Null Pointer Exception w kodzie

Question

Witam, ma ktoś pomysł dlaczego wyrzuca mi NPE w funkcji checkAdjacent(), skoro tablica fields jest zainicjalizowana w inicjalizeFields()?

import javax.swing.*;
import java.awt.*;

public class TicTacToe{
    public static void main(String[] args) {
        EventQueue.invokeLater(() -> {
            JFrame frame = new JFrame("Tic Tac Toe");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(new GameLogic());
            frame.pack();
            frame.setVisible(true);
        });
    }
}

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class GameLogic extends JPanel {
    private static final Font buttonFont = new Font("Dialog", Font.BOLD, 80);
    private String player;
    private JButton[] fields;

    public GameLogic() {
        setLayout(new GridLayout(3, 3));
        player = "X";
        initializeFields();
    }

    private void initializeFields() {
        fields = new JButton[9];
        for (JButton field : fields) {
            field = new JButton();
            field.setFont(buttonFont);
            field.addActionListener(new FieldListener());
            add(field);
        }
    }

    private boolean checkGameOver() {
        //check horizontal
        if (checkAdjacent(0, 1, 2))
            return true;
        else if (checkAdjacent(3, 4, 5))
            return true;
        else if (checkAdjacent(6, 7, 8))
            return true;

        //check vertical
        else if (checkAdjacent(0, 3, 6))
            return true;
        else if (checkAdjacent(1, 4, 7))
            return true;
        else if (checkAdjacent(2, 5, 8))
            return true;

        //check diagonal
        else if (checkAdjacent(0, 4, 8))
            return true;
        else if (checkAdjacent(2, 4, 6))
            return true;

        return false;
    }

    private boolean checkAdjacent(int first, int second, int third) {
        return !fields[first].getText().equals("")
                && fields[first].getText().equals(fields[second].getText())
                && fields[first].getText().equals(fields[third].getText());
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(600, 600);
    }

    private class FieldListener implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent event) {
            JButton clickedButton = (JButton) event.getSource();
            if (clickedButton.getText().equals("")) {
                clickedButton.setText(player);

                if (player.equals("X"))
                    player = "O";
                else
                    player = "X";
            }

            if (checkGameOver())
                JOptionPane.showMessageDialog(null, "Winner: " + player);
        }
    }
}

Błąd po kliknięciu na przycisk:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at GameLogic.checkAdjacent(GameLogic.java:54)
	at GameLogic.checkGameOver(GameLogic.java:29)
	at GameLogic.access$200(GameLogic.java:6)
	at GameLogic$FieldListener.actionPerformed(GameLogic.java:77)
	at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
	at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
	at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
	at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
	at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:269)
	at java.desktop/java.awt.Component.processMouseEvent(Component.java:6578)
	at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3343)
	at java.desktop/java.awt.Component.processEvent(Component.java:6343)
	at java.desktop/java.awt.Container.processEvent(Container.java:2259)
	at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:4961)
	at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2317)
	at java.desktop/java.awt.Component.dispatchEvent(Component.java:4793)
	at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4904)
	at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4539)
	at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4480)
	at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2303)
	at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2758)
	at java.desktop/java.awt.Component.dispatchEvent(Component.java:4793)
	at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:766)
	at java.desktop/java.awt.EventQueue.access$500(EventQueue.java:97)
	at java.desktop/java.awt.EventQueue$3.run(EventQueue.java:717)
	at java.desktop/java.awt.EventQueue$3.run(EventQueue.java:711)
	at java.base/java.security.AccessController.doPrivileged(Native Method)
	at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:89)
	at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:99)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:739)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:737)
	at java.base/java.security.AccessController.doPrivileged(Native Method)
	at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:89)
	at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:736)
	at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:199)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

Z góry dzięki za pomoc 

Answer 1

Proponuję takie ciało funkcji:

 

 private void initializeFields() {

        fields = new JButton[9];

        for (int i=0; i<9; i++) {

            fields[i] = new JButton();

            fields[i].setFont(buttonFont);

            fields[i].addActionListener(new FieldListener());

            add(fields[i]);

        }

    }

 

To powinno zadziałać.

Comments

Działa! Dzięki 

Możesz mi jeszcze napisać dlaczego? Pętla for each nie powinna przejść po każdym elemencie fields?

---

Cieszę się, że działa 

Nie jestem w stu procentach pewien, ale wydaje mi się, że field było kopią tablicy fields i działałeś na kopii, a nie na oryginale, dlatego tablica dalej była ustawiona na nulla (domyślnie jest na niego ustawiana).

 

 

Answer 2

https://stackoverflow.com/questions/29543889/how-to-fill-out-an-arraylist-with-for-each-loop-java/29544093

Lepiej (prosciej i bezpieczniej) użyć zwykłej pętli for.

Answer 3

Sprawdż czy to nie jest nullem --> fields[first];

Comments

Sprawdzałem już wcześniej if'em w funkcji checkAdjacent() i wszystkie elementy fields są nullem, a przecież zostały zainicjalizowane w inicjalizeFields().