我有以下代码:
public void refresh(){
View rootView=getView();
final SharedPreferences pref = this.getContext().getSharedPreferences("MyFav", 0);
final SharedPreferences.Editor editor = pref.edit();
Map<String,?> entries = pref.getAll();
final Set<String> keys = entries.keySet();}
此函数位于片段内,我从Activity调用它,如下所示: -
public void onBackPressed() {
super.onBackPressed();
FavLegislator fav =new FavLegislator();
if(frag.equals("true"))
fav.refresh();
finish();
}
活动中的共享偏好设置可能已更改 我只想在活动结束时获得更新的首选项列表。
我在Activity的后退按钮上收到以下错误:
11-27 10:06:24.126 31645-31645/com.app.congress.congressapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.app.congress.congressapp, PID: 31645
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference
at com.app.congress.congressapp.FavLegislator.refresh(FavLegislator.java:85)
at com.app.congress.congressapp.legislator_info.onBackPressed(legislator_info.java:466)
at com.app.congress.congressapp.legislator_info.onOptionsItemSelected(legislator_info.java:454)
at android.app.Activity.onMenuItemSelected(Activity.java:2912)
at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:361)
at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:147)
at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:100)
at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:100)
at android.support.v7.widget.ToolbarWidgetWrapper$1.onClick(ToolbarWidgetWrapper.java:194)
at android.view.View.performClick(View.java:5201)
at android.view.View$PerformClick.run(View.java:21163)
at android.os.Handler.handleCallback(Handler.java:746)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5443)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
有人可以告诉我为什么我会收到上述错误以及如何解决?
答案 0 :(得分:1)
super.onBackPressed();
调用会破坏/完成您的活动,您不能再使用属于此活动的任何对象,因此只需在结束时移动此调用,而无需调用finish()
public void onBackPressed() {
FavLegislator fav =new FavLegislator();
if(frag.equals("true"))
fav.refresh();
super.onBackPressed();
}
更改此
final SharedPreferences pref = this.getContext().getSharedPreferences("MyFav", 0);
到这个
final SharedPreferences pref = getActivity().getSharedPreferences("MyFav", 0);
注意:更好的选择是使用静态方法创建一个实用程序类,它将保存您的首选项数据,以便将上下文和数据传递给实用程序类并将其保存到首选项
public class Util {
public static void saveData(Context context,int i ){
// your preference code , change parameters accordingly
}
}
来自您活动的称之为
Util.saveData(this,5);
答案 1 :(得分:1)
您无法调用new FavLegislator()
并希望它具有上下文。
这似乎是一个Activity或Fragment类。当你创建一个new
时,没有附加Context,所以getContext()
返回null。
如果您在返回时尝试刷新其他视图,请使用onResume
方法