Java: linked list of items problem

Question

I have used linked lists before with Strings, doubles, etc., and they always worked exactly as expected. But now I am forming a linked list of items, and whenever I add a new element to the list, all objects in the list apparently become equal to the last object.

The essential code is as below:

import java.util.*;

public class Global
{

  static public LinkedList<StockInfo> DiaryStocks = new LinkedList<StockInfo>();

  static public class StockInfo    //info related to each stock in diary
  {
    String recordDate;             
    String ticker;
    int   status;                  
    String buyDate;
    String sellDate;
    double buyPrice;
    double sellPrice;
    double nmbrShares;
  }//StockInfo

//The following function places the Diary data for a stock in the arraylist

 static public void AddDiaryData(StockInfo thisdata)
  {
    String tckr;
    int i;

    DiaryStocks.add(thisdata);

    for (i = 0; i < DiaryStocks.size(); i++)   //this is debug code
    {
      tckr = DiaryStocks.get(i).ticker;
    }
  } 
}

As I said, when single stepping through the debug code near the bottom, each time I add a new item to the list, the list size grows as it should, but the tckr item only corresponds to the last item added.

Any insights into this puzzle would be greatly appreciated.

John Doner

Can you clarify what is meant by "but the tckr item only corresponds to the last item added"? Each time through the loop, you overwrite the tckr variable so after the loop, it would always be set to the last item. Are you saying it's assigned the same value each time through the loop? And are we to assume your data has different values for this variable? — Jonathan Wood
– Jonathan Wood, Commented Dec 24, 2010 at 16:20

Bozho · Accepted Answer · 2010-12-24 16:28:10Z

2

The problem is outside the code your provide. It is most likely that you are adding the same instance of StockInfo. Perhaps you have something like:

StockInfo info = new StockInfo();
for (...) {
   info.setFoo(..);
   info.setBar(..);
   AddDiaryData(info);
}

You should not reuse instances like that. You should create a new instance each time.

As a sidenote - method names in Java should start with lowercase letter.

edited Dec 24, 2010 at 16:28

answered Dec 24, 2010 at 16:21

Bozho

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thkala · Accepted Answer · 2010-12-24 16:23:13Z

1

From the symptoms you are describing, it seems as if you are always adding a reference to the same StockInfo object instance to your list, rather than a reference to a new copy each time.

When that object is updated with the contents of the new entry, all list entries appear to change to reflect that latest entry.

This problem lies outside the code snippet that you posted, perhaps in the caller of the AddDiaryData method.

answered Dec 24, 2010 at 16:23

thkala

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user467871 · Accepted Answer · 2010-12-24 16:30:52Z

0

Ooops.

Deep Copy please search it

DiaryStocks.add(thisdata);

you should create new StockInfo() then add to the list otherwise you add the reference and it equalize all the reference of items to the last one

answered Dec 24, 2010 at 16:30

user467871

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