Calling Instance of ArrayList in Static Method

Question

EDIT: I am rewriting this question, to hopefully make the code more explicit.

I have a class, Pred, with 2 variables.

public class Pred {

    private String str;
    private ArrayList<Keys> keysArr;

    public Pred(String s) {
        this.str = s;
        keysArr = new ArrayList<Keys>;
    }

    public void setKeysArr(ArrayList<Keys> kArr) {
        this.keysArr.addAll(kArr);
    }
}

In the extract() method, I loop through another array, get some variables, and use these to make new Preds.

public static Collection<Pred> extractPause(Keys[] kseArr) {

    Collection<Pred> predArray = new ArrayList<Pred>(); 

    // create method variables
    ArrayList<Keys> keysArray ka = new ArrayList<Keys>();
    String s = "";

    // loop through an array
    for (int i; i < someArray.size(); i++) {
        s = "Sam"+i;
        ka.add(key1+i);
        ka.add(key2+i);

        // create new instance of Pred
        Pred p = new Pred(s);
        p.setKeysArr(ka);
        predArray.add(p);

        // reset variables (not sure if that's necessary)
        s = "";
        ka.clear();

    }
    return predArray;
}

When I go to print the array of Preds, I run into a weird situation, where I'm getting the array values of only the last Pred in the predArray.

public void print() {
    for (Pred p : predArry)
        System.out.println(p.str+" "+p.get(p.keysArr.size()-1).getKeys());
}

Output:

Sam1 key12 key2  
Sam2 key12 key22

Should be:

Sam1 key11 key21  
Sam2 key12 key22

JB Nizet · Accepted Answer · 2012-12-15 16:40:17Z

3

Probably because when you create your list of Foo instances, you do

Foo foo = new Foo();
for (int i = 0; i < 10; i++) {
    foo.setBarStr("hello " + i);
    foo.setKeyArray(...);
    list.add(foo);
}

instead of doing

for (int i = 0; i < 10; i++) {
    Foo foo = new Foo();
    foo.setBarStr("hello " + i);
    foo.setKeyArray(...);
    list.add(foo);
}

Remember that a list contains references to objects, and not copies of objects. So if you add the same Foo instance 10 times to the list, the list will contain 10 references to this unique instance, which will of course contain its last modified state.

answered Dec 15, 2012 at 16:40

JB Nizet

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3 Comments

Adam_G Over a year ago

Thanks @JB, but this isn't the issue. For example, when I call f.getBarStr(), it calls the correct instance. It seems like it's specific to the ArrayList.

JB Nizet Over a year ago

Then you've put the same ArrayList instance in every Foo instance you have created. The problem is the same. If you want each foo to have a different list, you must create a new ArrayList for each foo. ANyway, the problem is in the code creating the objects. Show us this code if you can't find where the problem is.

Adam_G Over a year ago

Ok, thanks. If the problem is not unique to ArrayLists, then I'll keep debugging normally. Unfortunately, I can't share the code, as it contains too many proprietary algorithms.

mehmetakcay · Accepted Answer · 2015-01-20 15:25:45Z

1

I think the problem is in your print function. I don't know what get or getKeys functions do, but I think you're getting keys from the same object even though there are other Pred objects exists with different key values.

System.out.println(p.str+" "+p.get(p.keysArr.size()-1).getKeys());

If you have a typo here and have

predArry.get(...)

in your code, you're constantly getting the same Pred object, and that explains why you keep getting the same key values. Because index p.keysArr.size()-1 doesn't change for either one of those objects. p.keysArr.size()-1 is always 1 in above example and you keep getting the same Pred object from the array of Pred objects i.e. PredArry

answered Jan 20, 2015 at 15:25

mehmetakcay

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