Functioning of intsanceof in this code.

You are correct. In your example, obj is not an instance of Mountain. If we use just your code.. obj would be null.

But if we work within the context of the lesson you just completed.. and we have a method that takes an Object as a parameter.. instanceof would work fine. Consider the following code:

Mountain mountain = new Mountain();
System.out.println(isMountain(mountain));

public boolean isMountain(Object obj) {
    if(obj instanceof Mountain) return true;
    else return false;
}

In this case.. the return result from isMountain would be true.

If i took your example code above.. it would need to be something like this.. for obj to be an instance of Mountain:

Mountain m = new Mountain(); 
Object obj = m; // this is the important part.. now obj is a Mountain object.
if(obj instanceof Mountain) 
{ 
    Mountain mou = (Mountain)obj; 
}

In this example.. we are "up casting" so you do not have to explicitly cast the "m" object to Mountain when assigning it to "obj"

Hope this helps!