Q Liu Q Liu - 29 days ago 14
Java Question

HashSet vs. ArrayList

So I have a custom class Class that will have a set of another custom class Students. So it will look something like this:

public class Class {
private Set<Student> students;

// other methods
}


Now I will be adding and removing many students to the set students and i will also be changing many of the private fields of a student already in the set of students.

QUESTION: What data structure should I use to best implement this? Since I will be changing the property of the Student objects in set student (thereby changing the hashcodes) should I use an ArrayList instead?

Answer

What data structure should I use to best implement this? Since I will be changing the property of the Student objects in set student (thereby changing the hashcodes) should I use an ArrayList instead?

If the hashcodes for the set elements are liable to change, then you should NOT be using a HashSet. (If you do, the data structure will break, and elements in the set are liable to go missing.)

But I doubt you should be using ArrayList either, because if hashcode() is sensitive to changes to the object, then equals(Object) will most likely be too. And that means that contains(...) and similar methods won't be able to find objects.

I think you should be using a Map type, and using a "student identifier" as the key.

(You could also override hashcode and equals so that equality means that two objects have the same id. But that makes equals(Object) useless for other purposes.)