LIST - update Id numbers

Question

Hi
I have List<group> with groupid. I want to update or sort id the group id
I have written this code., but not working can anyone explain tome
public void updateid()
          for(int i= 1 ;i < g_list.size();i++)
               g_list.get(i).setGroupId(new BigDecimal(i));
     }

Answer 1

Note that the list index starts at zero, not one!
It is also conventions that classes are capitalized (Group, not group).
You can sort using Collections.sort(), in your case you need a custom comparator:
Collections.sort(g_list, new Comparator<Group>() {
  public int compare(Group group1, Group group2) {
   if(group1 != null && group2 == null) {
    // group 1 is bigger
    return 1;
  else if(group1 == null && group2 != null) {
    // group1 is smaller
    return -1;
// needs extra null checks here if group id can be null
// if group id really are decimals and not just ints, need to do rounding here
   return group1.getGroupID().subtract(group2.getGroupID()).intValue();
});