共享首选项的 Android 空指针异常 [英] Android Null Pointer Exception for shared Preferences

问题描述

我正在尝试在共享"首选项中保存一个整数值，并在应用程序重新启动时再次加载它.

I am trying to save a integer value in the Shared preferences and load it again when the application is restarted.

我在我的主要活动中创建了一个共享首选项，并尝试在单击菜单项时将整数值保存在另一个菜单侦听器类中.Menu 侦听器类位于不同的包中，Menu Listener 类扩展了 Main Activity.

I have created a shared preferences in my main activity and trying to save integer value in a another Menu Listener class on menu item click. The Menu listener class is in a different package and the Menu Listener class extends the Main Activity.

但是当我尝试这样做时，我收到了 Null 指针异常.

But I am getting a Null pointer exception when I try to do that.

我也浏览了这个链接！.但它似乎对我没有帮助.

I have also gone through this link!. But it doesn't seem to be helping me.

这是我的代码:

主要活动:

    protected SharedPreferences mPrefs;
protected SharedPreferences.Editor mEditor;
protected Context  main_activityclass_context; //EDIT
public static int count=0;

@Override
protected void onCreate(final Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);


     mPrefs = this.getSharedPreferences("THEME", 0);
     mEditor = mPrefs.edit();
    }

菜单监听器类:

    case  mainActivity.MENU_ITEM_THEME_DARK:
    {

        mainActivity.count=1;

        mPrefs =main_activityclass_context.getSharedPreferences("THEME", 0); //EDIT
        mEditor = mPrefs.edit();
        mEditor.putInt("Theme_count", mainActivity.count).commit();


    break;
    }

我在菜单侦听器的行上收到空指针异常:

I am getting a Null pointer exception on the lines in the Menu Listener :

       mPrefs = getSharedPreferences("THEME", 0);
        mEditor = mPrefs.edit();    

我的 Logcat:

      08-03 10:54:03.650: E/AndroidRuntime(31258): FATAL EXCEPTION: main
      08-03 10:54:03.650: E/AndroidRuntime(31258): java.lang.NullPointerException
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at               android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:153)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at de.exb.ptpt.tablet.menu.TabletMenuListener.onMenuItemClick(TabletMenuListener.java:215)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:144)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:874)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at com.android.internal.view.menu.MenuPopupHelper.onItemClick(MenuPopupHelper.java:156)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at android.widget.AdapterView.performItemClick(AdapterView.java:292)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at android.widget.AbsListView.performItemClick(AbsListView.java:1058)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.widget.AbsListView$PerformClick.run(AbsListView.java:2514)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.widget.AbsListView$1.run(AbsListView.java:3168)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.os.Handler.handleCallback(Handler.java:605)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.os.Handler.dispatchMessage(Handler.java:92)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.os.Looper.loop(Looper.java:137)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.app.ActivityThread.main(ActivityThread.java:4424)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at java.lang.reflect.Method.invokeNative(Native Method)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at java.lang.reflect.Method.invoke(Method.java:511)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at dalvik.system.NativeStart.main(Native Method)

有人可以帮我吗?

谢谢.

推荐答案

其中一个好方法是，您可以创建一个辅助类来声明、保存和检索首选项.您可以参考以下代码示例:

One of the good ways is, you create a helper class to declare, save and retrive preferences. You can refer following code sample:

public class PreferencesHelper {

private SharedPreferences sharedPreferences;
private Editor editor;

public PreferencesHelper(Context context) {
this.sharedPreferences = getPreferences(MODE_PRIVATE);    
this.editor = sharedPreferences.edit(); }

public String GetPreferences(String key) {
    return sharedPreferences.getString(key, "");
}

public void SavePreferences(String key, String value) {
editor.putString(key, value);    
editor.commit();  
}
}  

在活动的 onCreate 事件中，您可以创建 PreferencesHelper 的对象并获取所有保存的首选项，例如:

In onCreate event of an activity, you can create of object of PreferencesHelper and get all saved preferences like:

Prefs = new PreferencesHelper(getApplicationContext());  

Prefs.GetPreferences("YOURARGUMENT");  

并且您可以保存首选项，例如:

and you can save the preferences like:

Prefs.SavePreferences(key, value);  

希望对您有所帮助.

这篇关于共享首选项的 Android 空指针异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！