Flatten nested arrays. (Java)

Question

I'm struggling to create the right logic to flatten an array. I essentially want to duplicate parent rows for each child item in a nested array. The number of nested arrays could vary. I've been creating Java lists bc I find them easy to work with, but open to any solution. The nature of this problem is I'm starting with some nested JSON that I want to convert into a flat csv to load into a database table. Thanks for the help.

Example:

[1,2,[A,B,[Cat,Dog]],3]

I've created the above as a List. Each item is either a string or another List.

Result:

[1,2,A,Cat,3],
[1,2,A,Dog,3],
[1,2,B,Cat,3],
[1,2,B,Dog,3]

Here's what I have so far. Obviously not working.

private static List<List<String>> processData(List<String> row, List<Object> data, List<List<String>> rowList) {
    List<List<String>> tempRowList = new ArrayList<List<String>>();
    for (Object i : data) {
        if (i instanceof List<?>) {
            flattenArray((List<Object>) i, row, rowList);
        } else {
            for (List<String> r : rowList) {
                r.add(i.toString()); //add item to all rows
            }
        }
    }
    return rowList;

private static void flattenArray(List<Object> arrayObject, List<String> rowToCopy, List<List<String>> rowList) {

    for (Object x : arrayObject) {

        if (x instanceof List<?>) {
            for (List<String> t : rowList) {
                flattenArray((List<Object>) x, t, rowList);
            }

        } else {
            List<String> newRow = new ArrayList<String>(rowToCopy); //copy row

            List<Object> t = new ArrayList<Object>();
            t.add(x);

            List<List<String>> innerRowList = new ArrayList<List<String>>();
            innerRowList.add(newRow);

            processData(newRow, t, innerRowList); //pass in new copied row. object to add, 
            rowList.add(newRow);

        }

    }
    rowList.remove(rowToCopy);
}

And i set everything up like this.

public static void main(String[] args) {

    List<Object> data = new ArrayList<Object>();
    List<List<String>> rowList = new ArrayList<List<String>>();

    data.add("1");
    data.add("2");

    List<Object> l1 = new ArrayList<Object>();
    l1.add("A");
    l1.add("B");        
    List<Object> l2 = new ArrayList<Object>();
    l2.add("dog");
    l2.add("cat");
    l1.add(l2);
    data.add(l1);

    data.add("3");

    List<String> r0 = new ArrayList<String>();
    rowList.add(r0);
    System.out.println(data);
    rowList = processData(r0, data, rowList);
    System.out.println(rowList);
}

ok, added my code. I feel like I've way over complicated this. — Andrew
– Andrew, Commented Oct 24, 2016 at 13:02
That's fine, now we can help go through what problems you have — Ash
– Ash, Commented Oct 24, 2016 at 13:08
[1,2,[A,B,[Cat,Dog]],3] - will [1,2,[A,B],[Cat,Dog],3] result in exactly same output? Looks quite arbitrary that it behaves same way. — Artur Biesiadowski
– Artur Biesiadowski, Commented Oct 24, 2016 at 14:11
yes, both of those should yield the same output. Essentially 4 arrays. — Andrew
– Andrew, Commented Oct 24, 2016 at 14:55

GROX13 · Accepted Answer · 2016-10-24 14:02:53Z

2

I think this should work for you if you're using Java 8:

    List<Object> a = new ArrayList<>();
    List<Object> a1 = new ArrayList<>();
    a1.add("v");
    a1.add("w");
    List<Object> a2 = new ArrayList<>();
    a2.add("ww");
    a.add("a");
    a.add("b");
    a.add("c");
    a.add("d");
    a.add(a1);
    a.add(a2);
    List<Object> b = new ArrayList<>();

    a.stream().flatMap(x -> x instanceof String ? Stream.of(x) : ((List) x).stream()).forEach(b::add);

    System.out.println(a);
    System.out.println(b);

b list will contain flattened a list. Output is:

[a, b, c, d, [v, w], [ww]]
[a, b, c, d, v, w, ww]

edited Oct 24, 2016 at 14:02

answered Oct 24, 2016 at 13:12

GROX13

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4 Comments

Andrew Over a year ago

I am using Java 8. I've only played around with streams a little bit. Was curious if they could help me here.

Andrew Over a year ago

I'm not sure how to use them though based on your example.

GROX13 Over a year ago

Andrew this should work. I know it's ugly, I'll try to prettify it ASAP.

Andrew Over a year ago

that seems to flatten it all into one level. I want an array for each item in the top level. See example and result at the top.

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