mapping an arraylist to another arraylist in java

Well, you currently have several problems which are probably confusing the situation:

1. There is no such class ArrayList<string>, I guess you mean List<string>
2. Currently your lists consist of a single element, which is a comma / space delimited string. You probably want something more like this:

List fruit = new List(new string[] {"apple", "grape", "banana" });
List nut = new List(new string[] {"pistachio", "chestnut", "walnut", "peanut" });
List vegetable = new List(new string[] {"broccoli", "carrot", "cabbage", "tomato" });

This gives you a list where each element is a nut, fruit or vegetable respectively.

Also your second list should probably look more like this:

List<int[]> combinations = new List<int[]>(
    new int[][]
    {
        new int[] {1, 1, 2},
        new int[] {2, 1, 2},
        new int[] {2, 3, 4},
        new int[] {3, 4, 4},
    });

I.e. conbinations is a list of combinations, where each combination consists of 3 integers - the index of each element in the list. (This is possibly a tad confusing and by no means the only option - ask if this bit isn't clear).

In face as arrays are 0-indexed in c#, in fact you probably want this instead:

List<int[]> combinations = new List<int[]>(
    new int[][]
    {
        new int[] {0, 0, 1},
        new int[] {1, 0, 1},
        new int[] {1, 2, 3},
        new int[] {2, 3, 3},
    });

This at least makes your data easier to work with, so the only questions remaining are:

• How do you get from what you have to the above? (I'll let you have a go at that yourself).
• What is it that you are trying to do?

Try Below code it works as expected let me know it it does not fulfill use case.

public static List<String> fruits = new ArrayList<String>();
public static List<String> nuts = new ArrayList<String>();
public static List<String> vegitables = new ArrayList<String>();

/**
 * @param args
 * @throws ParseException
 * @author Rais.Alam
 */
public static void main(String[] args) throws ParseException
{

    fruits.add("apple");
    fruits.add("grape");
    fruits.add("banana");

    nuts.add("pistachio");
    nuts.add("chestnut");
    nuts.add("walnut");
    nuts.add("peanut");

    vegitables.add("broccoli");
    vegitables.add("carrot");
    vegitables.add("cabbage");
    vegitables.add("tomato");

    System.out.println(getValue("1:1:2"));
    System.out.println(getValue("2:1:2"));
    System.out.println(getValue("2:3:4"));
    System.out.println(getValue("3:4:4"));

}

public static String getValue(String key)
{
    String returnString = "";
    String[] arr = key.split(":");
    returnString += fruits.get(Integer.parseInt(arr[0]) - 1) == null ? "" : fruits.get(Integer.parseInt(arr[0]) - 1) + ":";
    returnString += nuts.get(Integer.parseInt(arr[1]) - 1) == null ? "" : nuts.get(Integer.parseInt(arr[1]) - 1) + ":";
    returnString += vegitables.get(Integer.parseInt(arr[2]) - 1) == null ? "" : vegitables.get(Integer.parseInt(arr[2]) - 1);

    return returnString;

}

After running the program you will get below output

apple:pistachio:carrot
grape:pistachio:carrot
grape:walnut:tomato
banana:peanut:tomato