Java- CardLayout如何從正在切換的卡中切換卡片

Question

我想熟悉CardLayout，因此我正在製作一個模擬遊戲菜單。這個菜單應該有三個按鈕，但是佈局部分很簡單。

所以，我想要做的就是用一個帶有三個按鈕的菜單啓動它。單人遊戲按鈕應該將用戶看到的內容更改爲單個按鈕，這可以將其更改回原始菜單。

我在網上跟着一個例子，然後對此應用了相同的方法。但是，菜單本身就是一張卡片，它是更換卡片的命令來自哪裏，而不是一個單獨的容器。

每當我運行此我得到一個錯誤：

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，我可以做類似的事情與按鈕的標籤，但我只用了思想兩個按鈕，所以在那個階段它會花更長的時間。

我對此有何看法？你能糾正我在代碼中犯的任何錯誤嗎？

Answer 1

Whenever I run this i get an error

您的應用程序在這裏

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

拋出NPE因爲你尚未初始化GridBagConstraintsgbc。

而且，你看到兩個面板並排側的是，即使你創造了一個CardLayout的原因，您忽略使用你cardPanel。因此，您仍在使用JPanel的默認FlowLayout。你可以這樣做：

cardPanel = new JPanel(cl);