How to make a edittext box in a dialog

Use Activtiy Context

Replace this

  final EditText input = new EditText(this);

By

  final EditText input = new EditText(MainActivity.this);  
  LinearLayout.LayoutParams lp = new LinearLayout.LayoutParams(
                        LinearLayout.LayoutParams.MATCH_PARENT,
                        LinearLayout.LayoutParams.MATCH_PARENT);
  input.setLayoutParams(lp);
  alertDialog.setView(input); // uncomment this line

I know its too late to answer this question but for others who are searching for some thing similar to this here is a simple code of an alertbox with an edittext

AlertDialog.Builder alert = new AlertDialog.Builder(this); 

or

new AlertDialog.Builder(mContext, R.style.MyCustomDialogTheme);

if you want to change the theme of the dialog.

final EditText edittext = new EditText(ActivityContext);
alert.setMessage("Enter Your Message");
alert.setTitle("Enter Your Title");

alert.setView(edittext);

alert.setPositiveButton("Yes Option", new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog, int whichButton) {
        //What ever you want to do with the value
        Editable YouEditTextValue = edittext.getText();
        //OR
        String YouEditTextValue = edittext.getText().toString();
    }
});

alert.setNegativeButton("No Option", new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog, int whichButton) {
        // what ever you want to do with No option.
    }
});

alert.show();