DialogFragment argument and nullpointer exception

问题:

My class should pass an argument to DialogFragment, but my app crashes inside onCreate method (of dialog class) for a NullPointerException. Dialog fragment class portion of code:

public class ConfirmDialog extends DialogFragment {

public ConfirmDialog() {}

 ConfirmDialog newInstance(String f) {
    ConfirmDialog d = new ConfirmDialog();

    Bundle args = new Bundle();
    args.putString("FILE_NAME", f);
    d.setArguments(args);

    return d;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    file = getArguments().getString("FILE_NAME");
}

I have the nullpointer at this line:

file = getArguments().getString("FILE_NAME");

And i don't know why. I paste also the code calls the dialog

private void showConfirmDialog(String file) {
    FragmentManager fm = getSupportFragmentManager();
    ConfirmDialog dialog = new ConfirmDialog();
    Log.i("SHOWFILEACTIVITY", file);
    dialog.newInstance(file);
    dialog.show(fm, "fragment_confirm_dialog");
}

Here the "file" string is not null, i've check it with

Log.i("SHOWFILEACTIVITY", file);

回答1:

You're creating a ConfirmDialog via the constructor, then calling newInstance(), which creates another (proper) ConfirmDialog. However you then discard that proper instance.

To fix this:

Your newInstance() method should be static:

public static ConfirmDialog newInstance(String f) {
    ConfirmDialog d = new ConfirmDialog();

    Bundle args = new Bundle();
    args.putString("FILE_NAME", f);
    d.setArguments(args);

    return d;
}

And showConfirmDialog() should be changed so it uses the newInstance() method properly.

private void showConfirmDialog(String file) {
    FragmentManager fm = getSupportFragmentManager();
    Log.i("SHOWFILEACTIVITY", file);

    ConfirmDialog dialog = ConfirmDialog.newInstance(file);
    dialog.show(fm, "fragment_confirm_dialog");
}