Parsing Nested Array String

Parsing a nested-array to such a String is rather simple:

Arrays.deepToString(array).replace(" ", "");

Converting this String to an infinitely nested-list is a bit more tricky. The easiest solution would probably be to use recursion:

/**
 * The following code is only for demonstration purposes.
 * It does neither do any validation on the input String 
 * nor work with more than one digit numbers.
 */

static int index = 0; // the position in the String

Object buildList(String nestedList) {
    List<Object> list = new ArrayList<>();

    while (index < nestedList.length()) {
        char c = nestedList.charAt(index++);

        if (c == '[') // add a sub-list via a recursive call
                list.add(buildList(nestedList));
        else if (c == ']') // stop building the list
                 break;
        else if (c == ',') {} // do nothing
        else // add an element to the list
            list.add(c);
    }

    return list;
}

Example call:

System.out.println(buildList("[3,[4,3],5]")); // prints [3, [4, 3], 5]

Note:

Even though the above code does what (at least I think) you want to achieve, it may not be advisable to use such a data-structure in practice, since the access to the nested-lists is quite complicated and involves some casting.

A better solution would probably be to use some kind of tree-datastructure where each node has a list of values including links to other nodes. (see: http://en.wikipedia.org/wiki/Tree_(data_structure))