adding objects to linkedlist/ arraylist in java

Question

I created a storage class and use to as the data type for my arraylist/Linked list.

private LinkedList bid_history;

I have initialised this in my constructure as

bid_history=new LinkedList <Bid_History> ();

I add new items to the list using add as illustrated below

bid_history.add(new Bid_History(bid_count,unit_price,bid_success));

After 'n' iterations I checked the contents of the list and found out that the list has 'n' elements but they were the same. i.e. the last element i added occupied the entire list. It is as if i added a reference variable to the list?

Any idea where I might have made a mistake? I also used an arraylist, the same problem. I am guessing I did something wrong with access specifiers! but I am out of ideas.....

----Added ------- I use a recursive function

bid()
{
   int bid,quantity;
        bid_success=false;
        bid_count++;
        System.out.println("Starting to bid, Bid ID:"+bid_count);
        quantity=(int)(rated_power*duration/60);
        if(bid_history.isEmpty())
        {
            unit_price=10;
        }
        else
        {
            unit_price++;
        }
        bid=unit_price*quantity;
        //Sending the calculated bid
        send_res(unit_price,quantity,500);
        long startTimeMs = System.currentTimeMillis( );
        System.out.println("Time:"+startTimeMs);
        while(!(System.currentTimeMillis( )>(startTimeMs+2000)));
        System.out.println("Time at end:"+System.currentTimeMillis( ));

        bid_history.add(new Bid_History(bid_count,unit_price,bid_success));

        if(bid_success!=true)
        {
            bid();
        }
}

the printing code is as follows

int count=0,size;
size=bid_history.size();
while(count<size)
System.out.println(((Bid_History)bid_history.get(count++)).getBid_amount());

there doesn't seem to be anything wrong with the code you have shown. show your (edited) loop code. we need more info to help you. also, how do you know they are all the same? show us that code too — Bohemian
– Bohemian ♦, Commented Jul 3, 2011 at 8:28
Are you changing the values of the bid_count, unit_price, bid_success in the successive iterations? — Swaranga Sarma
– Swaranga Sarma, Commented Jul 3, 2011 at 8:32
It won't fix the problem you have, but you should declare your list as List<Bid_History> bid_history to make it a parameterized list. As it is now, it's a raw LinkedList (i.e. a list which could contain anything) — JB Nizet
– JB Nizet, Commented Jul 3, 2011 at 8:40
Also, please respect the Java naming conventions (no underscore), declare variables only when they're needed, and use Thread.sleep to sleep rather than a busy loop. — JB Nizet
– JB Nizet, Commented Jul 3, 2011 at 8:41

Jarek Potiuk · Accepted Answer · 2011-07-03 08:48:25Z

5

Another possibility is that BidHistory(count, price, success) is not doing the right job and not setting the right fields. I don't want to guess, but it might be that instead of having fields in BidHistory you are using static count/price/success fields in the class.

The constructor should look like ("this." is important):

public BidHistory(int count, float price, boolean success) {
    this.count = count;
    this.price = price;
    this.success = success;
}

answered Jul 3, 2011 at 8:48

Jarek Potiuk

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1 Comment

Gan Over a year ago

You are right jarek. i used static variables and used eclipse to generate the getters and setters and did not actually take a closer look.

Iñigo Beitia · Accepted Answer · 2011-07-03 08:31:21Z

0

The only explanation to your problem I can think of is that the values of bid_count, unit_price and bid_success are not changed in each iteration.

answered Jul 3, 2011 at 8:31

Iñigo Beitia

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Comments

Roland Illig · Accepted Answer · 2011-07-03 09:04:00Z

I propose the following changes to make the code easier:

private final List<BidHistory> bidHistory = Lists.newLinkedList();

The final makes sure that the list cannot be replaced by another list. The generic prevents you from accidentally adding incompatible objects to the list. It also makes looping over the list easier. The class Lists from Google Guava keeps the code short, since you don't have to mention the generic datatype twice.

All the fields in the BidHistory class should be made final, too, so you cannot change them later. Since this is about history, you should not be able to change the facts later anyway.

private void printHistoryForDebugging() {
  for (BidHistory bid : bidHistory) {
    System.out.println(bid.getBidAmount() + " (hashCode " + System.defaultHashCode(bid) + ")");
  }
}

I chose to print the defaultHashCode of each bid to check whether the objects are the same or not. For non-same objects the defaultHashCode is very likely to be different, too.

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