Java ArrayList acting up

Question

I am running in to an odd issue. I have a database with 6 different "bids". I am trying to extract the bids with the following code:

public ArrayList<ProjectBid> getProjectBids(String projectID)
    {
        ArrayList<ProjectBid> result = new ArrayList<ProjectBid>();

        ProjectBid bid = new ProjectBid();

        try{

        st = con.createStatement();
        String query = "SELECT * FROM bids WHERE projectID=\""+projectID+"\"";
        System.out.println(query);
        ResultSet rs = st.executeQuery(query);


        while (rs.next())
          {
            bid.setProjectID(projectID);
            bid.setBidID(rs.getString("bidID"));
            bid.setBidderEmail(rs.getString("bidder_email"));
            bid.setBidAmount(rs.getInt("bid_amount"));
            bid.setBidMessage(rs.getString("bid_message"));
            bid.setTimestamp(rs.getString("timestamp"));

            result.add(bid);
          }
          st.close();

        }
        catch(Exception ex){
            System.out.println("Exception: "+ex);
        }

        return result;          
    }

For some reason my ArrayList result is giving me 6 identical "bid" objects instead of adding each of the "bids" from my database individually. The code runs through the ResultSet as expected and the correct values for each run is printed correctly if I add "System.out.println(bid.getBidID);" inside my while-loop. Can anyone tell me what is wrong with my code?

Thank you

Steven Pauk · Accepted Answer · 2015-03-21 06:45:28Z

3

You should move ProjectBid bid = new ProjectBid(); inside the while loop. With the current code you are updating the same bid variable which is defined outside the while loop hence it gets the last record's value after the loop completes.

answered Mar 21, 2015 at 6:45

Steven Pauk

5,3285 gold badges34 silver badges45 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Aniket Thakur · Accepted Answer · 2015-03-21 06:44:09Z

3

Add

ProjectBid bid = new ProjectBid();

in your while loop. As of now you are just overwriting the same ProjectBid object. So each reference in you ArrayList is pointing to the same object value of which you keep changing in your loop. So at the end you will have all elements in ArrayList with data same as last row retrieved from the database.

    while (rs.next())
      {
        ProjectBid bid = new ProjectBid();
        bid.setProjectID(projectID);
        bid.setBidID(rs.getString("bidID"));
        bid.setBidderEmail(rs.getString("bidder_email"));
        bid.setBidAmount(rs.getInt("bid_amount"));
        bid.setBidMessage(rs.getString("bid_message"));
        bid.setTimestamp(rs.getString("timestamp"));

        result.add(bid);
      }

answered Mar 21, 2015 at 6:44

Aniket Thakur

69.4k43 gold badges290 silver badges299 bronze badges

8 Comments

user3586514 Over a year ago

Then I will create a new "bid" object for each row in my database. How will that effect my memory usage if I have 1000 bids? Shouldn't overriding the same object work just as fine?

Makoto Over a year ago

@user3586514: No. First, you're going to be overwriting existing data with the current approach (this answer is correct); second, the lifecycle of each new ProjectBid object lasts only until the end of the loop; each one will be eligible for garbage collection.

Aniket Thakur Over a year ago

No it will not. All add() method does it add a reference to the object you pass. So as I said earlier at the end of loop all your array list references will point to same object. You need separate objects to save your distinct database row entries.

Steven Pauk Over a year ago

@user3586514 You could define a variable outside the loop but you have to reassign it with a new operator inside the loop.

Tom Over a year ago

@user3586514 No it shouldn't and you already noticed that it doesn't work. And yes it effect your memory... a little bit. Don't care about that.

|

Samarth · Accepted Answer · 2015-03-21 06:55:09Z

1

Since the reference to the bid is always the same thereby, it is overwriting the bid with the current value that in turn is changing all the bids in the result list giving you the last bid added.

Instantiate the bid object inside your loop and this should work fine.

while (rs.next())
  {
    ProjectBid bid = new ProjectBid();
    ...
    ...
    ...
    result.add(bid);
  }

answered Mar 21, 2015 at 6:55

Samarth

614 bronze badges

Collectives™ on Stack Overflow

Java ArrayList acting up

3 Answers 3

Comments

8 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

8 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related