Is it possible to dynamically build a multi-dimensional array in Java?

Question

Suppose we have the Java code:

Object arr = Array.newInstance(Array.class, 5); 

Would that run? As a further note, what if we were to try something like this:

Object arr1 = Array.newInstance(Array.class, 2); Object arr2 = Array.newInstance(String.class, 4); Object arr3 = Array.newInstance(String.class, 4); Array.set(arr1, 0, arr2); Array.set(arr1, 1, arr3); 

Would arr1 then be a 2D array equivalent to:

String[2][4] arr1; 

How about this: what if we don't know the dimensions of this array until runtime?

Edit: if this helps (I'm sure it would...) we're trying to parse an array of unknown dimensions from a String of the form

[value1, value2, ...] 

or

[ [value11, value12, ...] [value21, value22, ...] ...] 

And so on

Edit2: In case someone as stupid as I am tries this junk, here's a version that at least compiles and runs. Whether or not the logic is sound is another question entirely...

Object arr1 = Array.newInstance(Object.class, x); Object arr11 = Array.newInstance(Object.class, y); Object arr12 = Array.newInstance(Object.class, y); ... Object arr1x = Array.newInstance(Object.class, y); Array.set(arr1, 0, arr11); Array.set(arr1, 1, arr12); ... Array.set(arr1, x-1, arr1x); 

And so on. It just has to be a giant nested array of Objects

Answer 1

It is actually possible to do in java. (I'm a bit surprised I must say.)

Disclaimer; I never ever want to see this code anywhere else than as an answer to this question. I strongly encourage you to use Lists.

import java.lang.reflect.Array; import java.util.*;  public class Test {      public static int[] tail(int[] arr) {         return Arrays.copyOfRange(arr, 1, arr.length);     }      public static void setValue(Object array, String value, int... indecies) {         if (indecies.length == 1)             ((String[]) array)[indecies[0]] = value;         else             setValue(Array.get(array, indecies[0]), value, tail(indecies));     }      public static void fillWithSomeValues(Object array, String v, int... sizes) {         for (int i = 0; i < sizes[0]; i++)             if (sizes.length == 1)                 ((String[]) array)[i] = v + i;             else                 fillWithSomeValues(Array.get(array, i), v + i, tail(sizes));     }      public static void main(String[] args) {          // Randomly choose number of dimensions (1, 2 or 3) at runtime.         Random r = new Random();         int dims = 1 + r.nextInt(3);          // Randomly choose array lengths (1, 2 or 3) at runtime.         int[] sizes = new int[dims];         for (int i = 0; i < sizes.length; i++)             sizes[i] = 1 + r.nextInt(3);          // Create array         System.out.println("Creating array with dimensions / sizes: " +                 Arrays.toString(sizes).replaceAll(", ", "]["));         Object multiDimArray = Array.newInstance(String.class, sizes);          // Fill with some          fillWithSomeValues(multiDimArray, "pos ", sizes);          System.out.println(Arrays.deepToString((Object[]) multiDimArray));       } } 

Example Output:

Creating array with dimensions / sizes: [2][3][2] [[[pos 000, pos 001], [pos 010, pos 011], [pos 020, pos 021]],  [[pos 100, pos 101], [pos 110, pos 111], [pos 120, pos 121]]]