Are reference objects with different types any different in Java?

Question

When saying:

  String str = "hello";
  Object obj = str;
  System.out.println(str==obj);

The result is true, because it points to the same objects in memory, which makes sense. But if I say:

obj.indexOf("h");

Or any subclass method, I get "cannot find symbol". They're still pointing to the same object, so what's going on during compile-time that makes reference objects of different types different from each other?

Answer 1

The Object type reference only knows about methods that are part of its public interface.

You have to cast if you know that Object reference is a String type:

int index = ((String) obj).indexOf("h");

Answer 2

Apples and Pears.

The identity check == is performed at runtime . And valid and fine.

The construct obj.indexOf... is a compile time error as class Object just does not have a method indexOf

If you tell the compiler (by means of casting) that obj contains a String, you can get valid code

((String)obj).indexOf("h");

Will compile again