Instance of not working properly??

Question

Hi. This might be hard to explain but I really dont understand why this is happening..... I have a superclass called Student, and subclasses call PostGraduate and UnderGraduate. When I create a student, depending on whether they entered PostGraduate or UnderGraduate during data input, a new instance of one of these two will be created as seen in the following code:
if(graduate=='P')
studentList.add(new PostGraduate(name.toUpperCase(), address.toUpperCase(), phone, gender, code.toUpperCase(), false));
if(graduate=='U')
studentList.add(new UnderGraduate(name.toUpperCase(), address.toUpperCase(), phone, gender, code.toUpperCase(), false));
}I am positive this is working because I put a System.out in each if statement and it would create them as either UnderGraduate or PostGraduate depending on what I chose, and their corresponding toString methods are being called correctly.
The problem I am now having is this: I have two methods, one called displayAllPostGrad() and one called displayAllUnderGrad();
In these methods I have the following code:
for(int i=0;i<studentList.size();i++)
if(studentList.get(i) instanceof PostGraduate);
System.out.println(" - " + studentList.get(i).getCode() + " " + studentList.get(i).getName());
foundStudent=true;
}Except obviously the other method has "instance of UnderGraduate".
When I run these methods, say for example I run the method displayAllPostGrad, it is displaying all my students, that is, even students that are an instance of UnderGrad. The same happens when I run the method for displayAllUnderGrad, it displays students that are PostGrads as well.
I cannot see anywhere in my code why this is happening.. any help would be great.
Many thanks

Answer 1

By the way, relying on instanceof is kind of poor. Sometimes it's unavoidable, but if you can redesign your program so that it doesn't have to do this, it would be good.
You could add the Student objects to two different lists (it's still the same object, just used twice, so there's little waste of resources), one list represents all students, and one list represents just postgraduates or just undergraduates.
You could also perhaps have a method in Student which basically says "add yourself to the appropriate list", and when you want to display student lists, call that method on all students.