How can I put a Java array inside itself?

Question

I'm attempting to create a Java object array and place the array inside itself at its second index (in order to represent a self-similar fractal with the array), but when I try to access theArray[1][1][0], I get this error:

Main.java:11: error: array required, but Object found.

This is what I've tried so far, and I'm not sure why it's not working:

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

class Main
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Object[] theArray = new Object[2];
        theArray[0] = "This array should contain itself at its second index.";
        theArray[1] = theArray; //Now I'm attempting to put the array into itself.
        System.out.println(theArray[1][1][0]) //Main.java:11: error: array required, but Object found
    }
}

Is it actually possible to put a Java array inside itself, as I'm attempting to do here?

Answer 1

theArray[1] is of compile-time type Object (since it comes from an array of Objects).

You need to cast it to Object[] to use it as an array.

---

The fundamental problem you're encountering is that although an array that contains itself is a perfectly valid object, it isn't a valid type.

You can nest array types arbitrarily deeply – Object[][][][][][][][][][][][][] is a valid type.
However, the "bottom level" of the type can't be an array.

You're trying to create a type which is an array of itself.
Using generics, that would be possible:

class Evil extends ArrayList<Evil> { }

Answer 2

You're running into a casting error since you've declared theArray to be an Array of Objects. As a result, you can't promise Java that theArray[1] is an Array--it could be any kind of Object. You'll need to break up your access to do what you want:

Object[] innerArray = (Object[]) theArray[1];
System.out.println(innerArray[0] == theArray[0]); // Always true since innerArray IS theArray
while (true) {
    // Careful! This loops forever!
    // set innerArray = innerArray[1] = theArray = theArray[1] = innerArray... 
    // all of these are the exact same object (but you have to tell Java their type every time)
    innerArray = (Object[]) innerArray[1]; 
    System.out.println(innerArray[0]);
}