ArrayLists, setting a value on one ArrayList changes a value on another

Question

As the title says, I have two ArrayLists. Strangely, setting a value on one arraylist changes that of another.

Some information: these are ArrayLists of type Entry, each of which holds an amount and a value (which is what you see in the parentheses). I'm trying to turn this array: [(5,2)(2,5)(3,2)]

into these [(1,2)] and [(4,2)(2,5)(3,2)]. i == 1 in this case, and remainingAmt == 1.

ArrayList<Entry> firstHalf = new ArrayList<Entry>();
    ArrayList<Entry> secondHalf = new ArrayList<Entry>();

    for(int j = 0; j<=i;j++){
        firstHalf.add(rleAL.get(j));
    }
    for(int k = i; k<rleAL.size(); k++){
        secondHalf.add(rleAL.get(k));
    }
    System.out.println(firstHalf);//gives (5,2)
    firstHalf.get(i).setAmount(remainingAmt);
    System.out.println(firstHalf);//gives (1,2) *CORRECT*
    secondHalf.get(0).setAmount(rleAL.get(i).getAmount() - remainingAmt);
    System.out.println(firstHalf);//gives (0,2) *WRONG*

Dave Newton · Accepted Answer · 2011-10-16 21:45:02Z

7

Nothing unusual there--if the underlying objects are shared between lists, a change in one object will be reflected in the other. If you need the lists to be independent, you need to use different objects in each list by either creating new objects from scratch, or copying the initial object to a new object in list #2.

answered Oct 16, 2011 at 21:45

Dave Newton

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Dave Newton Over a year ago

@dylandrop No problem--you'd be surprised how often that trips people up, no matter how experienced.

user207421 · Accepted Answer · 2011-10-17 03:50:35Z

0

The only problem here is in your title. You haven't 'set a value on one ArrayList' at all. You've set a value in an object which is a member of both ArrayLists, so of course it shows up in both.

answered Oct 17, 2011 at 3:50

user207421

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