Convert a list of array into an array of array

Question

I have a list like this:

List<MyObject[]> list= new LinkedList<MyObject[]>();

and on Object like this:

MyObject[][] myMatrix;

How can I assign the "list" to "myMatrix"?

I don't want to loop over the list and assign element by element to MyMatrix, but I want to assign it directly (with the oppurtune modifications) if possible. Thanks

Solution 1

You could use toArray(T[]).

import java.util.*;
public class Test{
    public static void main(String[] a){ 
        List<String[]> list=new ArrayList<String[]>();
        String[][] matrix=new String[list.size()][];
        matrix=list.toArray(matrix);
    }   
}

Javadoc

Solution 2

The following snippet shows a solution:

// create a linked list
List<String[]> arrays = new LinkedList<String[]>();

// add some trivial test data (note: arrays with different lengths)
arrays.add(new String[]{"a", "b", "c"});
arrays.add(new String[]{"d", "e", "f", "g"});

// convert the datastructure to a 2D array
String[][] matrix = arrays.toArray(new String[0][]);

// test output of the 2D array
for (String[] s:matrix)
  System.out.println(Arrays.toString(s));

Try it on ideone

Solution 3

Let us assume that we have a list of 'int' array.

List<int[]> list = new ArrayList();

Now to convert it into 2D array of type 'int', we use 'toArray()' method.

int result[][] = list.toArray(new int[list.size()][]);

We can generalize it further like-

List<T[]> list = new ArrayList();
T result[][] = list.toArray(new T[list.size()][]);

Here, T is the type of array.

Comments

[Lars]

stackoverflow.com...fill-a-array-with-list-data is a nearly a duplicate

[Fili]

With toArray() the editor says "incompatible types; found: array java.lang.Object[], required: array Item[][]". I tried to cast the result of toArray() but it give me a ClassCastException

[Nestor]

Did you try the toArray(T []) ?

[Harry Joy]

@Gressie: +1, good suggestion. By this @Fili might be able to do it.

[Fili]

It doesn't compile: "incompatible types; found: array MyObject[], required: array MyObject[][]"

[Nestor]

Actually, try it with a 0 by 0 matrix and it should still work ;)

[Nestor]

Clearer example of the solution

[MarcoS]

you don't need to know the size of the matrix: see for example my code here

[Fili]

I tried this solution even with some modification to the size of the matrix, but it doesn't compile :(

[Jeremy]

I knew I shouldn't have assumed modifying it real quick would break the compile. Fixed.

[Fili]

Thanks for the solution but it didn't compile. At end I solved with a loop.