Java 尝试生成警报对话框时出现空指针异常
尽管我选择不为ListAdapter使用单独的类,但我一直在尝试使用问题答案中给出的代码。 当我尝试启动我的活动(InfosActivity)时,应用程序崩溃,下面是日志:Java 尝试生成警报对话框时出现空指针异常,java,android,nullpointerexception,Java,Android,Nullpointerexception,尽管我选择不为ListAdapter使用单独的类,但我一直在尝试使用问题答案中给出的代码。 当我尝试启动我的活动(InfosActivity)时,应用程序崩溃,下面是日志: 05-02 11:50:22.195 7521-7521/com.example.uia59227.User_and_Car_Data E/AndroidRuntime: FATAL EXCEPTION: main
05-02 11:50:22.195 7521-7521/com.example.uia59227.User_and_Car_Data E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.uia59227.User_and_Car_Data, PID: 7521
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.uia59227.User_and_Car_Data/com.example.uia59227.User_and_Car_Data.InfosActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2849)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)
at com.example.uia59227.User_and_Car_Data.InfosActivity.<init>(InfosActivity.java:223)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1086)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2839)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
您能帮助我吗?问题是您在声明中实例化了ListAdapter。因此,实例化是在InfoActivity类的实例化过程中完成的(在完成实例化之前)。如果InfoActivity尚未完成实例化,
此
为null
。因此,当您执行getApplicationContext()
时,您正在执行此操作。getApplicationContext()
但是此
为空
在活动实例化之后,必须实例化ListAdapter。例如,您可以在onStart()方法中实例化Listadapter
此外,如本文所述,如果您需要活动上下文,请使用
this
而不是getApplicationContext()
发布完整的堆栈跟踪,并向我们显示NullPointerException指向哪一行,但我不确定异常发生在哪里导致错误的那一行是InfoActivity中的第223行然后是我的ListAdapter适配器的声明,但我不知道我能在一个活动中对此做些什么?您不需要使用getapplicationcontext,而是使用它,对于上下文@downvoter,您能提供反馈吗?因此,我可以改进我的回答。当我仍然有一个空点异常指向行holder.icon.setImageDrawable(drawable)代码>。在getview的定义中,我在parent
上有一个警告,说未注释的参数覆盖@NonNull parameter
,我不知道这是否是我的错误的原因。这意味着holder
或icon
为null<如果执行else和convertView,则code>holder
为null。getTag()返回null<如果您通过if和findViewById()
找不到具有R.id.icon
作为id的视图,或者您通过else,并且convertView.getTag()
返回的对象有一个为null的字段图标OK,这对我来说不容易理解,但我做到了,更改了convertView.findViewById(R.id.icon)代码>这是不存在的(我不明白为什么我没有任何错误)进入findViewById(R.id.Car)
中,它实际上存在,并且有效。谢谢
String[] items = {"airplanes", "animals", "cars", "colors", "flowers", "letters", "monsters", "numbers", "shapes", "smileys", "sports", "stars" };
ListAdapter adapter = new ArrayAdapter<String>(getApplicationContext(), R.layout.list_row, items) {
ViewHolder holder;
Drawable icon;
class ViewHolder {
ImageView icon;
TextView title;
}
public View getView(int position, View convertView, ViewGroup parent) {
final LayoutInflater inflater = (LayoutInflater) getApplicationContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (convertView == null) {
convertView = inflater.inflate(R.layout.list_row, null);
holder = new ViewHolder();
holder.icon = (ImageView) convertView.findViewById(R.id.icon);
holder.title = (TextView) convertView.findViewById(R.id.title);
convertView.setTag(holder);
} else {
// view already defined, retrieve view holder
holder = (ViewHolder) convertView.getTag();
}
Drawable drawable = ContextCompat.getDrawable(context,R.drawable.ic_person); //this is an image from the drawables folder
holder.title.setText(items[position]);
holder.icon.setImageDrawable(drawable);
return convertView;
}
};
quickReviewButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View arg0) {
AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(context);
alertDialogBuilder
.setTitle("Full report")
.setAdapter(adapter, new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int item) {
Toast.makeText(InfosActivity.this, "You selected: " + items[item], Toast.LENGTH_LONG).show();
dialog.dismiss();
}
});
// create alert dialog
AlertDialog alertDialog = alertDialogBuilder.create();
// show it
alertDialog.show();
}
});