Re: Make a simple 4 row by 4 column scratch pad.

Knute Johnson <>
Thu, 04 Mar 2010 21:12:38 -0800
On 3/4/2010 6:41 PM, bH wrote:

On Feb 25, 11:17 pm, bH<> wrote:

On Feb 25, 9:59 pm, "John B. Matthews"<nos...@nospam.invalid> wrote:

In article

  bH<> wrote:

I looked at Lew's comment again and went to the JTable tutorial. I
think that this might do it for the features explained how to edit
while main program is running. ... But I was hoping for something
simple than what is in the link above.

Yes, I think you need at least a renderer and editor to make it usable:


import java.awt.Color;
import java.awt.Component;
import java.awt.EventQueue;
import java.text.DecimalFormat;
import java.util.Arrays;
import javax.swing.DefaultCellEditor;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTable;
import javax.swing.JTextField;
import javax.swing.table.AbstractTableModel;
import javax.swing.table.DefaultTableCellRenderer;

public class TableGrid extends JPanel {

     private DecimalFormat df = new DecimalFormat("#0.00");
     private JTextField tf = new JTextField();

     public static void main(String[] args) {
         EventQueue.invokeLater(new Runnable() {

             public void run() {
                 JFrame f = new JFrame("TableGrid");
                 f.add(new TableGrid());

     public TableGrid() {
         TableModel dataModel = new TableModel();
         JTable table = new JTable(dataModel);
         table.setDefaultRenderer(Double.class, new DecRenderer(df));
         table.setDefaultEditor(Double.class, new DecEditor(tf, df));

     private static class TableModel extends AbstractTableModel {

         private static final int SIZE = 4;
         private Double[][] matrix = new Double[SIZE][SIZE];

         public TableModel() {
             for (Object[] row : matrix) {
                 Arrays.fill(row, Double.valueOf(0));

         public int getRowCount() {
             return SIZE;

         public int getColumnCount() {
             return SIZE;

         public Object getValueAt(int row, int col) {
             return matrix[row][col];

         public void setValueAt(Object value, int row, int col) {
             matrix[row][col] = (Double) value;

         public Class<?> getColumnClass(int col) {
             return Double.class;

         public boolean isCellEditable(int row, int col) {
             return true;

     private static class DecRenderer extends DefaultTableCellRenderer {

         DecimalFormat df;

         public DecRenderer(DecimalFormat df) {
             this.df = df;

         protected void setValue(Object value) {
             setText((value == null) ? "" : df.format(value));

     private static class DecEditor extends DefaultCellEditor {

         private JTextField tf;
         private DecimalFormat df;

         public DecEditor(JTextField tf, DecimalFormat df) {
    = tf;
             this.df = df;

         public Object getCellEditorValue() {
             try {
                 return new Double(tf.getText());
             } catch (NumberFormatException e) {
                 return Double.valueOf(0);

         public Component getTableCellEditorComponent(JTable table,
             Object value, boolean isSelected, int row, int column) {
             tf.setText((value == null) ? "" : df.format((Double) value));
             if (isSelected) tf.selectAll();
             return tf;


John B. Matthews
trashgod at gmail dot com

Hi John,
A really big thank you for your demo.
I am impressed by the coloring
on the cell chosen and the grid's 4 by 4 appearance.
With a very few modifications this scratch pad
is complete.
Making comment about my earlier questions
and answers that you helped me with, that is,
making sub images out of a larger image.
I have removed the control of the fixed grid.
I now move separate but shuffled sub images to
recombine into the original image.
It is highly graphical specific. Image movement is
not with a mouse but controls using
input from a JTextField(s) and a click on a custom drawing.
Your scratch pad helps me to keep track of
what numbered image to move when getting
to a stage of final alignment. There are 16
sub images.
Thanks again. I truly appreciate it.
bH- Hide quoted text -

- Show quoted text -

Hi All,
Taking a second look at the problem
of a Scratch pad, and at Roedy's suggestion
to write my own ("skinning the cat"),
this program runs and accomplishes the task of
making a "scratch sheet". The "0,1,2,... 15' numbers are
entered in the JTextField and based on where the mouse
click occurs, the grid and click locations and numbers are
maintained throughout the run of the program.
I am frustrated by the list of errors that
are displayed "during the run". I want help to correct them.
Your help is appreciated. First the program
and then the list of errors.

import java.awt.Point;
import java.awt.Color;
import java.awt.Container;
import java.awt.GridLayout;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.ActionEvent;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JTextField;

public class ScratchPadVer2 extends JFrame implements
   ActionListener {
   private Point clickPoint = null;
   private JButton closeBtn = new JButton();
   private int t = 20;
   private int[] locX = new int[t];
   private int[] locY = new int[t];
   private String []cellItem = new String[t];
   private JPanel infoPanel = new JPanel();

   private JLabel topLbl1 = new JLabel ("Enter Picture Number " );
   private JLabel topLbl2 = new JLabel ("Click on Pad Location" );
   private JTextField numberTf = new JTextField("0",2);
   private String holdString = " ";

   ScratchPadVer2() {

   public void init() {
     JPanel infoPanel= new JPanel();
     Container contentPane= getContentPane();
     contentPane.setLayout(new GridLayout(1,1));

     for (int i = 0;i<16;i++){
       locX[i] = 200 ;
       locY[i] = 200 ;
       cellItem[i] = "-1";
     this.addMouseListener(new MouseAdapter() {
       public void mousePressed(MouseEvent event){
         clickPoint = event.getPoint();

   public void actionPerformed(ActionEvent ae) {
     String cmd = ae.getActionCommand();
     if ("Close".equals(cmd)) {

   public void paint(Graphics g) {
     // draw the grid
     Graphics2D g2 = (Graphics2D)g;
     for (int xi = 1; xi<= 5; xi++) {
       for (int yi = 1; yi<= 5; yi++) {
         g.drawLine(xi * 60, 75, xi * 60, yi * 75);
         g.drawLine(60, yi * 75, xi * 60, yi * 75);
     holdString = numberTf.getText();
     if (numberTf.getText()!= null){
       g.drawString(holdString ,clickPoint.x,clickPoint.y);
       for (int ia = 1; ia<= 16; ia++) {
         int t = Integer.parseInt(holdString );
         if (ia == t){
           locX[ia] = clickPoint.x;
           locY[ia] = clickPoint.y;
           cellItem [ia] = holdString;

       for (int ib = 1; ib<= 16; ib++) {
         int xc = locX[ib];
         int yc = locY[ib];
         String str = cellItem[ib];
         if (cellItem[ib]!= "-1"){
           g.drawString(str ,xc,yc);

   public static void main(String[] args) {
     EventQueue.invokeLater(new Runnable() {
       public void run() {
         new ScratchPadVer2();
  Error listing:
  Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at ScratchPadVer2.paint(
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.seqPaintDirtyRegions(Unknown Source)
    at javax.swing.SystemEventQueueUtilities
$ Source)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at Source)

You've got a couple of things that a null when you first draw them.
clickPoint and str. Just put a check in the test before drawing that
they aren't null.

Your program is organized a little funny. It would be better to use a
JPanel to draw upon and put it in the JFrame. You can set the preferred
size of a component, such as a JPanel, then pack the frame around it.
If you leave the frame's default layout manager, BorderLayout, it will
cause the components inside the frame to resize to fit the frame.
Generally one wants to avoid using the paint() method with Swing
components and use paintComponent() instead. However, using a JFrame to
draw on requires the paint() call.

In addition you don't need all of the setVisible() calls, just one on
the frame will do. I generally organize my applications this way;

public class X extends JPanel implements whatever {
     // declare the references to my components that I may need in any
     // listeners here but don't create them here

     public X() {
         // create any components here
         // add the components
         // set a preferred size

     public void whateverAction() {

     public void paintComponent(Graphics g) {
         // draw on the component

     public static void main(String[] args) {
         EventQueue.invokeLater(new Runnable() {
             public void run() {
                 JFrame f = new JFrame();
                 X x = new X();


Knute Johnson
email s/nospam/knute2010/

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