How to Iterate through two ArrayLists Simultaneously?

Question

I Have Two Array Lists, Declared as:

ArrayList<JRadioButton> category = new ArrayList<JRadioButton>();
ArrayList<Integer> cat_ids = new ArrayList<Integer>();

Both of the these fields contain exactly, the Same No of Values, which are infact corresponding in Nature.

I know I can iterate over one of the loops like this:

for(JRadioButton button: category)
{
     if(button.isSelected())
     {
           buttonName = button.getName();
           System.out.println(buttonName);       
     }
}

But, I would like to iterate over both the LISTS simultaneously. I know they have the exact same size. How do I Do that?

basically this should be enough why do you want to iterate over ids? — RE60K, Apr 13 '13 at 07:33

Maroun · Accepted Answer · 2015-06-16T14:09:04.433

118

You can use Collection#iterator:

Iterator<JRadioButton> it1 = category.iterator();
Iterator<Integer> it2 = cats_ids.iterator();

while (it1.hasNext() && it2.hasNext()) {
    ...
}

edited Jun 16 '15 at 14:09

answered Apr 13 '13 at 07:19

Maroun

94,125
30
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241

score 21 · Answer 2 · edited Aug 08 '16 at 10:22

java8 style:

private static <T1, T2> void iterateSimultaneously(Iterable<T1> c1, Iterable<T2> c2, BiConsumer<T1, T2> consumer) {
    Iterator<T1> i1 = c1.iterator();
    Iterator<T2> i2 = c2.iterator();
    while (i1.hasNext() && i2.hasNext()) {
        consumer.accept(i1.next(), i2.next());
    }
}
//
iterateSimultaneously(category, cay_id, (JRadioButton b, Integer i) -> {
    // do stuff...
});

score 18 · Answer 3 · answered Jul 04 '14 at 08:36

If you do this often you may consider using a helper function to zip two lists into one pair list:

public static <A, B> List<Pair<A, B>> zip(List<A> listA, List<B> listB) {
    if (listA.size() != listB.size()) {
        throw new IllegalArgumentException("Lists must have same size");
    }

    List<Pair<A, B>> pairList = new LinkedList<>();

    for (int index = 0; index < listA.size(); index++) {
        pairList.add(Pair.of(listA.get(index), listB.get(index)));
    }
    return pairList;
}

You will also need a Pair implementation. Apache commons lang package has a proper one.

And with these you can now elegantly iterate on the pairlist:

ArrayList<JRadioButton> category = new ArrayList<JRadioButton>();
ArrayList<Integer> cat_ids = new ArrayList<Integer>();

for (Pair<JRadioButton, Integer> item : zip(category , cat_ids)) {
   // do something with JRadioButton
   item.getLeft()...
   // do something with Integer
   item.getRight()...
}

No. That's why the throw new IllegalArgumentException... line is there. It's not obvious what to do if the two lists have different sizes: Trim the longer one, or use placeholders to fill the shorter. Because the placeholder can depend on the type of the lists, it's better not being handled in a generic implementation. — Zoltan.Tamasi, Jul 15 '15 at 06:27

score 12 · Answer 4 · answered Apr 13 '13 at 07:13

12

Try this

ArrayList<JRadioButton> category = new ArrayList<JRadioButton>();
ArrayList<Integer> cat_ids = new ArrayList<Integer>();
for (int i = 0; i < category.size(); i++) { 
    JRadioButton cat = category.get(i);
    Integer id= cat_ids.get(i);
    ..
}

answered Apr 13 '13 at 07:13

Jaydeep Rajput

3,605
17
35

4

I would add i < category.size() && i < cat_ids.size() just in case – giorashc Apr 13 '13 at 07:16
1

Little out of scope, but always use interface when your are working with collections. List category = new ArrayList(); – Vaclav Stengl Jul 04 '17 at 11:02
1

Not a nice answer. Typically you only know the interface (List), the get(i) method makes this solution O(n^2). Avoid. – gabor Dec 10 '21 at 12:53

score 1 · Answer 5 · edited Apr 13 '13 at 08:01

1

Although you are expecting both sizes to be same, just to be on safer side get the sizes for both of them and make sure they are equal.

Let that size value be count. Then use generic for loop, iterate till count and acess the values as array indexes. If 'i' is the index, then acess as below in the for loop.

category[i] and cat_ids[i]

category[i].isSelected() and so on

edited Apr 13 '13 at 08:01

Trikaldarshiii

11,174
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67
95

answered Apr 13 '13 at 07:25

Rahul Sundar

480
8
26

score 1 · Answer 6 · answered Apr 13 '13 at 07:34

ArrayList<JRadioButton> category = new ArrayList<JRadioButton>();
ArrayList<Integer> cat_ids = new ArrayList<Integer>();
Iterator<JRadioButton> itrJRB = category.iterator();
Iterator<Integer> itrInteger = cat_ids.iterator();
while(itrJRB.hasNext() && itrInteger.hasNext()) {
    // put your logic here
}

How to Iterate through two ArrayLists Simultaneously?

6 Answers6

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