Question

I have many Arraylists having String objects , and I have a requirement to concatenate there values.
Eg:

ArrayList finalList = new ArrayList();
ArrayList catMe = new ArrayList();
ArrayList x = new ArrayList();
x.add("Green");
x.add("Red");
ArrayList y = new ArrayList();
y.add(" Apple");

//......

catMe.add(x);
catMe.add(y);

concatContents(catMe);  // Here i need to do 
                        // some concatenation magic.

so when finalList is printed:

finalList.get(0) // should show > "Green Apple"
finalList.get(1) // should show >  "Red Apple"

I know it looks easy if there are only two list X and Y… but I need it for n dimensions. Say if there is 3rd list

ArrayList z= new ArrayList();
z.add(" USA");
z.add(" Canada");
catMe.add(z);
concatContents(catMe);

Now finalList should show

Green Apple USA
Green Apple Canada
Red Apple USA
Red Apple Canada

Do i need recursion? Unable to think how to implement though! Do any java master there have a solution?

Answer 1

Here’s a recursive answer. Just cooked it up, so no guarantees on quality… 🙂

public ArrayList<String> concatLists(ArrayList<ArrayList<String>> list) {
    ArrayList<String> catStrs = new ArrayList<String>();
    int len = list.size();
    if (len == 1) {
        catStrs.addAll(list.get(0));
        return catStrs;
    }
    ArrayList<String> myStrs = list.get(0);
    ArrayList<ArrayList<String>> strs = new ArrayList<ArrayList<String>>();
    strs.addAll(list.subList(1, len));
    ArrayList<String> retStrs = concatLists(strs);
    for (String str : myStrs) {
        for (String retStr : retStrs) {
            catStrs.add(str+retStr);
        }
    }
    return catStrs;
}