Adding array list to another arraylist

Question

I am trying to add array list in another array list, and returning final array list. I have 4 records in table, when i am trying to display, the first set of record only 4 coming times.

 ArrayList<String> al = new ArrayList<String>();
 ArrayList<ArrayList<String>> list = new ArrayList<ArrayList<String>>();
 try
 {
 conn=com.sample.DBConnection.getDBConnection();
 String query= null;
 query=com.sample.RRRConstants.REPORT_SUMMERY;
 st = conn.createStatement();
 rs=st.executeQuery(query);
 Map<String, Object[]> summ = new HashMap<String, Object[]>();
 while(rs.next())
 {
 al.add(rs.getString("sched_id"));
 al.add(rs.getString("sched_name"));
 al.add(rs.getString("gen_date"));
 al.add(rs.getString("gen_time"));
 al.add(rs.getString("generated_by"));
 al.add(rs.getString("gen_version"));
 al.add(rs.getString("gen_status"));
 list.add(al); 
 }
 }
 catch(Exception e)
 {
 e.printStackTrace();
 }

Displaying :

 ArrayList<ArrayList<String>> a1 =rs.getSummary();
 ArrayList<String> a2 = new ArrayList<String>();
 for (int i=0 ; i <a1.size() ; ++i)
 {
 a2 = a1.get(i);
 System.out.println(i +" record "+a2.get(0));
 System.out.println(i +" record "+a2.get(1));
 System.out.println(i +" record "+a2.get(2));
 System.out.println(i +" record "+a2.get(3));
 System.out.println(i +" record "+a2.get(4));
 System.out.println(i +" record "+a2.get(5));
 System.out.println(i +" record "+a2.get(6));
 } 

Answer 1

You're creating a single ArrayList<String>, and adding a reference to that list to your ArrayList<ArrayList<String>> several times.

You need to create a new object within your loop, so that each iteration creates an independent list:

while(rs.next())
{
 ArrayList<String> al = new ArrayList<>();
 ...
}

However, I'd recommend not using ArrayList<String> for this anyway - given that you've got very specific fields, why not create your own type with properties for schedule ID, schedule name etc? That's likely to be a lot easier to work with than a list of strings where you're associating each index with a particular meaning.

Answer 2

Change your while Statement, and reinstantiate al like this

while(rs.next()) {
 ArrayList<String> al = new ArrayList<String>();
 al.add(rs.getString("sched_id"));
 al.add(rs.getString("sched_name"));
 al.add(rs.getString("gen_date"));
 al.add(rs.getString("gen_time"));
 al.add(rs.getString("generated_by"));
 al.add(rs.getString("gen_version"));
 al.add(rs.getString("gen_status"));
 list.add(al); 
}

Otherwise you have only a single ArrayList instance, which contains everything, while your usecase demands one ArrayList for every Loop.

Answer 3

You are adding same list every time in loop. try to create a new list every time. like this:

while(rs.next())
 {
 ArrayList<String> al = new ArrayList<String>();
 al.add(rs.getString("sched_id"));
 al.add(rs.getString("sched_name"));
 al.add(rs.getString("gen_date"));
 al.add(rs.getString("gen_time"));
 al.add(rs.getString("generated_by"));
 al.add(rs.getString("gen_version"));
 al.add(rs.getString("gen_status"));
 list.add(al); 
 }