If something is an instance of a class why then cast it into that class?

If you look at the method in the challenge, it says something to the effect of:

public String methodName(Object obj) { ...

this is indicating that the method accepts any Object as an argument. Everything in java is an object, it's the highest and least specific class in the language, so it doesn't really tell us much about the input going into the method. For the code we're using, we're assuming that either a string or blogpost is coming into the method, but methods can be used in different ways later, and the compiler knows that, so it won't do anything to an argument that could be anything without us promising what it can expect to get. I think this is a big reason for casting, to narrow down a vague data type of a higher class, so the compiler can know what to expect. Just because we know we'll be passing strings and blogposts to the method doesn't mean the compiler knows. It needs specifics. thus we cast the object into a more specific data type so the compiler knows what it's dealing with, like this:

String title = (String) obj;

(String) is there to let the compiler know that we can now assume obj is a String. Notice that we couldn't before, and since it could be a String or BlogPost, we need some if statements using instanceof to determine what data type obj is before we cast it appropriately. Also remember to take scope into account, as if you declare title in a scope we can't return outside of an if test, you'll get a nullpointerexception error, I believe. This is why we often declare our variables outside of the operation we initialize them, so they have a wider scope. This looks sort of like this:

String title;
    if(obj instanceof String) {
       title = (String) obj;
    }
return title;

this allows us to manipulate the variable outside of the scope of the if statement. Also, please remember that the compiler is dumb, so initialize the variable to a default value that tells us something about what could possibly have gone wrong if we didn't get a string or a blogpost, such as:

String title = "Bad Data type";

put all these pieces together and do it for BlogPost, and you should be golden. Happy coding!

Nicolas