Thread: How to set (minimum) panel size for panel with a Graphics2D object only?

I am writing a program to solve Sudoku puzzles. I create the user interface using AWT/Swing.

One panel of the main window (JFrame object) contains a Graphics2D component only (a grid for the sudoku puzzle). The problem is that this panel is too small for its contents when it is created. I have tried many things to make it bigger (like the setSize method of the panel), but to no avail. Who can tell me how I can make the panel with a single Graphics2D object larger?

Below is the code for the main window, which is a JFrame object. I set its size to 400 x 400 to have ample room for all panels. I then create two panels: a sudoku panel that contains the sudoku grid and a button panel that contains two buttons. The button panel has a flow layout.

I then create the main panel. This has a vertical BoxLayout. Finally I add the sudoku panel and the button panel to the main panel and the main panel to the frame.

public class ViewController_Graphics extends JFrame {
    JButton solveButton = new JButton("Solve");
    JButton nakedSinglesButton = new JButton("Naked Singles");
 
        public ViewController_Graphics(SudokuModel model) {
        	super("Sudoku");
        	setSize(400,400);
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          setLookAndFeel();
          // create sudoku panel
          SudokuPanel sudokuPanel = new SudokuPanel();
          // create button panel
          JPanel buttonPanel = new JPanel();
          buttonPanel.setLayout(new FlowLayout(FlowLayout.LEFT));
          buttonPanel.add(solveButton);
          buttonPanel.add(nakedSinglesButton);
          //create main panel
        	JPanel mainPanel = new JPanel();
        	mainPanel.setLayout(new BoxLayout(mainPanel, BoxLayout.Y_AXIS));
        	mainPanel.add(sudokuPanel);
        	mainPanel.add(buttonPanel);
        	// show frame
        	add(new JScrollPane(mainPanel));
          setVisible(true);
    }
}

Below is the code for the SudokuPanel class.

    class SudokuPanel extends JPanel {
        final int X_OFFSET = 10;
        final int Y_OFFSET = 10;
        final int WIDTH = 30;
        final int HEIGHT  = 30;
        final float LIGHTSTROKE = 1.0f;
        final float BOLDSTROKE = 2.0f;
 
        public void paintComponent(Graphics comp) {
            Graphics2D comp2D = (Graphics2D)comp;
            comp2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            comp2D.setColor(Color.black);
            for (int x = 0; x < 9; x++) {
                for (int y = 0; y < 9; y++) {
                    int xPos = X_OFFSET + (x * WIDTH);
                    int yPos = Y_OFFSET + (y * HEIGHT);
                    comp2D.setStroke(new BasicStroke(LIGHTSTROKE, BasicStroke.CAP_SQUARE, BasicStroke.JOIN_MITER));
                    comp2D.draw(new Rectangle2D.Double(xPos, yPos, WIDTH, HEIGHT));
                    if (((x % 3) == 0) && ((y % 3) == 0)) {
                        comp2D.setStroke(new BasicStroke(BOLDSTROKE, BasicStroke.CAP_SQUARE, BasicStroke.JOIN_MITER));
                        comp2D.draw(new Rectangle2D.Double(xPos, yPos, WIDTH * 3, HEIGHT * 3));
                    }
                }
            }
        }
 
    }

When I run the program, the following Window shows up:



As you can see, the button panel is exactly in the middle (and it remains in the middle, no matter how I resize the window). And there is not enough room for the sudoku panel.

How can I make enough room for the sudoku panel when the window is created?

Thanks a lot! It works!

You could also make your custom Panel overwrite its getPreferredSize() method, or use a layout that manages your components better.

Yes, I read that later. Which is the preferred method, override getPreferredSize() or use setMinimumSize() ?

The preferred approach would be to use an appropriate LayoutManager and/or multiple containers to present the interface you desire.

Its usually a bad habit to set a minimum size. Overwriting getPreferredSize is okay for your custom components but its not always possible, sometimes the preferred size is just the maximum.
As Greg said, a good layout manager would be the better choice.