Question

I’m terribly unfamiliar with swing, but here I’m trying to make a “clacker” game where each time two dice are rolled, either you click the two (or 1 if they’re the same number) buttons representing the two numbers or just one button representing the sum of the two numbers. If a button is clicked, visible is set to false. I have a JButton array to hold the 12 buttons, but every time a reference is made to the array buttons, it throws a NullPointerException. I’ve tried putting the code making the JButton array within the constructor and actionPerformed, but it still throws the same exception.

Here is the code:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Random;

public class Clacker implements ActionListener
{
    JPanel panel;
    JFrame frame;
    JLabel dieFace1, dieFace2, numTurns;
    JButton[] buttons;
    JButton rollDie, reset;
    String turn="Turns: ";
    int turns, numCovered;

public Clacker()
{
    frame = new JFrame("clacker");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    panel=new JPanel();
    panel.setBorder(BorderFactory.createEmptyBorder(50,30,50,30));
    panel.setBackground(Color.gray);
    panel.setLayout(new GridLayout(5,10,1,1));

    dieFace1=new JLabel(new ImageIcon("die3.jpg"));
    dieFace1.setAlignmentX(JLabel.RIGHT_ALIGNMENT);
    dieFace1.setBorder(BorderFactory.createEmptyBorder(0,0,10,0));
    panel.add(dieFace1);

    dieFace2=new JLabel(new ImageIcon("die3.jpg"));
    dieFace2.setAlignmentX(JLabel.RIGHT_ALIGNMENT);
    dieFace2.setBorder(BorderFactory.createEmptyBorder(0,0,10,0));
    panel.add(dieFace2);

    rollDie=new JButton("Roll Die");
    rollDie.setAlignmentX(JButton.RIGHT_ALIGNMENT);
    rollDie.addActionListener(this);
    panel.add(rollDie);

    numTurns=new JLabel(turn+turns);
    numTurns.setAlignmentX(JLabel.LEFT_ALIGNMENT);
    numTurns.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));
    panel.add(numTurns);

    setArray();

    reset=new JButton("Reset");
    reset.setAlignmentX(JButton.LEFT_ALIGNMENT);
    reset.addActionListener(this);
    panel.add(reset);

    frame.setContentPane(panel);

    frame.pack();
    frame.setVisible(true);
}

public void setArray()
{
    JButton[] buttons = new JButton[12];
    for (int i=0; i<12; i++)
    {

        buttons[i]= new JButton(i+1+"");
        buttons[i].setActionCommand(i+"");
        buttons[i].addActionListener(this);
        panel.add(buttons[i]);
        buttons[i].setEnabled(false);
        System.out.println(buttons[i]); 
    }
}

public void actionPerformed(ActionEvent event)
{
    if (event.getActionCommand().equals("Roll Die"))
    {   
        Random asdf = new Random();
        int newRoll= asdf.nextInt(6)+1;
        dieFace1.setIcon(new ImageIcon("die"+newRoll+".jpg"));

        Random asdf2 = new Random();
        int newRoll2= asdf2.nextInt(6)+1;
        dieFace2.setIcon(new ImageIcon("die"+newRoll2+".jpg"));

        int sum=newRoll+newRoll2;
        turns++;
        numTurns.setText(turn+turns);

        for (int i=0; i<7; i++)                                    ////here                          
        {
            if (i==newRoll)
                buttons[i].setEnabled(true);
            if (i==newRoll2)
                buttons[i].setEnabled(true);
        }

        for (int i=7; i<13; i++)
        {
            if (sum==i)
                buttons[i].setEnabled(true);
        }

        String eventName = event.getActionCommand();
        for (int b=1; b<13; b++)
        {
        if (eventName.equals(buttons[b].getActionCommand()))       ////here            
            {
                if (Integer.parseInt(eventName)>6)
                {
                    buttons[b].setText("");
                    numCovered++;
                    buttons[b].setVisible(false);
                    for (int i=0; i<7; i++)
                        buttons[i].setEnabled(false);
                }
            }   
        }

        for (int i=0; i<buttons.length; i++)                       ////here                            
            buttons[i].setEnabled(false);

        if (numCovered==12)
        {
            rollDie.setEnabled(false);
            numTurns.setText("Win! "+turns+" turns");
        }

    }

    else if (event.getActionCommand().equals("Reset"))
    {
        turns=0;
        numTurns.setText(turn+turns);
        rollDie.setEnabled(true);
        numCovered=0;
        for (int i=0; i<buttons.length; i++)                       ////here                             
        {
            buttons[i].setVisible(true);
            buttons[i].setEnabled(false);
        }
    }
}

private static void runGUI()
{ JFrame.setDefaultLookAndFeelDecorated(true); Clacker game = new Clacker();}

public static void main(String[] args)
{ javax.swing.SwingUtilities.invokeLater(new Runnable() {public void run() { runGUI();}});}
}

Answer 1

In setArray you’re declaring a local variable instead of referencing the class member variable. Just change

JButton[] buttons = new JButton[12];

to

this.buttons = new JButton[12];