android - 需要解释 : getText() throws error on getActitvity()ut not on view

Question

我是 android 开发的初学者，希望得到以下解释，

我正在尝试在自定义对话框中获取用户输入。下面的代码有效。但是如果我更换

final EditText editText = (EditText) view.findViewById(R.id.editText);

与

final EditText editText = (EditText) getActivity().findViewById(R.id.editText);

我在 setPositiveButton() 的 getText() 语句中得到 nullPointerException。

此外，任何关于为什么我在声明时但在阅读文本 (getText()) 时没有出错的解释都将不胜感激。

public class FireMissilesDialogFragment extends DialogFragment{
@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {

    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
    LayoutInflater layoutInflater = getActivity().getLayoutInflater();
    View view = layoutInflater.inflate(R.layout.edit_menu, null);

    final EditText editText = (EditText) view.findViewById(R.id.editText);
    builder.setView(view)
            .setMessage("Fire food missile?")
            .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    Toast.makeText(getActivity(), editText.getText().toString(), Toast.LENGTH_SHORT).show();
                }
             })
            .setNegativeButton("No", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    Toast.makeText(getActivity(), "Why not?", Toast.LENGTH_SHORT).show();
                }
            });
    return builder.create();
}

Answer 1

i get nullPointerException at getText() statement in setPositiveButton().

In addition, any explanation of why i don't get error while declaring but while reading the text (getText()) would be appreciated

findViewById 在第二种情况下返回 null，因为您要查找的小部件 TextView 属于您扩充并设置为对话框 View 的布局而不是 Activity 的 Activity 。由于返回的引用为 null，当您尝试访问其成员之一时，getText() 在这种情况下，将抛出 NullPointerException