My problems about reload table

Question

hi guys,
i hava some problems now ,but i think they are easy for u.i need your help,thanks.
1.in my mini application i save the data of JTable in txt format.i have a File menu,and the menuitems are New , Open ,Save ,Save As ,and exit.When i trigger the New or Open events how can i know whether another table is opening and editing.if so ,i must make a dialog pop up to inquire the user whether save the editing table.
2.how can i know the editing table has been saved, so when i modified the table and trigger the Save event the table saves while the JFileChooser dialog dont pop up.
3.when i open a saved table,the blank cells display null.i want to replace the null with the String "".i tried
FileInputStream fin = new FileInputStream(filename);
InputStreamReader isr = new InputStreamReader(fin);
BufferedReader br = new BufferedReader(isr);
StringTokenizer st1 = new StringTokenizer(br.readLine(), "|");
while( st1.hasMoreTokens() )
     columnNames.addElement(st1.nextToken());
while ((aLine = br.readLine()) != null)
     StringTokenizer st2 = new StringTokenizer(aLine, "|");
     Vector row = new Vector();
     while(st2.hasMoreTokens())
     String s = st2.nextToken();
     if (s!=null)
          row.addElement(st2.nextToken());
     else
          row.addElement("");     }
data.addElement( row );
br.close();
and the table's coustructor is
model = new DefaultTableModel(data,columnNames);
table = new JTable(model);
it failed,how can i do?
Can u help me,guys?
thank you in advance.

Answer 1

1. This is really up to how you've programmed your application. In effect you've simply got to determine if there's a JTable in your UI (you probably know the container it will be in so just need to check its children)
2. Your table model will have to support the notion of being "updated". If your table model contains a flag that indicates whether it has been saved you can then set the flag when you save it and reset it when a table cell is updated. This table model can also save the File to which the table model was saved.
3. Are you sure your text file doesn't contain the string "null" anywhere? Null values in table models are usually rendered as the empty string anyway - perhaps the problem is with how you saved the data?
Hope this helps