Why is the swing gui behaving strange?

I just started making my Java application on Minesweeper. The idea is a GridLayout JPanel inside a JFrame. But after executing the program I get some weird window. There is this strange gray corner on the upper left corner. And the tiles don’t all show. Somehow hovering the mouse over them shows.

JFrame frame = new JFrame("MineSweeper");
        JPanel panel = new JPanel();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setPreferredSize(new Dimension(600, 540));
        frame.add(panel);
        panel.setPreferredSize(new Dimension(540, 540));
        panel.setLayout(new GridLayout(numRows, numCols));
        
        for(int y=0; y<numRows; y++) {
            for(int x=0; x<numCols; x++) {
                Tile t = new Tile(x, y);        
                panel.add(t);
                field[y][x] = t;
                
            }
        }
        
        frame.pack();
        panel.setVisible(true);
        frame.setVisible(true);

The program produced this ugly abomination.

How to make the grid all show at once? And how to remove that gray spot on upper left corner.

Note: I just started Swing GUI so I know practically nothing. Try not to make it to complicated.

Answer

I have tried to rewrite this layout code, not complicated at all 🙂

public class MineSweeper extends JFrame {

    public static void main(String[] args) {
        int columns = 10, rows = 10;
        int cellSize = 50;

        JPanel board = new JPanel();
        board.setLayout(new GridLayout(rows, columns));
        for (int i = 0; i < columns; i++) {
            for (int j = 0; j < rows; j++) {
                board.add(new Button("x"));
            }
        }

        MineSweeper mineSweeper = new MineSweeper();
        mineSweeper.setDefaultCloseOperation(EXIT_ON_CLOSE);
        mineSweeper.setContentPane(board);
        mineSweeper.setSize(cellSize * columns, cellSize * rows);
        mineSweeper.setResizable(false);
        mineSweeper.setVisible(true);
    }
}

So I think may be your problem is about setPreferredSize() call, or your Tile class.