1. why is not a constant 2. put reference

Question

Hello!
I have two questions.
1. It is 2 classes - A and B. In A I define variable:
final int number = 1;
In class B ( I create it so - B(A aa) ) I try to use this variable:
case aa.number: .... - And I receive compilation error in this string - "constant expression required". Why variable number don't accepted as constant?
2. And my second problem. I have some classes CreateTab, TabModel, MainProg. MainProg call CreateTab, CreateTab uses TabModel for creation JTable, class TabModel extends AbstractTableModel. In MainProg I define an array:
Object [][] tabData = { {"", "", ""} };
This array is used in TabModel :
private Object [][] data = mp.tableData;
Who can hint me how I have to put reference of tabData from MainProg to TabModel? I understanded that constructor TabModel don't use in this case - I tried to do so:
in MainProg: new CreateTab(this),
in CreateTab: TabModel tm = TabModel(mainProg);
in TabModel: TabModel (MainProg mainProg) {
mp = mainProg;
private Object [][] data = mp.tableData; but in this string I receive "Null pointer exception".
What do I do incorrectly?
Thanks.

Answer 1

Your variable mp hasn't been set when that line is encountered.
If I understand you correctly, your code is like
class TabModel extends AbstractTableModel {
    private MainProg mp;
    private Object [][] data = mp.tableData;
    TabModel (MainProg mainProg) {
        mp = mainProg;
}The assignment to data happens before the assignment to mp; when your constructor is invoked, before
executing any code in the constructor itself, member initialization will occur. At that point, the member
variable has not been set yet. You could instead do
class TabModel extends AbstractTableModel {
    private MainProg mp;
    private Object [][] data;
    TabModel (MainProg mainProg) {
        mp = mainProg;
        data = mp.tableData;
}: jay