typecast list to string using hibernate in java

If your List gantttablecolnameList really have String then try this :

for(int i2=0; i2<gantttablecolnameList.size(); i2++)
{
  columnname= (String) (gantttablecolnameList.get(i2));
}

You are doing this :

(List)gantttablecolnameList.get(i1)

Assuming your List gantttablecolnameList contains String ,gantttablecolnameList.get(i1) will give you an Object which is actually a String instance for the index specified by the runtime value of i1 . So , in a nutshell you are trying this :

 (List)"someString";

which is impossible.

Try

 columnname= (String) ((List)gantttablecolnameList).get(0));

in (List)gantttablecolnameList.get(i1)) the gantttablecolnameList.get(i1) will give you the element ..not the list.

But really you have to check null conditions.Yours is as of now NUll PATTERN

Several things:

1. Downcast to List <String> instead of the raw type you have now;
2. Iterate over the list using the enhanced for loop. Don't do index-based iteration;

3. Once you have dealt with the above, your code will be simple enough to make the bug simply evaporate:

    for (String columname : (List<String>) ganttablecolumnlist) ...