CODEWITHSUNDEEP
javaswingjframejpanelstructure
Corey
Corey

Reputation: 152

Java: How to use a cardholder with multiple classes

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.

The main method points to this class

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();  
}

}

This is the board class, one of the JPanels I'm trying to switch between

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;
}
}

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.

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;
}
}

Like I said, I'm getting a null pointer exception. It is occuring on this line of the Skeleton() class

menuSet = menu.getMenuset();

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.

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.

Upvotes: 0

Views: 175

Answers (1)

ajb
ajb

Reputation: 31699

Your 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.

Upvotes: 3

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