how to get colored line to differentiate text field array in swing

I have developed one frame on which I used GridBagLayout to arrange textfields of 12X12. i.e., total 144 textfields on frame. Now I want to differentiate these text fields with colored line after each 3 columns and three rows as shown in the following diagram. I tried in many ways, but I couldn't find the solution. Please suggest. Below is the some part of my code. Thanks in advance.

1 2 3 | 4 5 6 | 7 8 9 | 10 11 12
      |       |       |
1 2 3 | 4 5 6 | 7 8 9 | 10 11 12 
      |       |       |
1 2 3 | 4 5 6 | 7 8 9 | 10 11 12
-------------------------------    
1 2 3 | 4 5 6 | 7 8 9 | 10 11 12
      |       |       |
1 2 3 | 4 5 6 | 7 8 9 | 10 11 12
      |       |       |
1 2 3 | 4 5 6 | 7 8 9 | 10 11 12
--------------------------------    |
1 2 3 | 4 5 6................
. .
. .
.

Please consider the each number as one textfield in diagram.

JTextField jt[][]=new JTextField[12][12];


for(int i=0;i<jt.length;i++)
        {
            for(int j=0;j<jt.length;j++)
            {

                jt[i][j] = new JTextField(1);


                constraints.gridx=j;
                consraints.gridy=i;
                gridbag.setConstraints(jt[i][j],cons);
                c.add(jt[i][j]);
                                jt[i][j].setHorizontalAlignment(JTextField.CENTER);
                jt[i][j].setFont(new Font("TimesNewRoman",Font.BOLD,14));
                jt[i][j].setDocument(new JTextFieldLimit(2));
            }
        }

Solution

You could us a JSeparator or, break each group of 3x3 fields into there own separate panes and use a LineBorder.

So long as you've setup you fields properly, you should be able to get the compound panels/LineBorder to work

UPDATE

Sorry, it should have been MatteBorder :P

Matte Border

GridBagConstraints gbc = new GridBagConstraints();
gbc.fill = GridBagConstraints.BOTH;
gbc.gridx = 0;
gbc.gridy = 0;

for (int row = 0; row < 4; row++) {

    gbc.gridx = 0;

    add(buildGroup(0, 0, 1, 1), gbc);
    gbc.gridx++;
    add(buildGroup(0, 0, 1, 1), gbc);
    gbc.gridx++;
    add(buildGroup(0, 0, 1, 1), gbc);
    gbc.gridx++;
    add(buildGroup(0, 0, 1, 0), gbc);
    gbc.gridy++;

}

public JPanel buildGroup(int top, int left, int bottom, int right) {

    JPanel panel = new JPanel(new GridBagLayout());
    panel.setBorder(new MatteBorder(top, left, bottom, right, Color.RED));
    GridBagConstraints gbc = new GridBagConstraints();
    gbc.insets = new Insets(2, 2, 2, 2);
    gbc.fill = GridBagConstraints.HORIZONTAL;
    for (int row = 0; row < 3; row++) {
        for (int col = 0; col < 3; col++) {

            JTextField field = new JTextField(8);
            gbc.gridx = col;
            gbc.gridy = row;

            panel.add(field, gbc);

        }
    }

    return panel;

}

Now, obviously, you need to figure out how you'd seed your fields, but basically, I'd just pass in the fields you want to be used (such as 2D array for example).

Or with separators :P

Separators

for (int row = 0; row < 9; row++) {

    gbc.gridwidth = 1;
    gbc.weightx = 0;

    int verSplit = 0;

    for (int col = 0; col < 12; col++) {

        gbc.gridx++;

        add(new JTextField(8), gbc);

        verSplit++;
        if (verSplit == 3) {

            verSplit = 0;

            gbc.gridx++;

            if (horSplit % 3 == 0) {

            gbc.gridheight = 3;
            gbc.fill = GridBagConstraints.VERTICAL;
            add(new JSeparator(JSeparator.VERTICAL), gbc);

            gbc.fill = GridBagConstraints.HORIZONTAL;
            gbc.gridheight = 1;

            }

        }

    }

    horSplit++;

    gbc.gridx = 0;

    if (horSplit == 3) {

        horSplit = 0;
        gbc.gridy++;
        gbc.gridwidth = GridBagConstraints.REMAINDER;
        gbc.weightx = 1;
        add(new JSeparator(JSeparator.HORIZONTAL), gbc);

    }

    gbc.gridy++;

}

Or variations on the same theme

UPDATED with field management

// Build the array of fields, I used a col by row matrix
JTextField fields[][] = new JTextField[12][12];
for (int col = 0; col < 12; col++) {

    for (int row = 0; row < 12; row++) {

        fields[col][row] = new JTextField(col + "x" + row, 8);

    }

}

// Build the groups...
for (int row = 0; row < 12; row++) {

    gbc.gridx = 0;

    int col = 0;

    add(buildGroup(fields, col, row, 0, 0, 1, 1), gbc);

    col += 3;
    gbc.gridx++;
    add(buildGroup(fields, col, row, 0, 0, 1, 1), gbc);

    col += 3;
    gbc.gridx++;
    add(buildGroup(fields, col, row, 0, 0, 1, 1), gbc);

    col += 3;
    gbc.gridx++;
    add(buildGroup(fields, col, row, 0, 0, 1, 0), gbc);

    gbc.gridy++;
    row += 2; // This is important, don't miss this ;)

}

public JPanel buildGroup(JTextField[][] fields, int startCol, int startRow, int top, int left, int bottom, int right) {

    JPanel panel = new JPanel(new GridBagLayout());
    panel.setBorder(new MatteBorder(top, left, bottom, right, Color.RED));
    GridBagConstraints gbc = new GridBagConstraints();
    gbc.insets = new Insets(2, 2, 2, 2);
    gbc.fill = GridBagConstraints.HORIZONTAL;
    for (int row = 0; row < 3; row++) {
        for (int col = 0; col < 3; col++) {

            // Get the field to use for this cell 
            JTextField field = fields[col + startCol][row + startRow];
            gbc.gridx = col;
            gbc.gridy = row;

            panel.add(field, gbc);

        }
    }

    return panel;

}