Imagine writing a secret note to a friend. If anyone reads it on the way, the secret is out. To protect it, you convert it into a scrambled, unreadable message (Ciphertext) using a secret code (Key).

src/ ├── Main.java # 程序入口 ├── model/ │ └── Contact.java # 联系人 JavaBean ├── components/ │ ├── ButtonAction.java # 接口：按钮行为 onPress() │ ├── PhoneButton.java # 抽象类 extends JButton（按钮基类） │ ├── NumberButton ...