Java- CardLayout如何从正在切换的卡中切换卡片

Question

我想熟悉CardLayout，因此我正在制作一个模拟游戏菜单。这个菜单应该有三个按钮，但是布局部分很简单。

所以，我想要做的就是用一个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，这可以将其更改回原始菜单。

我在网上跟着一个例子，然后对此应用了相同的方法。但是，菜单本身就是一张卡片，它是更换卡片的命令来自哪里，而不是一个单独的容器。

每当我运行此我得到一个错误：

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，我可以做类似的事情与按钮的标签，但我只用了思想两个按钮，所以在那个阶段它会花更长的时间。

我对此有何看法？你能纠正我在代码中犯的任何错误吗？

Answer 1

Whenever I run this i get an error

您的应用程序在这里

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

抛出NPE因为你尚未初始化GridBagConstraintsgbc。

而且，你看到两个面板并排侧的是，即使你创造了一个CardLayout的原因，您忽略使用你cardPanel。因此，您仍在使用JPanel的默认FlowLayout。你可以这样做：

cardPanel = new JPanel(cl);