java - JButton 监听器调用类中的方法给出 NullPointerException

Question

这里是java新手，我正在尝试使用GUI进行我的第一个项目。我有一个包含 gamePlay() 和 userTurn() 的 GameConsole 类。我有一个 ButtonListener 类，它使用调用 userTurn() 的 actionListener 构造按钮。但是，每次按下按钮时，我都会收到 NullPointerException。为什么会发生这种情况以及我该如何解决它？

相关代码:

public static final void main (String[] args){
    GameConsole game = new GameConsole();
    game.gamePlay();
}

public class GameConsole {
    public Player user;
    public Player dealer;
    public Deck playingDeck;
    ValidateInput validate = new ValidateInput();

public void gamePlay(){
    //get info from user
    user.addToHand(playingDeck.getTopCard());
    dealer.addToHand(playingDeck.getTopCard());
    user.addToHand(playingDeck.getTopCard());
    dealer.addToHand(playingDeck.getTopCard());

    userTurn();
}

public void userTurn(){
    boolean turn = true;
    do{   //the code breaks at this point. On the first round of gameplay I get the exception pasted
          //below. The second round (after the button is pressed) I get an 
          //Exception in thread "AWT-EventQueue-0      bringing me back here
        JOptionPane.showMessageDialog(null, "The cards you were dealt are: \n" + user.printHand());

        if(user.sumOfHand() == 21){  //no need to continue if user has 21
            System.out.println("You win this round.");
            break;
        } else if (user.sumOfHand() > 21){
            System.out.println("You have busted. You lose this round.");
            break;
        }


        String HSInput = null;
        for(int x = 0; x < 1;){
        HSInput = JOptionPane.showInputDialog("\nThe dealer is showing a " + dealer.getTopCard()
                + "\nYou are currently at " + user.sumOfHand()
                + "\n\nWhat would you like to do?\nHit (H) or Stay (S)?");
        if(validate.containDesiredString(HSInput, "HhSs")) //only accept h and s either caps
            x++;
        }


        if(HSInput.equalsIgnoreCase("H")){  //if the user wants to stay
            user.addToHand(playingDeck.getTopCard()); //deal new card then repeat loop
        } 
        else {
            turn = false;
            }
    } while (turn == true);
}

还有 ButtonListener 类...

public class ButtonListener implements ActionListener {

    JButton newRoundButton;

    public ButtonListener(JPanel endGamePanel){

        newRoundButton = new JButton("Start another round");
        newRoundButton.setPreferredSize(new Dimension(150, 50));
        endGamePanel.add(newRoundButton);
        newRoundButton.addActionListener(this);

    }


    @Override
    public void actionPerformed(ActionEvent e) {        
        if(e.getSource() == newRoundButton){
            System.out.println("Hi there for a new round");
                     //I know that this method is entered into because this line will print
            GameConsole game = new GameConsole();
            game.userTurn();
        }
    }

我非常迷失方向，在过去的两天里一直在原地踏步。现在程序甚至不会进入 userTurn()。我不知道我做了什么，因为几个小时前这还不是问题。不管怎样，我的最终问题是我无法让 ButtonListener 调用 userTurn() 类。为什么我现在会收到 userTurn() 的 NullPointerException 异常？我该怎么做才能让 ActionListener 调用 userTurn() 而不给出 NullPointerException？

-绝望地迷失......感谢任何帮助!

编辑:堆栈跟踪线程“main”jav 中出现异常

a.lang.NullPointerException
    at blackjackControls.GameConsole.userTurn(GameConsole.java:180)
    at blackjackControls.GameConsole.gamePlay(GameConsole.java:119)
    at blackjackControls.Main.main(Main.java:7)

因为它不再进入 userTurn() 我无法发布跟踪。它确实声明它是 AWT 中的一个线程

ButtonListener 类已与现有 ResultPane 类合并。完整代码为:

public class ResultPanel extends JFrame {

    JFrame frame = new JFrame("Blackjack Game");
    JPanel endGamePanel = new JPanel();
    ImageIcon image;
    JLabel lbl;
    JButton newRoundButton;
    JButton endButton;

    public ResultPanel(){

        frame.setSize(800, 600);
        frame.getContentPane().add(endGamePanel);   //add the panel to the frame
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);       

        JButton newRoundButton= new JButton("Start another round");
        newRoundButton.setPreferredSize(new Dimension(150, 50));
//      newRoundButton.addActionListener();
        newRoundButton.addActionListener(new ActionListener(){

            @Override
            public void actionPerformed(ActionEvent e) {

                System.out.println("Hi there for a new round");
                GameConsole game = new GameConsole();
                game.userTurn();
            }
        });


        frame.setVisible(true);
    }

    public void winGame(){
        image = new ImageIcon(getClass().getResource("win.jpg"));
        lbl = new JLabel (image);
        endGamePanel.add(lbl);

        frame.setVisible(true);
    }

Answer 1

据我所知，您在主程序中创建了一个新的 GameConsole 对象，因此我假设这是整个游戏运行的对象。如果是这种情况，为什么您的按钮会创建一个新的 GameConsole 对象？我假设您收到 NullPointerException ，因为您的按钮在没有 User 的新 GameConsole 对象上调用 userTurn() 方法尚未对象，因此 user.sumOfHand() 例如会抛出异常。

您的按钮应该调用您在 main 中创建的 GameConsole 对象上的 userTurn 方法，而不是在新对象上调用。

代码建议:

JButton newRoundButton= new JButton("Start another round");
newRoundButton.setPreferedSize(new Dimension(150, 50));
newRoundButton.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("Hi there for a new round");
        game.userTurn();
    }
});
endGamePanel.add(newRoundButton);

这是假设构建您的面板的任何类都知道您的 GameConsole 对象。如果此类是 GameConsole 对象本身，只需删除 game。 如果在您的 GameConsole 对象中创建了该类的对象，只需传递您的GameConsole 对象指向该类，并在该类构造函数中使用参数 GameConsole game ，如果需要，请将其作为该类中的字段。要从该对象传递您的 GameConsole 对象，请传递 this。