界面切换就是scene的改变，程序运行启动类会初始化一个stage，它会在程序的整个生命周期存在，scene是场景信息，界面的元素通过场景来呈现。然后把scene交给stage显示出来。

1. 封装界面地址枚举类

@AllArgsConstructor
@Getter
public enum PageIndexEnum {

    LOGIN_PAGE("登录页", "login.fxml", 500, 300),
    MAIN_PAGE("主界面", "main.fxml", 1000, 700);

    private String pageName;

    private String pageFile;

    private Integer width;

    private Integer height;
}

2. 界面切换工具类

public class StageUtil {

    private static Stage primaryStage;

    public static void set(Stage state) {
        primaryStage = state;
    }

    public static Stage get() {
        return primaryStage;
    }

    public static void forward(PageIndexEnum pageIndexEnum) {
        try {
            Parent mainPage = FXMLLoader.load(Objects.requireNonNull(ClassLoader.getSystemClassLoader().getResource(pageIndexEnum.getPageFile())));
            Stage primaryStage = StageUtil.get();
            primaryStage.getIcons().add(new Image("https://img1.baidu.com/it/u=1855883715,2143013249&fm=253&fmt=auto&app=138&f=PNG?w=278&h=277"));
            primaryStage.setScene(new Scene(mainPage, pageIndexEnum.getWidth(), pageIndexEnum.getHeight()));
        } catch (Exception e) {}
    }
}

3. 启动类

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        PageIndexEnum page = PageIndexEnum.MAIN_PAGE;
        Parent root = FXMLLoader.load(Objects.requireNonNull(getClass().getClassLoader().getResource(page.getPageFile())));
        primaryStage.setTitle("狗语IM聊天系统");
        primaryStage.getIcons().add(new Image("https://img1.baidu.com/it/u=1855883715,2143013249&fm=253&fmt=auto&app=138&f=PNG?w=278&h=277"));
        primaryStage.setScene(new Scene(root, page.getWidth(), page.getHeight()));
        primaryStage.show();
        StageUtil.set(primaryStage);
    }
    public static void main(String[] args) {
        launch(args);
    }
}