First of all, be very careful not to confuse conversion with casting. They may share the surface syntax, but are very different processes.
In Java you can downcast an Object to any type, but at runtime you'll get a ClassCastException if the object is not in fact compatible with the target type. This happens at the bytecode level: there is a bytecode instruction dedicated to downcasting.
Child c = (Child) new Object();
will unconditionally result in a ClassCastException.
Double d = 3.3; // note: no explicit casting needed
will perform autoboxing into an instance of Double. So here, a new instance is actually created.
A normal, successful dowcast may look like this:
Object o = "a";
String s = (String)o;
Here, no objects are created: only the value of o is copied into s. The value is a reference.
