Adding element in two dimensional ArrayList

outerList.get(0).set(1, 17);

with outerList being a List<List<Integer>>.

Remember that 2-dimensional arrays don't exist. They're in fact arrays or arrays.

 ArrayList<ArrayList<String>> data = new ArrayList<ArrayList<String>>();
 data.add(new ArrayList<String>());
 data.get(0).add("String");

ArrayList<ArrayList<String>> contains elements of type ArrayList<String>

Each element must be initialised

These elements contain elements of type String

To get back the String "String" in the 3-line example, you would use

String getValue = data.get(0).get(0);

the way i found best and convinient for me was to declare ur 2d arrayList and then also a nornal mono-dimension array.

ArrayList<ArrayList<String>> 2darraylist = new ArrayList<>();
ArrayList<String> 1darraylist=new ArrayList<>();

then fill the '1D'array list and later add the 1D to the 2D array list.

1darraylist.add("string data");
2darraylist.add(idarraylist);

this will work as long as your problem is simply to add to elements to the list. if u want to add them to specific positions in the list, the the .get().set(); is what u wanna stick to.

ArrayList<ArrayList<Integer>> FLCP = new ArrayList<ArrayList<Integer>>();
FLCP.add(new ArrayList<Integer>());
FLCP.get(0).add(new Integer(0));

Each element must be instantiated. Here the outer ArrayList has ArrayList element, and first you need to add an element to reference it using get method.

Some additional notes; after reading other answers and comments:

1> Each element must be instantiated; initialization is different from instantiation (refer to flexJavaMysql's answer)

2> In Java, 2-dimensional arrays do exist; C# doesn't have 2D arrays (refer to JB Nizet's answer)

String[] myList = {"a","b","c","d"};
ArrayList<ArrayList<String>> data = new ArrayList<ArrayList<String>>();
data.add(new ArrayList<String>());
int outerIndex =0;
int innerIndex =0;
for (int i =0; i<list.length; i++) {
     data.get(outerIndex).add(innerIndex, list[i]);
     innerIndex++;
 }
System.out.println(data);

Simple for loop to add data to a multidimensional Array.
For every outer index you need to add

data.add(new ArrayList<String>());

then increment the outer index, and reset the inner index.
That would look something like this.

public static String[] myList = {"a", "b","-","c","d","-","e","f","-"};

public static ArrayList<ArrayList<String>> splitList(String[] list) {

    ArrayList<ArrayList<String>> data = new ArrayList<ArrayList<String>>();
    data.add(new ArrayList<String>());
    int outerIndex =0;
    int innerIndex =0;
    for (int i=0; i<list.length; i++) {
        System.out.println("will add: " + list[i]);
        if(!list[i].contains("-")) {
            System.out.println("outerIndex: " + outerIndex +" innerIndex: "+ innerIndex);
            data.get(outerIndex).add(innerIndex, list[i]);
            innerIndex++;
        } else {
            outerIndex++;    // will move to next outerIndex
            innerIndex = 0;  // reset or you will be out of bounds
            if (i != list.length-1) {
                data.add(new ArrayList<String>());  // create an new outer index until your list is empty
            }
        }
   }  
    return data;
}
public static void main(String[] args) {
    System.out.println(splitList(myList));
}