Re: Confused with the ArrayList

On Thu, 23 Aug 2007 05:21:26 -0700, dshankhoon@gmail.com wrote, quoted
or indirectly quoted someone who said :


I have converted your snippet to an SSCCE.
 see http://mindprod.com/jgloss/sscce.html
I have also added some generics to make it clearer
 what you are doing.
 I have renamed your temp variables to make it clear
they are distinct.
I have added some labelling of the output.

You did not ask a question. Presumably you were expecting different
output than what you got. To me there were no surprises. Perhaps my
comments will explain what was troubling you.

import java.util.ArrayList;
import static java.lang.System.out;

public class CopyConfusion
   {
   /**
    * main startup method
    * @param args not used
    */
   public static void main ( String [] args )
      {
      ArrayList<ArrayList<String>> original = new
ArrayList<ArrayList<String>>();
      ArrayList<String> copy = new ArrayList<String>();
      copy.add("Test1");
      copy.add("Test2");
      original.add(copy);
      copy = new ArrayList<String>();
      copy.add("Test3");
      copy.add("Test4");
      // this adds the entire ArrayList as a single element, not the
individual Strings.
      original.add(copy);

      // original contains two ArrayLists at this point, each with two
Strings.
      out.println(">>>temp1");
      for ( int i=0;i<original.size();i++ )
         {
         final ArrayList<String> temp1 = original.get(i);
         for ( int j=0;j<temp1.size();j++ )
            {
            out.println(temp1.get(j));
            }
         }
      // output is
      // >>>temp1
      // Test1
      // Test2
      // Test3
      // Test4

      // The following code modifies the ArrayLists in the original.
      // Perhaps you were expecting get to hand you a clone rather
      // than the actual element.
      // Note that the for loop starts at 1 rather than 0.
      // This means you will leave original[0] intact.
      for ( int i=1;i<original.size();i++ )
         {
         final ArrayList<String> temp2 = original.get(i);
         temp2.set(0,"Test5");
         temp2.set(1,"Test6");
         }

      out.println(">>>temp3");
      for ( int i=0;i<original.size();i++ )
         {
         final ArrayList<String> temp3 = original.get(i);
         for ( int j=0;j<temp3.size();j++ )
            {
            out.println(temp3.get(j));
            }
         }
      // output is
      // >>>temp3
      // Test1
      // Test2
      // Test5
      // Test6

      }
   }

--
Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com