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POJava: How to use a cardholder with multiple classes
 

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  1. POJava: How to use a cardholder with multiple classes
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    <p>I'm programming in Java, trying to use a cardholder in order to switch between 2 JPanels which are each an extension of their own class. I think I understand the basic concepts but I am having errors in my current revision, when calling the classes. I'm getting a null pointer exception and I think it's a structural problem but I'm not sure how or why.</p> <p>The main method points to this class</p> <pre><code>public class Skeleton implements ActionListener{ JPanel cardHolder; CardLayout cards; String cardA = "A"; String cardB = "B"; JPanel Jboard; JPanel Jmenu; JFrame frame2; Board board; Menu menu; boolean menuSet; boolean boardSet; Timer timer; public class Switcher implements ActionListener{ String card; Switcher(String card){ this.card = card; } @Override public void actionPerformed(ActionEvent e) { cards.show(cardHolder, card); } } public Skeleton(JFrame frame){ JPanel menu = new Menu(); JPanel board = new Board(); JFrame frame2 = frame; timer = new Timer(5, this); timer.start(); cardHolder = new JPanel(); cards = new CardLayout(); cardHolder.setLayout(cards); cardHolder.add(menu, cardA); cardHolder.add(board, cardB); frame2.add(cardHolder); frame2.revalidate(); frame2.setVisible(true); } public JFrame getSkeleton(){ return frame2; } public JPanel getCardHolder(){ return cardHolder; } public void checkStatus(){ if (menuSet == true){ new Switcher(cardB); boardSet = false; } if (boardSet == true){ new Switcher(cardA); menuSet = false; } } @Override public void actionPerformed(ActionEvent e) { menuSet = menu.getMenuset(); boardSet = board.getBoardset(); checkStatus(); } } </code></pre> <p>This is the board class, one of the JPanels I'm trying to switch between</p> <pre><code>public class Board extends JPanel{ boolean boardset; Menu menu = new Menu(); public Board(){ setBackground(Color.WHITE); } public JPanel getPanel(){ return this; } public void setBoardset(boolean x){ boardset = x; } public boolean getBoardset(){ return boardset; } } </code></pre> <p>Here is the other JPanel class, which contains a button used to switch to the other JPanel class. This is also the original starting JPanel used.</p> <pre><code>public class Menu extends JPanel implements ActionListener{ boolean menuset; public Menu(){ setBackground(Color.BLACK); JButton button = new JButton("hello"); button.addActionListener(this); this.add(button); } public JPanel getPanel(){ return this; } @Override public void actionPerformed(ActionEvent e) { menuset = true; } public void setMenuset(boolean x){ menuset = x; } public boolean getMenuset(){ return menuset; } } </code></pre> <p>Like I said, I'm getting a null pointer exception. It is occuring on this line of the Skeleton() class</p> <pre><code>menuSet = menu.getMenuset(); </code></pre> <p>The line above is right after the actionPerformed event above (from the timer), and I have tested the timer a little, it works doing basic print statements but whenever I try to use the 'menu' or 'board' instance inside the actionPerformed, I get this null pointer exception.</p> <p>I would appreciate any advice. I get the idea that the way I'm doing this may be a little convoluted. If anyone has any suggestions on a better way to do this it would also be helpful. My main goal is to be able to call 2 separate classes from one main class containing a cardholder. That way I can separate the code in order to keep everything isolated and in order.</p>
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    1. USmKorbel
    1. USCorey Smith
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    1. COYour Skeleton class has a "menu" member but it isn't set anywhere that I can see. The constructor declares its own "menu" local variable, which is local to the constructor and hides the member. Setting "menu" inside the constructor won't set the member. I don't see anywhere else where the "menu" member is set, unless I've missed something or unless another class in the same package is setting it.
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      1. POJava: How to use a cardholder with multiple classes
      1. USajb
    2. COThanks, looks like I needed to initialize it above.
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    3. CO@ajb If you post that as an answer and notify me, I'll upvote it.
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      1. POJava: How to use a cardholder with multiple classes
      1. UStbodt
 

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