13

First of all, apologies for not being able to supply any sourcecode. My project is pretty big and linking everything would be impractical, and I have not been able to scale down the problem which is exceedingly annoying. I will do my best to explain it here.

I am dynamically creating new instances of a class on each loop in my code. This instance is dynamically given a couple of properties while in the loop, 'name' for example. At the end of each loop, the newly generated instance is added to an ArrayList held in a another, 3rd, class.

The problem however is that when a new element is added, for whatever reason, all previous elements change to match exactly the latest. My guess is that the ArrayList is creating a reference to the dynamically created element so that whenever it changes, they all change, but I do not know how to fix this.

I would be grateful for any advice and apologies again for the quality of this explanation. I will post any specific piece of the code you may wish to see

As Requested - XmlHandler.java - http://pastebin.com/mGmWt1RD ParsedDataSet.java = http://pastebin.com/k1xb3KBe Content.java = http://pastebin.com/UxiL2f9q

Just to cut down on your comprehension time - The project is an epub reader. The XMLHandler is being called from a SAX parser in another class not shown. The XMLHandler is used 3 different times for 3 different XML sets so there is some clutter there.

The problem lies with the 'toc' ArrayList. The 'toc', or TableOfContents, holds the Contents instances to be referenced later (not shown). I am trying to pass data each new instance of 'Content' and then pass that into the static ArrayList

Mark D
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2 Answers2

26

I've seen folks report this kind of problem many times, and it always comes down to this: you're actually not creating a new instance, but instead using the same one for each iteration of the loop. It's an easy mistake to make, especially if you're coming from a language with different copy semantics. There are a number of different ways you can make this mistake; if you edit your question to show the loop code, I'm sure I'll be able to explain what's happening.

OK, now that you've added the code: the problem is that in "Content", all the data member are marked "static". In Java, that means that there's one variable shared by all objects -- i.e., the variable has the same value for every object. SO in fact you are creating many Content objects to put in the ArrayList, but they all look identical! Remove those "static" attributes from Content's data members, and you'll be all set.

Ernest Friedman-Hill
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  • Yes, show us the looping and object creation code. – Steve Kuo Apr 10 '11 at 01:11
  • OK, the problem is that in "Content", all the data member are marked "static". In Java, that means that there's one variable shared by all objects -- i.e., the variable has the same value for every object. SO in fact you *are* creating many Content objects to put in the ArrayList, but they all look identical! Remove those "static" attributes from COntent's data members, and you'll be all set! – Ernest Friedman-Hill Apr 10 '11 at 01:28
  • Sorry for not getting back to you sooner, had to sleep. Thank you! – Mark D Apr 10 '11 at 10:49
  • Sorry to bother you again, but now all the 'Content' properties are turning blank and I'm not sure why – Mark D Apr 10 '11 at 21:23
  • Looking at `XmlHandler`, I think the logic starting at line 33 is suspect. I think whenever you hit a "navPoint" element, *or any element contained inside a navPoint*, you're going to create a new `Content`. This will actually result in a lot of `Content` objects getting created, having one parameter set, and then thrown away. The constructor call at line 38 should occur if and only if `qName` equals "navPoint"; it should definitely *not* occur if `inNavPoint` is already true, because that will throw the previous "live" `Content` object away. – Ernest Friedman-Hill Apr 10 '11 at 21:35
  • I can't thank you enough :) I got rid of my faulty logic and replaced it with your correct simpler system, of course it works perfectly now! This is the new XmlHandler file if your interested - http://pastebin.com/WuQb4nWr. I still have to use a inNavPoint Check, note sure why, but it works! – Mark D Apr 10 '11 at 22:00
  • it worked for me .. :) – Kushal Dec 18 '15 at 07:13
17

ArrayList just stores reference of elements. Ensure that your code looks like:

ArrayList list = new ArrayList<>();
loop(...) {
    MyObject newOne = new MyObject();
    newOne.setXXX(xxx);
    list.add(newOne);
}

Wrong code:

ArrayList list = new ArrayList<>();
MyObject sameOne = new MyObject();
loop(...) {
    sameOne.setXXX(xxx);
    list.add(sameOne);
}
buræquete
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卢声远 Shengyuan Lu
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