Method overriding and object casting in java

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Java

<pre>class Alpha
{
    public String doStuff(String msg)
    {return msg;}
}
class Beta extends Alpha
{
    public String doStuff(String msg)
    { return msg.replace('a','e');}
}
public class Main extends Beta
{ public String doStuff(String msg)
{
    return msg.substring(2);
}
  public static void main (String[]args)
  {
    List <Alpha> strs=new ArrayList<>();
    strs.add(new Alpha());
    strs.add(new Beta());
    strs.add(new Main());
    for(Alpha t:strs)
    {    
        System.out.println(t.doStuff("Java"));
    }
}
}

output
Java
Jeve
va

What I have tried:

my understanding is that :

In the for loop,
first iteration,
the anonymous object of Alpha and object 't' of Alpha will be pointing to the same location so we get the output as Java

but in the second iteration,
the properties of the anonymous object of Beta will be assigned to 't' which I expected will throw an error but the program produces the output.

Could anyone explain what's happening in the second and third iterations ?