From an ArrayList to a String []

Question

I've implemented a method that return the result of a query on a SQL database. I just want the method to retur only a String[] which is the result of a query that select a column on the db. Here my code:

public class DBConnection {
private static Connection con;
private static Statement st;
private static ResultSet rs;

    try
    {   
        Class.forName("com.mysql.jdbc.Driver");
        con = DriverManager.getConnection("jdbc:mysql://localhost:3306/database","user","password");
        st = con.createStatement();
    }

    catch (Exception ex)
    {
        System.out.println("Error: "+ex);
    }
public ArrayList<String[]> doQuery (String query)
{
      ArrayList<String[]> v = null;
      String [] record;
      int columns = 0;
      try {
         Statement stmt = con.createStatement();    
         ResultSet rs = stmt.executeQuery(query);   
         v = new ArrayList<String[]>();
         ResultSetMetaData rsmd = rs.getMetaData(); 
         columns= rsmd.getColumnCount();            

         while(rs.next()) {   
            record = new String[columns]; 
            for (int i=0; i<colonne; i++) record[i] = rs.getString(i+1); 
            v.add( (String[]) record.clone() );
         }
         rs.close();     
         stmt.close();   
      } catch (Exception e) { e.printStackTrace(); }

      return v;
   }    

this method return an ArrayList object that contains the result of a query. Now, the question is: how can I have from this ArrayList object a String[] object that contains ONLY a column of the result?

(As information : The String[] object will be inserted in a JComboBox object)

Answer 1

why not to call v.toArray(new String[0])?

Answer 2

I assume your question has two components: a) you want to return a string array and b) you want to return only a single column.

The answer to a) has already been given or at least hinted at.
The answer to b) would require you to know the name of the column you want to return or adjust the query.

You might change your method to something like this:

public String[] doQuery (String query, String columnName) //columnName not needed if you know the index of the column or if the name is always the same, in which case it could be some constant
{
  List<String> v = new ArrayList<String>();

  try {
     Statement stmt = con.createStatement();    
     ResultSet rs = stmt.executeQuery(query);           

     while(rs.next()) {    
        v.add( rs.getString(columnName) ); //or rs.getString(1); if you know the column is the first in the query's result
     }
     rs.close();     
     stmt.close();   
  } catch (Exception e) { e.printStackTrace(); }

  return v.toArray(new String[v.size()]); 
}    

A few notes:

• You'd have to ensure that the column has the name you want to query it with, i.e. you can't do select columnA from ... and then call rs.getString("columnB");. If you don't know the name but know the index of the column in the resultset, use rs.getString(x); instead, where x is the one-based index.

• instead of v.toArray(new String[v.size()]); you could also use v.toArray(new String[0]);. The difference between the two is that the former returns the array you pass as a parameter whereas the latter creates a new array internally and returns that.

Answer 3

pasted the solution given in link Converting 'ArrayList<String> to 'String[]' in Java

ArrayList<String> list = new ArrayList<String>();
String[] array = list.toArray(new String[list.size()]);

(or)

ArrayList<String> arrayList = new ArrayList<String>(); 
Object[] ObjectList = arrayList.toArray();
String[] StringArray = Arrays.copyof(ObjectList,ObjectList.length,String[].class);

Answer 4

On an ArrayList you can call toArray() to get an Array of its values.

This would look like this:

// create an ArrayList
ArrayList<String> theArrayList = new ArrayList<String>();
theArrayList.add("aString");
theArrayList.add("anotherString");

// get its contents as an Array
String[] theArray = new String[theArrayList .size()];
theArray = theArrayList.toArray(theArray);

You can look up more details in the Java Docu for ArrayList.toArray().