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)); }

Jon Skeet · Accepted Answer · 2015-06-10 09:30:42Z

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.

Iwan1993 · Accepted Answer · 2015-06-10 09:31:02Z

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.

Sachin Gupta · Accepted Answer · 2015-06-10 09:31:31Z

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); }

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