該方法就是設定圖示方法 setIconImage()

public class Login extends JFrame{
    private JLabel nameLabel = new JLabel("登入名:");
    private JLabel passwordLabel = new JLabel("密碼:");
    private JTextField nameTextField = new JTextField();
    private JPasswordField passwordPasswordField = new JPasswordField();
    private JButton btnLogin = new JButton("登入");
    private JButton btnCancle = new JButton("取消");

    private Master master = new Master();

    public Login(){
        this.setTitle("寵物管理系統");
        this.setSize(500,500);
        this.setLocationRelativeTo(null);
        this.setVisible(true);
        this.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        this.getContentPane().setBackground(new Color(255, 255, 255));
        // 這一條就是
        this.setIconImage(new ImageIcon("src/cn/jbit/epet/image/dog.jpg").getImage());
        this.
        init();

    }