Java – restrict digital input jtextfield

I'm working on a Sudoku solver. I'm trying to limit the number of cells users can enter and can only enter numbers. At present, it also accepts letters It works as follows: if the user enters "11125455876", it will only use the last number entered (in this case, "6") I want to limit the number of individual cells that users can enter to 1 and accept only the numbers 1-9 I'm not sure what I should do So maybe it just overwrites the previous numbers or something. If I press a letter key as "a", nothing will happen

This is my code:

public PanelGUI(int size) {

    super(size);
    NumberFormat f = NumberFormat.getInstance();
    f.setMaximumIntegerDigits(1);
    f.setMinimumIntegerDigits(0);
    f.setGroupingUsed(false);
    cells = new JTextField[size][size];
    panel.setLayout(new GridLayout(size,size));
    for (int i = 0; i < size; i++) {
        for (int j = 0; j < size; j++) {
            cells[i][j] = new jformattedtextfield(f);
            cells[i][j].setEditable(true);
            cells[i][j].setFont(boldFont);
            cells[i][j].setBorder(BorderFactory.createMatteBorder(1,1,Color.white));
            cells[i][j].setHorizontalAlignment(JTextField.CENTER);
            panel.add(cells[i][j]);
        }
    }
}

As you can see, I am currently using digital format

Solution

NumberFormat f = NumberFormat.getInstance();

NumberFormat f = NumberFormat.getInstance();
f.setMaximumIntegerDigits(1);
f.setMinimumIntegerDigits(0);
f.setGroupingUsed(false);

You want to use maskformat instread:

MaskFormatter f = new MaskFormatter( "#" );
f.setValidCharacters("123456789");