How to make ArrayList that work as two dimentional array in java?

Question

I want to make arrayList object in java that work as two dimentional array. My question is how can we access value from specific dimention from arrayList.

in two diment

Answer 1

An arraylist is not an object to make a 2 dimentional arrays. However you can use it anyway : You can use :

new ArrayList<ArrayList<Object>>; //or
new ArrayList<Object[]>; 

But you should implement your own matrix class because you will probably have some check to do and a function get(int row, int column) would be cool

Answer 2

The solution like List<List<..>> is slow then you should use one dimention array like

    // Two dimentions: m and n

        List<String> arr = new ArrayList<String>(m*n);
        for (int i=0; i< m; ++i) {
           for (int j=0; j<n; ++j) {
             String str=arr.get(i*n + j);
             //You code here
           }
        }

Answer 3

Also consider Table collection provided by Google Guava library. ArrayTable is an implementation based on 2D array.

Answer 4

Memory is an important consideration here.

It can be acceptable to model a 2D (or higher dimension) array using a 1D container. (This is how the VARIANT SAFEARRAY of Microsoft's COM works.) But, consider this carefully if the number of elements is large; especially if the container allocates a contiguous memory block. Using something like List<List<... will model a jagged-edged matrix and can fragment your memory.

With the 1D approach, you can use the get(index) method on the ArrayList appropriately transformed:

Given the (i)th row and (j)th column, transform using index = i * rows + j where rows is the number of rows in your matrix.

Answer 5

You mean something like a List in a List??

May be something like...

List<List<...>> twoDList = new ArrayList<>();

i want to make a List, in which each List key contains another List inside it

It should more like you want some kind of Map, which is basically a key/value pair.

Map<String, List<String>> mapValues = new HashMap<>(25);
List<String> listOfValues = ...;
//...
mapValues.put("A unique key for this list", listOfValues);

//...

List<String> thatListOfValues = mapValues.get("A unique key for this list");

Answer 6

List<List<Integer>> list = new ArrayList<List<Integer>>();
list.add(new ArrayList<Integer>());
list.add(new ArrayList<Integer>());
list.get(0).add(5);
list.get(1).add(6);

for(List<Integer> listiter : list)
{
    for(Integer integer : listiter)
    {
        System.out.println("" + integer);
    }
}

This way you can get the items like

list.get(1).get(0); //second dimension list -> integer

EDIT:

Although it is true that you can use a Map if you are trying to use numeric indices for example for each list, like so:

Map<Integer, List<YourObject>> map = new HashMap<Integer, List<YourObject>>();
map.put(0, new ArrayList<YourObject>());
map.put(5, new ArrayList<YourObject>());
map.get(0).add(new YourObject("Hello"));
map.get(5).add(new YourObject("World"));

for(Integer integer : map.keySet())
{
    for(YourObject yourObject : map.get(integer))
    {
         yourObject.print(); //example method
         System.out.println(" ");
    }
}

Although even then the accessing of Lists would be the same as before,

map.get(0).get(1); //List -> value at index

Obviously you don't need to use Integers as the generic type parameter, that's just a placeholder type.