This transition from class-based design to message-based design is a turning point in your design career. The message-based perspective yields more flexible applications than does the class-based perspective. Changing the fundamental design question from “I know I need this class, what should it do?” to “I need to send this message, who should respond to it?” is the first step in that direction. You don’t send messages because you have objects, you have objects because you send messages.
