"Argument is not an array" exception when setting array value using Java reflection

Question

I need to create an array at running time according to the values of the input array tableParameter.

An example of my code is as follows:

int[] tableParameter = new int[dimension + 1];
tableParameter[0] = N;
for(int i = 1; i < tableParameter.length; i++)
    tableParameter[i] = i;
Object myArray = Array.newInstance(int.class, tableParameter);
//set the index 1 of the array with the value 100
Array.setInt(myArray, 1, 100);

The exception occurs in the last line of the code above:

Exception in thread "main" java.lang.IllegalArgumentException: Argument is not an array

But when I used the

System.out.println(myArray.getClass().getCanonicalName());

to verify the class of myArray, it prints out int[][][][], which means myArray is definitely an array type.

So why the JVM throws the exception that myArray is not an array?

Answer 1

It may be a misleading exception, but it's definitely correct to complain. If the array is an int[][][][], it doesn't make sense to set the element at index 1 to value 100. For example, this won't compile:

int[][][][] array = new int[1][1][1][1];
array[0] = 100; // Nope...

You can only set an element to an int if it's an actual int[]. So if dimension is 0 (meaning you end up with a 1-dimensional array) and if N is 2 or more, the code works fine. To take reflection out of the creation part:

import java.lang.reflect.Array;

class Test {
    public static void main(String[] args) throws Exception {
        int[] array1 = new int[10];
        Array.setInt(array1, 1, 100); // Works fine

        int[][] array2 = new int[10][10];
        Array.setInt(array2, 1, 100); // Throws IllegalArgumentException
    }
}