Java: How to initialize a fixed length List of List?

Question

I have a list of String[] that contains some data returned from a database

List<String[]> x;

Now for each "x" I have to store its relative Ys. (Imagine that "x" contains the elements returned from a "SELECT DISTINCT xColumn1, xColumn2 FROM table" and now for each element I have to store the data from another query).

Y's elements are going to be stored here:

List<List<String[]>> yElements;

I've read that I can declare a fixed length list, but I don't know how. This is what I have done:

yElements = Arrays.asList(new List<String[]>[x.sixe()]);

Netbeans told me "generic array creation" error.

I also want to tell you that I MUST have a fixed lenght list to store a List of String[ ] in a specific index of this List of Lists. (If you can show me how to do with an array it would be great, too!) Thanks.

Looks like you'd be better of with a bean to keep things together: `List` with `class XandY { String xColumn1; String xColumn2; Something y;}` — Thilo, Nov 25 '12 at 02:01

score 0 · Answer 1 · edited May 23 '17 at 11:48

You cannot create an instance of an array of a generic type using new.

One alternative is this:

import java.lang.reflect.*;
import java.util.*;

public class Test
{
    public static void main( String args[] )
    {
        List<String[]> x = ...;
        List<List<String[]>> yElements = Arrays.asList(
                (List<String[]>[])Array.newInstance(List.class, x.size()));
    }
}

Related question: How to create a generic array in Java?. (The example there is about creating an instance of an array of a generic type parameter ... but the same approach applies here.)

However, I think that this whole question is based on an incorrect assumption.

I also want to tell you that I MUST have a fixed lenght list to store a List of String[ ] in a specific index of this List of Lists.

You don't HAVE TO have a fixed sized list. From the computational perspective, your code would work just fine with a non-fixed sized list ... provided that you don't add or remove list elements. (In fact, using Arrays.asList to wrap an array won't stop some other code trying to add / remove elements ...) Anyway ... if you just make the implementation type ArrayList<ArrayList<ArrayList<String>>>, then the generic array creation problem goes away.

In addition, I suspect that it is incorrect to use x.size() as the size of yElements. The size of yElements probably should be determined by the number of x instances there are going to be, not the size of a given x instance.

I "have to" because of my implementation in threads and, yes, x.size() is the correct length but I have to admit that probably I was not so much clear to describe my problem :) BTW, Thank you for your time. The first part of your comment is very interesting! — Immortal, Nov 25 '12 at 23:15

score 0 · Accepted Answer · answered Nov 25 '12 at 02:58

If you want your lists to be absolutely fixed size, you could use the native arrays. They can be multi-dimensional so for example you can have String[][] x or `String[][][] y'.

Honestly however, your approach is a bit confusing and not that crisp from a design point of view.

Why not, similarly to as was suggested in the comment, have an object which has both columns (xColumn1, xColumn2), and then have the Y elements in a separate object, which can then be associated with the first one?

so:

class XDetails
{
   String xColumn1;
   String xColumn2;

   YDetails relatedY;
   ...
}

class YDetails
{
   ... fields of Y ...

}

Then you can have an array or List<XDetails>

This is what I've done: `class A { String[] xColumn; private Yelements[] yElements; } class Yelements { List yElements; } ` Now works like a charm. Thank you! — Immortal, Nov 25 '12 at 23:21

Java: How to initialize a fixed length List of List?

2 Answers2