I tried @Shant Dashjian's solution but I am getting the following exception.

"main" java.lang.reflect.InaccessibleObjectException: Unable to make field transient java.lang.Object[] java.util.ArrayList.elementData accessible: module java.base does not "opens java.util" to unnamed module @2a139a55

Here is my piece of code, could you please help me?

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

public class ArrayListExample {
    public static void main(String[] args) throws Exception  {
        List l = new ArrayList<>();
        l.add(1);
        System.out.println(cap(l));
        
    }
    public static int cap(List a) throws Exception {
        Field f = ArrayList.class.getDeclaredField("elementData");
        f.setAccessible(true);
        return  (( Object[] ) f.get(a)).length;
    }
}