How to directly cast array of Objects to array of Integer without modifications

Question

I have an Objects array filled up with Strings:

Object[] array = new Object[count];
for (int i = 0; i < array.length; i++) {
  array[i] = String.valueOf(i);
}

Then I refill it with Integers:

for (int i = 0; i < array.length; i++) {
  array[i] = Integer.parseInt(array[i]);
}

I want to return an object of type Integer[]. When I do:

return (Integer[]) array;

or

return Integer[].class.cast(array);

I get Exception in thread "main" java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Ljava.lang.Integer; ([Ljava.lang.Object; and [Ljava.lang.Integer; are in module java.base of loader 'bootstrap').

I don't want to use streams, copying array or any other modifications or transformations. Just direct casting, maybe even via reflection.

Are there any options?

Answer 1

It's not an array, but you can do this with a generic list and unchecked casting using a wildcard list: List<?>. You can even cast it when it contains objects of different types, it'll throw a ClassCastException when you try to access them as the generic type.

ArrayList<Object> arr = new ArrayList<Object>();
arr.add(1);
arr.add("xyz");

@SuppressWarnings("unchecked")
ArrayList<Integer> casted = (ArrayList<Integer>) (ArrayList<?>) arr;

System.out.println(casted.get(0)); // Prints 1
System.out.println(casted.get(1)); // Prints "xyz"
        
Integer first = casted.get(0); // Works!
// Integer second = casted.get(1); // Throws ClassCastException at runtime
// String secondStr = casted.get(1); // Doesn't compile, 'incompatible types'
String secondStrGood = (String) (Object) casted.get(1); // Works!

This is because ArrayLists just use an Object[] array instead of a generic T[] array, and casts on get (sort of...)

---

If you try to be sneaky and retrieve the internal array backing an ArrayList through reflection, no dice there. It's just a normal Object[] array as previously mentioned, so you get the same casting issue.

Field privateField = casted.getClass().getDeclaredField("elementData"); 
privateField.setAccessible(true); 

// java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;
Integer[] array = (Integer[]) privateField.get(casted); 

---

If you try to be even sneakier and try this trick with a generic class instead of a List<?>, it still doesn't work.

public class Test<T> {
    public T[] array;
}
    
public static void main(String args[]) {
    Object[] objArray = new Object[] {1, "string"};

    Test<Object> test = new Test<Object>();
    test.array = objArray;
        
    @SuppressWarnings("unchecked")
    Test<Integer> testInteger = (Test<Integer>) (Test<?>) test;

    // java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;
    Integer[] intArray = testInteger.array;
}

---

TL;DR: best you can do is with Collections