ArrayList added item is replaced by the next item in for loop

Question

I am facing a very strange situation. I add an object to an arrayList in a loop, but it is replaced by the next object. Actually second item is duplicated. ( It replaces the first item as well as inserts another object to the ArrayList.)

This is my code. I have done the debugging and included the comments where needed. Could someone point out why this happens? I am taking the object details from the database and those are working as expected.

public class Serv
{
    @Autowired
    GrpHeader objGrpHeader;

    @Autowired
    CompPesoOutgoingMsg objMsg;

    @Autowired
    OutwardMessage objOutwardMessage;

    public List<OutwardMessage> outgoingMessagesAsSingleTrx()
    {
        List<OutgoingMsg_Obj> trxList = myRepo.getTrx("5");
        List<OutwardMessage> myTrxList = new ArrayList<>();


        for (OutgoingMsg_Obj outgoingMsg : trxList)
        {
            BigDecimal trxAmt = outgoingMsg.getIntrBkSttlmAmt().getTrxn_amt();
            trxAmt = (trxAmt).divide(new BigDecimal(100));

            GrpHeader grpHeader = objGrpHeader;
            CompPesoOutgoingMsg outMsg2 = objMsg;
            OutwardMessage objOutwardMessage2 = objOutwardMessage;

            outgoingMsg.setRmtInf(objRmtInf);
            outgoingMsg.setPmtTpInf(objPmtTpInf);

            outMsg2.setHeader(grpHeader);
            outMsg2.setCdtTrfTxInf(Arrays.asList(outgoingMsg));
            objOutwardMessage2.setObjMsg(outMsg2);

            **//Here, Correct object details are printed**
            log.info("outwardMsg 100 {} ", objOutwardMessage2);

            //Add Item to the list
            myTrxList.add(objOutwardMessage2);

            for (OutwardMessage outwardMsgx : myTrxList)
            {
                //1. When this loop executed first time, first object details are printed
                //2. When printed second time, first added object is no more. And second added object is there twice.
                log.info("outwardMsg 101 {} ", outwardMsgx);
            }
        }
        return myTrxList;
    }
}

You use a single `objOutwardMessage`. You only mutate that single object. (PS.: please drop the Hungarian notion, it makes things very hard to read) — Johannes Kuhn, Jun 02 '20 at 09:53
Thanks for the answer.I am sorry about the format.Any idea how to fix it? I am kinda new to springboot — Ran_Macavity, Jun 02 '20 at 09:57
Does this answer your question? [Why does my ArrayList contain N copies of the last item added to the list?](https://stackoverflow.com/questions/19843506/why-does-my-arraylist-contain-n-copies-of-the-last-item-added-to-the-list) — Ivar, Jun 02 '20 at 09:58

score 1 · Accepted Answer · answered Jun 02 '20 at 09:55

1

You have a single reference. By setting the objOutwardMessage2to objOutwardMessageyou are just changing the data inside the reference.

answered Jun 02 '20 at 09:55

fjsv

705
10
23

Thanks for the answer. Any idea how to fix it? I am kinda new to springboot – Ran_Macavity Jun 02 '20 at 09:57
1

For what I can gather, you should create a new instance of `OutwardMessage`instead of: `OutwardMessage objOutwardMessage2 = objOutwardMessage;` doing this. Depending on what you are trying to achieve you may need something else since the `OutwardMessage` is a bean. If you need the data inside that object, take a look at: [this question](https://stackoverflow.com/questions/2624165/how-to-copy-an-object-by-value-not-by-reference) – fjsv Jun 02 '20 at 10:03

Sreejith · Answer 2 · 2020-06-02T14:43:10.640

1

Since no new object is created for each iteration, the same objOutwardMessage2 value is getting replaced each time.

Try

OutwardMessage objOutwardMessage2 = new OutwardMessage();

and copy the value of objOutwardMessage to the newly created objOutwardMessage2.

edited Jun 02 '20 at 14:43

answered Jun 02 '20 at 10:02

Sreejith

506
5
7

Thanks for the answer. But 'new' keyword cannot be used since I am creating a Spring boot app. – Ran_Macavity Jun 02 '20 at 10:08
@Ran_Macavity Spring Boot doesn't prohibit you from using the `new` keyword. – Ivar Jun 02 '20 at 10:10
Yes. There is nothing against creating new instances. You not even can create new instances but also new beans by anotating methods that return _new stuff_ – fjsv Jun 02 '20 at 10:11
@Ivar yes I know. I will try if this fails. Thanks – Ran_Macavity Jun 02 '20 at 10:13

ArrayList added item is replaced by the next item in for loop

2 Answers2