java arraylist of arraylists

Question

hi when I wanted to have something like vector of vectors (elements shall be ordered of course) I thought of having arraylist of arraylists but in C++ i would do this to access the element v.at(i).at(j)=5; and when i need to add new element v.at(i).push_back(value); so how do i do this in java ? because I can only access the outer arraylist but i don't know how to add new elements...and is there a better way to simulate C++ vector of vectors in java ??

ps (it's not a matrix 3*2 for ex but each arraylist may have different size)

Answer 1

Declare an ArrayList of ArraList<Integer> s:

ArrayList<ArrayList<Integer>> v = new ArrayList<ArrayList<Integer>>();
v.add(new ArrayList<Integer>());

v.get(0).add(new Integer(5));
v.get(0).add(new Integer(10));
System.out.println(v.get(0).get(0)); // => 5
System.out.println(v.get(0).get(1)); // => 10

Answer 2

List<List<Integer>> l = new ArrayList<List<Integer>>();

// initialize the inner lists
for (int i = 0; i < 10; i++) 
    l.add(new ArrayList<Integer>());


// now you can use it as you would like
l.get(i).add(5);
l.get(i).set(0, 3);

Answer 3

You can have List<List<Integer>> in Java. accessing the element would be list.get(i).get(j) . Adding, likewise: list.get(i).add(var) . Also take a look at the set(..) method

Answer 4

I can't comment on previous comments yet so i'll post it here.

As netbeans says it's not necessary to write

ArrayList< ArrayList< Integer>> v = new ArrayList< ArrayList< Integer>>();

because the arguments are redundant; instead you can write:

ArrayList< ArrayList< Integer>> v = new ArrayList<>();