(text);
        alert.setPositiveButton("Change", new DialogInterface.OnClickListener() {

            public void onClick(DialogInterface dialog, int which) {
                    String stringData = currentPassword.getText().toString().trim();
                    String stringNew = newPassword.getText().toString().trim();
                    dataReturned = myFolder.getString("passwordKey", "");
                    if(dataReturned.equals(stringData)) {                       
                        String newData = newPassword.getText().toString().trim();
                        SharedPreferences.Editor editor = myFolder.edit();
                        editor.putString("passwordKey", newData);
                        editor.commit();
                        dataReturned  = myFolder.getString("passwordKey", "couldn't load data");
                        Toast.makeText(context, "Password changed", Toast.LENGTH_SHORT).show();
                        currentPassword.setText("");
                        newPassword.setText("");
                    }

                    else {
                        Toast.makeText(context, "Incorrect Password", Toast.LENGTH_LONG).show();
                        currentPassword.setText("");
                        newPassword.setText("");
                    }
                }
            });
        alert.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {

            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub
                dialog.cancel();
            }
            });
        alert.show();
        ;

        return true;
    }

});

Preference notification_pref = (Preference) findPreference("notification");
notification_pref.setOnPreferenceClickListener(new OnPreferenceClickListener() {

    public boolean onPreferenceClick(Preference preference) {
        // TODO Auto-generated method stub
        NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context)
        .setSmallIcon(R.drawable.device_access_secure)
        .setOngoing(true)
        .setContentTitle("Obstruct")
        .setContentText("Start Stealth Mode");

        Intent resultIntent = new Intent();

        TaskStackBuilder stackBuilder = TaskStackBuilder.create(context);
        stackBuilder.addNextIntent(resultIntent);

        PendingIntent resultPendingIntent = stackBuilder.getPendingIntent(0, PendingIntent.FLAG_UPDATE_CURRENT);
        mBuilder.setContentIntent(resultPendingIntent);

        NotificationManager mNotificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
        mNotificationManager.notify(0, mBuilder.build());
        return true;
    }

});
}
}

这是logcat：

11-23 15:23:36.191: E/AndroidRuntime(17168): FATAL EXCEPTION: main
11-23 15:23:36.191: E/AndroidRuntime(17168): java.lang.NullPointerException
11-23 15:23:36.191: E/AndroidRuntime(17168):    at com.mshaw.avanos8.adfree.SettingsFragment$1.onPreferenceClick(SettingsFragment.java:42)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.preference.Preference.performClick(Preference.java:941)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.preference.PreferenceScreen.onItemClick(PreferenceScreen.java:202)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.widget.AdapterView.performItemClick(AdapterView.java:292)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.widget.AbsListView.performItemClick(AbsListView.java:1071)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.widget.AbsListView$PerformClick.run(AbsListView.java:2527)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at  android.widget.AbsListView$1.run(AbsListView.java:3181)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.os.Handler.handleCallback(Handler.java:605)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.os.Handler.dispatchMessage(Handler.java:92)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.os.Looper.loop(Looper.java:154)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at android.app.ActivityThread.main(ActivityThread.java:4894)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at java.lang.reflect.Method.invokeNative(Native Method)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at java.lang.reflect.Method.invoke(Method.java:511)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
11-23 15:23:36.191: E/AndroidRuntime(17168):    at dalvik.system.NativeStart.main(Native Method)

我已经尽力解决了这个问题。 我怀疑应该归咎于背景，但我不确定。

Answer 1

看起来您的变量上下文为空，请尝试使用getActivity（）而不是上下文，或者通过在preferencefragment的构造函数中传递打开首选项的活动的上下文，然后将其分配给变量。

Answer 2

不用在类上定义context变量，而是在onCreate内将其定义为

final Context context = this;

Answer 3

只需确保将“ addPreferencesFromResource”放在片段类的getPreferenceScreen之前，例如：

public static class MyFragment extends PreferenceFragment {

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

        addPreferencesFromResource(R.xml.pref_general);

        PreferenceScreen root = this.getPreferenceScreen();

    }
}