Call toString() when convert String to Object [duplicate]

Question

Here is my example code:

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

I found the source code of Object, and the toString() method is:

public String toString() {   
   return getClass().getName() + "@" + Integer.toHexString(hashCode());
} 

I thought the result of the example shound be the address of this Object, like [B@15db9742 , after I convert str to Object , but it still print hello. Why? Shoundn't obj use the method of Object? Can Anyone explain the principle of it to me?

Answer 1

This is polymorphism (specifically, runtime polymorphism). It doesn't matter what the type of your reference to the object is (Object or String), as long as that type has toString (so your code compiles), the toString that will be used is the one the object itself actually has, not necessarily the one provided by the type of your reference. In this case, the object is a String no matter what the type of your reference to it is, so String#toString is used.