Re: Newbie Question - ArrayLists and methods

Row 0 is changing because the parameters are being passed by reference.

You send in the list of items, and it simultaneously affects both the
to-be-returned value and the input (rows 1 and 0, respectively). To keep
it from changing, you want to create a copy of the row, which is what
the first line of my method does.

public class Matriculate
{
  List< List <Integer> > table
    = new ArrayList< ArrayList <Integer> > ();

 public Matriculate()
 {
   // the table will now contain zero rows
   assert table.size() == 0;

   // here row is declared the one and only time
   // and initialized for the first of more than one time
   List <Integer> row = new ArrayList <Integer> ();
   row.add( 1 );
   row.add( 2 );
   row.add( 3 );
   row.add( 5 );

   table.add( row );

   // the table will now contain one row
   assert table.size() == 1;

   row = new ArrayList <Integer> ();
   // notice - re-used, not re-declared
   // the variable 'row' now points to a whole
   // new ArrayList
   row.add( 1 );
   row.add( 2 );
   row.add( 4 );
   row.add( 8 );
   row.add( 16 );

   table.add( row );

   // the table will now contain two rows
   assert table.size() == 2;
 }
 // now the variable 'row' is out of scope
 // the closing curly brace killed it

 public List< List <Integer>> getTable()
 {
   return Collections.unmodifiableList( table );
 }
}