Java loop strange behaviour

Question

I have this split method inside which I have a loop. This loop is running 4 times but it should run 5 times. Any idea why is it behaving like this?

public static <T> List<List<T>> split(List<T> bigCollection, int maxBatchSize) {
        List<List<T>> result = new ArrayList<List<T>>();

        if (CollectionUtils.isEmpty(bigCollection)) {
            // return empty list
        } else if (bigCollection.size() < maxBatchSize) {
            result.add(bigCollection);
        } else {
            for (int i = 0; (i + maxBatchSize) <= bigCollection.size(); i = i + maxBatchSize) {
                result.add(bigCollection.subList(i, i + maxBatchSize));
            }

            if (bigCollection.size() % maxBatchSize > 0) {
                result.add(bigCollection.subList((int) (bigCollection.size() / maxBatchSize) * maxBatchSize,
                    bigCollection.size()));
            }
        }

        return result;
    }

    public static void main(String[] args) {
        List<String> coll = new ArrayList<String>();
        coll.add("1");
        coll.add("2");
        coll.add("3");
        coll.add("4");
        coll.add("5");
        coll.add("6");
        coll.add("7");
        coll.add("8");

        System.out.println(split(coll, 2));
    }

Output - [[1, 2], [3, 4], [5, 6], [7, 8]]

According to me this code should break when loop runs the fifth time and it tries to perform sublist function.

Dude, there are only 4 steps (5 according to you)! Why don't you run it in the debugger? — dotvav
– dotvav, Commented Aug 17, 2015 at 8:11
your statement (i + maxBatchSize) <= bigCollection.size() check i for 2,4,6,8 and in the fifth run for 10 agains 8 and then stops. Whats wrong ? — MrT
– MrT, Commented Aug 17, 2015 at 8:14
If you can't easily look at your loop code and see how many time it runs it means you code is too complicated, try creating some intermediate variables with good names to make it easier to understand. — codebox
– codebox, Commented Aug 17, 2015 at 8:15

Dimitri Mockelyn · Accepted Answer · 2015-08-17 08:14:18Z

4

When you're at iteration 4, your loop condititoin is like this :

(i + maxBatchSize) <= bigCollection.size()
 6 + 2             <= 8

So you're going in. But the fifth iteration doesn't respect this condition anymore, because it's like this :

(i + maxBatchSize) <= bigCollection.size()
 8 + 2             <= 8

Don't forget that your condition isn't only on i but on i + maxBatchSize

answered Aug 17, 2015 at 8:14

Dimitri Mockelyn

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Tunaki · Accepted Answer · 2015-08-17 08:16:27Z

1

The following for loop

for (int i = 0; (i + maxBatchSize) <= bigCollection.size(); i = i + maxBatchSize) {
    result.add(bigCollection.subList(i, i + maxBatchSize));
}

goes from 0 to bigCollection.size() - maxBatchSize by steps of maxBatchSize. In your example, bigCollection has size 8 and maxBatchSize is 2, so the loop goes from 0 to 6 by steps of 2. In total, that makes 4 steps : 0, 2, 4 and 6.

The following if

if (bigCollection.size() % maxBatchSize > 0)

is not executed because 8 % 2 = 0 (so the subList is not performed).

answered Aug 17, 2015 at 8:16

Tunaki

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