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)
3 Answers
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).
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 ) );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
}
