Question

i have a parent class called Course, and two child class PostgradCourse and UndergradCourse.
i have a hashmap HashMap courses; i store all the postgradCourse and undergradCourse objects in the hashmap.

i want to retrieve an undergradCourse object from the hashmap using the key.
Course course = courses.get(courseCode);
then i want to call a method in the UndergradCourse class, setUnits() method
course.setUnits();
but the compiler say cannot find symbol- method setUnit()

im pretty sure the problem is the compiler is looking for a method setUnit() in the Course class instead of UndergradCourse class

i did this but its not working
UndergradCourse course = courses.get(courseCode);
results in incompatible type

so how can i retrieve undergradCourse object from the hashmap as an undergradCourse object instead of course object?
so then i can call a method inside the child class

thanks in advance

Answer 1

Assuming you program in Java, you can use the instanceof operator the determine whether the object is of a certain type.

Course course = courses.get(courseCode)
if(course instanceof UndergradCourse)
{
    UndergradCourse undergradCourse = (UndergradCourse)course;
    // do you stuff with the undergrad course
}

Note: Depending on the actual type of the object is breaking object oriented programing. It’s not considered good style.