Write a method that merges two array lists, alternating elements from both array lists

Iterators seem to do the trick most easily

public static <T> ArrayList<T> merge(Collection<T> a, Collection<T> b) {
    Iterator<T> itA = a.iterator();
    Iterator<T> itB = b.iterator();
    ArrayList<T> result = new ArrayList<T>();

    while (itA.hasNext() || itB.hasNext()) {
        if (itA.hasNext()) result.add(itA.next());
        if (itB.hasNext()) result.add(itB.next());
    }

    return result;
}

Without iterators:

public static <T> ArrayList<T> merge(List<T> a, List<T> b) {
    ArrayList<T> result = new ArrayList<T>();
    int size = Math.max(a.size(), b.size());

    for (int i = 0; i < size; i++) {
        if (i < a.size()) result.add(a.get(i));
        if (i < b.size()) result.add(b.get(i));
    }

    return result;
}

Note, I've relaxed the method signature a bit. If you're implementing the merging using iterators, Collection (or even Iterable) will do. Otherwise, List will do. There is no need to require ArrayList as a method argument type

Without Iterator:

public static ArrayList merge(ArrayList a, ArrayList b) {
    int c1 = 0, c2 = 0;
    ArrayList<Integer> res = new ArrayList<Integer>();

    while(c1 < a.size() || c2 < b.size()) {
        if(c1 < a.size())
            res.add((Integer) a.get(c1++));
        if(c2 < b.size())
            res.add((Integer) b.get(c2++));
    }
    return res;
}

Try this:I implemented using Array.

public static void main(String[] args) {
    int[] first = { 1, 4, 9, 16 };
    int[] second = { 9, 7, 4, 9, 11 };
    int[] merge = new int[first.length + second.length];
    int j = 0, k = 0, l = 0;
    int max = Math.max(first.length, second.length);
    for (int i = 0; i < max; i++) {
        if (j < first.length)
            merge[l++] = first[j++];
        if (k < second.length)
            merge[l++] = second[k++];
    }
    System.out.println(Arrays.toString(merge));
}

Output:

[1, 9, 4, 7, 9, 4, 16, 9, 11]