Java-CardLayout 如何从正在切换的卡片中切换卡片

Question

我想熟悉 CardLayout，所以我正在制作一个模拟游戏菜单。该菜单应该有三个按钮，但布局部分很简单。

所以，我想做的是用带有三个按钮的菜单启动它。单人游戏按钮应该将用户看到的内容更改为单个按钮，这样可以将其更改回原始菜单。

我按照网上的一个例子，然后应用了相同的方法。然而，菜单本身是一张卡片，更改卡片的命令将来自它，而不是一个单独的容器。

每当我运行这个时，我都会收到错误:

public class GameMenuCards extends JFrame{

private int currentCard = 1;
private JPanel cardPanel;
private CardLayout cl;
private GridBagConstraints gbc;

public GameMenuCards(){
    initUI();
}

public void initUI(){

    //set the properties for the window
    setTitle("Game Menu With Cards");
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setExtendedState(MAXIMIZED_BOTH);

    cardPanel = new JPanel();
    cl = new CardLayout();

    JPanel game = new JPanel();
    game.setBackground(Color.BLACK);

    //the menu panel
    JPanel menu = new JPanel();
    menu.setLayout(new GridBagLayout());
    menu.setBackground(Color.BLACK);

    cardPanel.add(menu, "1");
    cardPanel.add(game, "2");

    //set up the buttons for the menu
    JButton single = new JButton("Single Player");
    single.setPreferredSize(new Dimension(300, 30));
    single.setBackground(Color.GRAY);
    single.setForeground(Color.CYAN);
    single.setBorder(BorderFactory.createLineBorder(Color.CYAN, 3));

    JButton multi = new JButton("Multi Player");
    multi.setPreferredSize(new Dimension(300, 30));
    multi.setBackground(Color.GRAY);
    multi.setForeground(Color.CYAN);
    multi.setBorder(BorderFactory.createLineBorder(Color.CYAN, 3));

    JButton score = new JButton("High Scores");
    score.setPreferredSize(new Dimension(300, 30));
    score.setBackground(Color.GRAY);
    score.setForeground(Color.CYAN);
    score.setBorder(BorderFactory.createLineBorder(Color.CYAN, 3));

    gbc = new GridBagContraints();

    //add everything to the menu
    gbc.insets = new Insets(35, 50, 0, 50);

    gbc.gridwidth = 1;
    gbc.gridheight = 1;
    gbc.gridx = 1;
    gbc.gridy = 1;

    menu.add(single, gbc);

    gbc.gridx = 1;
    gbc.gridy = 2;

    label(menu);

    gbc.gridx = 1;
    gbc.gridy = 3;

    menu.add(multi, gbc);

    gbc.gridx = 1;
    gbc.gridy = 4;

    label(menu);

    gbc.gridx = 1;
    gbc.gridy = 5;
    gbc.insets = new Insets(35, 50, 35, 50);

    menu.add(score, gbc);

    single.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent event){
            currentCard = 2;
            cl.show(cardPanel, "" + (currentCard));
        }
    });

    JButton returnBut = new JButton("Back To Menu");
    returnBut.setPreferredSize(new Dimension(300, 30));
    returnBut.setBackground(Color.GRAY);
    returnBut.setForeground(Color.CYAN);
    returnBut.setBorder(BorderFactory.createLineBorder(Color.CYAN, 3));

    returnBut.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent event){
            currentCard = 1;
            cl.show(cardPanel, "" + (currentCard));
        }
    });

    game.add(returnBut);

    getContentPane().add(cardPanel);

}

public void label(Container c){
    JLabel j1 = new JLabel();
    j1.setPreferredSize(new Dimension(300, 40));
    j1.setBackground(Color.BLACK);
    c.add(j1, gbc);
}

public static void main(String[] args) {

    SwingUtilities.invokeLater(new Runnable(){
        public void run(){
            GameMenuCards gm = new GameMenuCards();
            gm.setVisible(true);
        }
    });

}


}

我知道我可以用标签上的按钮做类似的事情，但我只想到了两个按钮，所以在那个阶段它会花费更长的时间。

我的处理方式正确吗？你能纠正我在代码中犯的错误吗？

Answer 1

Whenever I run this i get an error

您的应用程序在此处抛出 NPE

gbc.insets = new Insets(35, 50, 0, 50);

因为您尚未初始化 GridBagConstraints gbc。

此外，您看到两个面板并排的原因是，即使您创建了 CardLayout，您也忽略了将其用于您的 卡片面板。因此，您仍然使用 JPanel 的默认 FlowLayout。你可以这样做:

cardPanel = new JPanel(cl);