Adding multiple Arraylist items into one ArrayList item

Question

I have two ArrayLists and want to put a range of 5 items into one item from one ArrayList into the other. My current code for that looks like this:

fragenArrayPack.add(fragenArray.get(0) + fragenArray.get(1) + fragenArray.get(2) + fragenArray.get(3) + fragenArray.get(4));
fragenArrayPack.add(fragenArray.get(5) + fragenArray.get(6) + fragenArray.get(7) + fragenArray.get(8) + fragenArray.get(9));

This works but is very impractical. Is there a faster/better way to do this?

Answer 1

Why you want to add them with "+". May be you will create constructor with 5 objects or simply add with for loop

Answer 2

You want something like that

public int sumRangeOfArrayList(int startIndex, int lastIndex, ArrayList<Integer> myNumberArrayList) {
    if (startIndex < 0 || lastIndex < 0 || startIndex > myNumberArrayList.size() - 1 || lastIndex > myNumberArrayList.size() - 1 || startIndex > lastIndex) {
        throw new RuntimeException("wrong input");
    }
    int sum = 0;
    for (int i = startIndex; i <= lastIndex; i++) {
        sum += myNumberArrayList.get(i);
    }
    return sum;
}

Beware I did not test my code, I have just put it together. But you want a method like that, which will calculate the sum and is reusable.

Answer 3

Assuming your lists contains Integer and you are using Java 8, you can use streams:

public static Integer addRange(ArrayList<Integer> list, Integer startingIndex, Integer amount) {
    return list.stream().skip(startingIndex).limit(amount).reduce(0, Integer::sum);
}

fragenArrayPack.add(addRange(fragenArray, 0, 5));
fragenArrayPack.add(addRange(fragenArray, 5, 5));

For String concatenation :

public static String concatRange(ArrayList<String> list, Integer startingIndex, Integer amount) {
    return list.stream().skip(startingIndex).limit(amount).reduce("", String::concat);
}

fragenArrayPack.add(concatRange(fragenArray, 0, 5));
fragenArrayPack.add(concatRange(fragenArray, 5, 5));

And if you want you can parameterize the function to pass to the reduce method and made the method generic