It obviously has no idea what you mean by 'textboxB' or 'textboxA' (or 'convertString' or 'codeKey'). Maybe you should show the code of the 'Cipher' class as well as the code of the class that the line you gave is in.

import greenfoot.*; /** * a world to demonstrate Ceasar Ciphering */ public class CipherWorld extends World { private Textbox textboxA, textboxB; // the text boxes private int codeKey; // the cipher key /** prepares world */ public CipherWorld() { super(380, 500, 1); addObject(textboxA = new Textbox(), getWidth()/2, 40); addObject(textboxB = new Textbox(), getWidth()/2, 80); } /** processes keyboard input and maintains string values of textboxes */ public void act() { String key = Greenfoot.getKey(); if (key != null) { if ("space".equals(key)) key = " "; if ("backspace".equals(key) && textboxA.getText().length() > 0) { String text = textboxA.getText(); text = text.substring(0, text.length()-1); textboxA.setText(text); } if ("escape".equals(key)) textboxA.setText(""); if ("up".equals(key)) codeKey = (codeKey+1)%26; if ("down".equals(key)) codeKey = (codeKey+25)%26; if (key.length() == 1) textboxA.setText(textboxA.getText()+key); }

No -- as you only gave a condensed version of the error message you received. (for one thing)