java - Java JFrame Exception in thread "AWT-EventQueue-0" java.util.EmptyStackException even though stack is initialize first

Question

I am creating a number puzzle game in 3 by 3 grid layout panel. First i random 8 numbers and push them to a stack and then pop their value and set them as button text. The application runs fine but sometimes i have this error printed out: Exception in thread "AWT-EventQueue-0" java.util.EmptyStackException at java.util.Stack.peek(Stack.java:102) at java.util.Stack.pop(Stack.java:84)

and it points to this line of code: buttons[i].setText(randomStack.pop() + "");

I know that it is something with the multi-threading but i can not figure out, please help me.

    public Frame() {

        JPanel panel = new JPanel();
        Stack<Integer> randomStack = new Stack();

        while(randomStack.size() < 8) {
            int n = new Random().nextInt(8) + 1;

            if(!randomStack.contains(n)) {
                randomStack.push(n);
            }
        }


        panel.setLayout(new GridLayout(3, 3));
        panel.setPreferredSize(new Dimension(200 ,200));
        buttons = new JButton[9];

        for(int i = 0; i < 9; i++) {
            buttons[i] = new JButton();
            buttons[i].setSize(30, 30);
            panel.add(buttons[i]);

        }

        Random random = new Random();
        int n = random.nextInt(10);
        for(int i = 0; i < 9; i++) {
            if(i != n) {
               buttons[i].setText(randomStack.pop() + "");
            }
        }

        add(panel, BorderLayout.NORTH);

    }

    public static void main(String args[]) {

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Frame().setVisible(true);
        }
    });
}
}

Ivan · Accepted Answer · 2019-05-15 14:29:10Z

2

You put 8 elements and you try to get 9 elements from stack. This is why you get an exception. This has nothing to do with multithreading since your code doesn't start any additional threads. And this code

Random random = new Random();
int n = random.nextInt(10);

might result in n getting value 9 and in this case your for-loop will iterate 9 times

edited May 15, 2019 at 14:29

answered May 15, 2019 at 14:25

Ivan

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3 Comments

Thuan Nguyen Over a year ago

no i only get 8 elements out. I random a number that i will not pop the stack to that buttons array index.

Ivan Over a year ago

for(int i = 0; i < 9; i++) this will provide 9 iterations

Thuan Nguyen Over a year ago

oh i get it. It may random number 9 and my array's max index is 8 so it will try to pop 9 elements out. Thank you very much.

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