Re: ArrayList.add problem

From:

"Proton Projects - Moin" <mohd.mohideen@gmail.com>

Newsgroups:

comp.lang.java.help

Date:

17 Jan 2007 21:53:41 -0800

Message-ID:

<1169099621.563207.318380@11g2000cwr.googlegroups.com>

There are more simpler way to do these things...by having a common
method and calling it in a loop fby passing different objects....

On Jan 18, 10:39 am, "Ook" <Ook Don't send me any freakin' spam at
zootal dot com delete the Don't send me any freakin' spam> wrote:

Ahh..I get it. I thought when I did wordPairList.add, it added it by value,
not by reference, or IOW it would implicitly created a new instance in the
list and copied the values from tempWordPair to the new entry in the list.
That explains a lot! Thanks - this was a roadblock. This list will hold data
read in from a file, and my actuall class structure is more complex then
what I posted here. This code was trimmed down to illustrate the issue I was
seeing. The actual code is much bigger, and I saw this behavior in several
places, and for the same reason.

"Proton Projects - Moin" <mohd.mohid...@gmail.com> wrote in messagenews:1169097355.170578.267120@s34g2000cwa.googlegroups.com...

Hi,

The reason for the following output is

[0] - ccc
[1] - ccc
[2] - ccc

When the tempWordPair object is created, WordPair tempWordPair = new
WordPair();, it will hold the reference(nothin but the memory location)
to the object, say tempWordPair holds the memory location as 12121. Now
in that location, firstWord variable is set to "aaa". Now u r adding
the reference to the Arraylist, nothing but 12121.

Again u have changed the firstWord to "bbb" and added to the arraylist
nothing but 12121.

Again u have changed the firstWord to "ccc" and added to the arraylist
nothing but 12121.

Finally, when u iterate the Arraylist, u will retrieving the same
object at the same reference,...so u always get the value "ccc"

To overcome this...try to create new object each time, before u set the
firstWord...

import java.util.*;

public class Assignment1
{
static class WordPair
{
String firstWord;
}
public static void main(String[] args)
   {
       WordPair tempWordPair = new WordPair();
       ArrayList<WordPair> wordPairList = new ArrayList<WordPair>();

       tempWordPair.firstWord = "aaa";
       wordPairList.add( tempWordPair );

tempWordPair = new WordPair();
       tempWordPair.firstWord = "bbb";
       wordPairList.add( tempWordPair );

tempWordPair = new WordPair();
       tempWordPair.firstWord = "ccc";
       wordPairList.add( tempWordPair );

       System.out.println("[0] - " + wordPairList.get(0).firstWord);
       System.out.println("[1] - " + wordPairList.get(1).firstWord);
       System.out.println("[2] - " + wordPairList.get(2).firstWord);
   }
}

On Jan 18, 9:18 am, "Ook" <Ook Don't send me any freakin' spam at
zootal dot com delete the Don't send me any freakin' spam> wrote:

I think this is a bit tougher then the last one I posted (which was quite
simple). I think I'm missing something fundamental. This code runs, but
when
I run it and it prints out the contents of the list it shows that all
items
in the list have the same value as the last item I added. I expect to see
in
the console:

[0] - aaa
[1] - bbb
[2] - ccc

But instead I see

[0] - ccc
[1] - ccc
[2] - ccc

------------------------------------------------------

import java.util.*;

public class Assignment1
{
    public static class WordPair
    {
        String firstWord;
    }

    public static void main(String[] args)
    {
        WordPair tempWordPair = new WordPair();
        ArrayList<WordPair> wordPairList = new ArrayList<WordPair>();

        tempWordPair.firstWord = "aaa";
        wordPairList.add( tempWordPair );

        tempWordPair.firstWord = "bbb";
        wordPairList.add( tempWordPair );

        tempWordPair.firstWord = "ccc";
        wordPairList.add( tempWordPair );

        System.out.println("[0] - " + wordPairList.get(0).firstWord);
        System.out.println("[1] - " + wordPairList.get(1).firstWord);
        System.out.println("[2] - " + wordPairList.get(2).firstWord);
    }

}