Basic exception handling is required. It's your responsibility to ensure that any thrown exceptions are handled gracefully and do not crash the program. Any program you write should be thoroughly tested to the best of your abilities. Common exceptions include but may not be limited to: Pressing backspace on an empty text box Dividing and modding by 0 Evaluating an empty textbox Evaluating an invalid expression (5=, 5 =, 5 2=, 5..0 2=, 5 =, etc