Why new Alertdialog.Builder(getApplicationContext()). create() does not work?

Question

I have been observing this problem for some time, and I always have to maintain an instance to Activity current, rather than maintaining only one instance to Context, and until then did not understand why, since the method AlertDialog.Builder(Context context), requires only one instance of Context and not of Activity.

So if I make the following code:

AlertDialog alert = new AlertDialog.Builder(getApplicationContext()).create();
alert.setTitle("Title");
alert.setMessage("Message");
alert.show();

The following error running is generated:

04-22 13:06:55.585: E/AndroidRuntime(3014): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.appteste/com.example.appteste.MainActivity}: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application

Already if I pass the Activity as a parameter instead of Context, everything works as expected:

AlertDialog alert = new AlertDialog.Builder(this).create();
alert.setTitle("Title");
alert.setMessage("Message");
alert.show();

I tested in the following versions of Android: 4.0.3 and 4.4.2

My doubts are as follows:

1. This is a problem (Bug) of Android?
2. I’m doing something wrong?
3. Why does this happen?

Obs: For testing purposes, the codes presented were executed within the method onCreate(Bundle savedInstanceState) of Activity.

Answer 1

1 - This is a problem (Bug) of Android?

Apparently not: getApplicationContext() should be used if you need a context whose life cycle is separated from the current context , which is linked to the process life time rather than the current component. More details in the documentation.

2 - I’m doing something wrong?

Probably yes: So how are you using one Activity and creating their AlertDialog within it, the context of that Alert will be your Activity. Based on this answer.

3 - Why does this happen?

The reasoning is that the AlertDialog doesn’t have a life outside of his Activity.

So it needs the context in which it was created, like the getApplicationContext() is different from this from your charity you will have this Exception.

Answer 2

As far as I know, there is no getApplicationContext() or shall not apply to AlertDialog.Builder. Try to use this minimized code made by me:

AlertDialog.Builder(this)
    .create()
    .setTitle("titulo")
    .setMessage("msg")
    .setPositiveButton("funcao 1",
                       new DialogInterface.OnClickListener()
       {
          @Override public void onClick(DialogInterface _dialog, int _which)
          {
             //codigo
          }
       })
    .show();

Answer 3

This is an example of a Alertdialog.

AlertDialog.Builder ADBuilder = new AlertDialog.Builder(MainActivity.this);
ADBuilder.setIcon(R.drawable.ic_dialog_info);
ADBuilder.setTitle("TÍTULO DO ALERTA");
ADBuilder.setMessage("MENSAGEM DO ALERTA");
ADBuilder.setPositiveButton(android.R.string.yes, new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog, int which) {
        // CÓDIGO QUE SERÁ EXECUTADO SE O USUÁRIO PRESSIONAR O BOTÃO SIM
    }
});
ADBuilder.setNegativeButton(android.R.string.no, new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog, int which) {
        // CÓDIGO QUE SERÁ EXECUTADO SE O USUÁRIO PRESSIONAR O BOTÃO NÃO
    }
});
ADBuilder.show();