How do I initialize a two-dimensional List statically?

Question

How can I initialize a multidimensional List statically?

This works:

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

But I'd like to init the list with some static lists like: (1,2,3), (4,5,6) and (7,8,9)

Answer 1

This is an old answer, but things have changed a bit. For java 9+ this can be done using the List.of() method which returns an immutable List which is a subclass of AbstractImmutableList.

import java.util.List;

List<List<Integer>> list = List.of(
                               List.of(1, 2, 3),
                               List.of(4, 5, 6),
                               List.of(7, 8, 9)
                           );

For older version of java or if one needs a mutable List the old answer still works:

2011 answer

If you create a helper method, the code looks a bit nicer. For example

public class Collections {
    public static <T> List<T> asList(T ... items) {
        List<T> list = new ArrayList<T>();
        for (T item : items) {
            list.add(item);
        }

        return list;
    }
}

and then you can do (with a static import)

List<List<Integer>> list = asList(
                             asList(1,2,3),
                             asList(4,5,6),
                             asList(7,8,9),
                           );

Why I don't use Arrays.asList()

Arrays.asList() returns a class of type java.util.Arrays.ArrayList (it's an inner class of Arrays). The problem I've found is that it's VERY easy to think that one is using a java.lang.ArrayList, but their implementation are very, very different.

The comment above is also old, for java 9+ use List.of(E...elem).

Answer 2

You can do it this way:

import static java.util.Arrays.*;

...

List<List<Integer>> list = asList(
    asList( 1, 2, 3 ),
    asList( 4, 5, 6 ),
    asList( 6, 7, 8 ) );

Answer 3

You can do so by adding a static block in your code.

private static List<List<Integer>> list = new ArrayList<List<Integer>>();
static {
  List<Integer> innerList = new ArrayList<Integer>(3);
  innerList.add(1);
  innerList.add(2);
  innerList.add(3);
  list.add(innerList);
  //repeat
}