Passing TableModel to a JTable

Question

<b>I created a personal tableModel class.
I called it with that code :</b>
tableModels[0]=new MyTableModel(data1);
<b>and any time I have this exception:</b>
Exception in thread "main" java.lang.NullPointerException:
at MyTableModel.getRowCount(MyTableModel.java:25)
at javax.swing.table.DefaultTableModel.setNumRows(DefaultTableModel.java
:317)
at javax.swing.table.DefaultTableModel.<init>(DefaultTableModel.java:105
at javax.swing.table.DefaultTableModel.<init>(DefaultTableModel.java:70)
at MyTableModel.<init>(MyTableModel.java:11)
at SwingWinc.layoutComponents(SwingWinc.java, Compiled Code)
at SwingWinc.buildGUI(SwingWinc.java:194)
at SwingWinc.<init>(SwingWinc.java, Compiled Code)
at SwingWinc.main(SwingWinc.java:504)
<b>Why ? please help.
Follow the MyTableModel class </b>
import javax.swing.table.*;
import javax.swing.*;
import java.util.*;
import java.awt.*;
public class MyTableModel extends DefaultTableModel
final String[] columnNames={"Analisi Eseguite","Risultati","Valori di riferimento"};
Object[][] matrixTable={
{"GLICEMIA","150 mg/dl","( 60-110 mg/dl )"},
{"AZOTEMIA","33 mg/dl","( 20- 50 mg/dl )"},
{"COLESTEROLO TOT","291 mg/dl","( 130-200 mg/dl )"},
{"COLESTEROLO HDL"," 60 mg/dl","( 40- 70 mg/dl )"},
{"COLESTEROLO LDL","100 mg/dl","( 0-150 mg/dl )"},
{"Glicemia","88 mg/dl","( 60-110 mg/dl )"},
{"Glicemia","88 mg/dl","( 60-110 mg/dl )"},
{"Glicemia","88 mg/dl","( 60-110 mg/dl )"},
{"Glicemia","88 mg/dl","( 60-110 mg/dl )"},
{"Glicemia","88 mg/dl","( 60-110 mg/dl )"},
{"Glicemia","88 mg/dl","( 60-110 mg/dl )"}} ;;
public MyTableModel(Object [][] matrix)
DefaultTableModel tableModel=new DefaultTableModel(matrix,columnNames);
matrixTable=matrix;
public int getColumnCount()
return columnNames.length;
public int getRowCount()
return matrixTable.length;
public String getColumnName(int col)
return columnNames[col];
public Object getValueAt(int row, int col)
return matrixTable[row][col];
public Class getColumnClass(int c) {
return getValueAt(0, c).getClass();
public boolean isCellEditable(int row, int col) {
//Note that the data/cell address is constant,
//no matter where the cell appears onscreen.
if (col < 2) {
return false;
} else {
return true;
public void setValueAt(Object value, int row, int col)
matrixTable[row][col] = value;
fireTableCellUpdated(row, col);
I called it with that
tableModels=new MyTableModel(data1);

Answer 1

Hi,
is that data1 reference, that you pass, initialized or do you pass perhaps a null-reference there?
Another thing - in your constructor, you never call the super-constructor in order to construct the underlying DefaultTableModel in the correct way. Instead you instantiate a new DefaultTableModel with your parameters, that will exist only in this block and so do really nothing usefull.
The normal way to call the super-constructor is in your case:
public MyTableModel(Object [ ] [ ] matrix) {
super(matrix,columnNames);
tableMatrix=matrix; } // see comments belowAlso it is not correct, to hold an own matrixTable within the DefaultTableModel and overwrite the TableModel interface methods - the DefaultTableModel has an own Vector of Vector structure to hold the table-data - if you pass for example an Object [ ] [ ] in its constructor, the DefaultTableModel builds its dataVector right after it and copy the values for this array into the dataVector.
If you want to use your own datamodel in Object [ ] [ ] form, then I would suggest to make your MyTableModel extend AbstractTableModel - but if you want to use DefaultTableModel and its methods, you should leave the storage of the data to the DefaultTableModel and its dataVector,
that uses a Vector, that contains Vectors, that each contain the celldata of one row. The methods of DefaultTableModel are designed to use that dataVector, they will not work with or even use your matrixTable array, except that the original data is copied into the dataVector, if you change the constructor in the way, I have posted above.
So my advice is, to consult the online tutorial for that - it is really good and you will see, that creating an own TableModel by subclassing AbstractTableModel is relative easy to do. You have the basic steps done in your implementation (if it would be no DefaultTableModel, that would be a good start for that). So, I guess, you will figure it out, If you read the tutorial about the use of JTable. Start here:
http://java.sun.com/docs/books/tutorial/uiswing/components/table.html
greetings Marsian