Android : AlertDialog from another class

In mainActivity I have

public void showDialog(String title, String message, String accept, String denied){
    AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
    builder.setTitle(title);
    builder.setMessage(message);
    builder.setCancelable(true);

    builder.setPositiveButton(accept, new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.dismiss();
        }
    });

    builder.setNegativeButton(denied, new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.dismiss();
        }
    });

    builder.create().show();
}

It works inside main activity, but I am traing call this from another activity, so I wrote

MainActivity o = new MainActivity();
o.showDialog("t", "t", "t", "t");

And app is crashing. I tried also wrote "this", "getAplicationContext()" instead of "MainActivity.this".