Could Someone Explain Interfaces in English Please?

Question

Hi Everyone-
I'm new to Java. I've been reading "Learning Java" from O'Reilly. From what I understand, Interfaces are sort of a structural framework that lists certain methods. Any class that implements an interface must contain this "structure" and any objects intantiated using this class will be of the interface type? Is this correct? If not can someone explain interfaces in laymen's terms and the what is the usefulness of implementing Interfaces?
Thanks.
SA

Answer 1

I'm new to Java. I've been reading "Learning Java"
from O'Reilly. From what I understand, Interfaces are
sort of a structural framework that lists certain
methods. Any class that implements an interface must
contain this "structure" ...OK so far...
and any objects intantiated
using this class will be of the interface type? Is
this correct? It depends what you mean by "interface type". An interface can be said to define a type. So a class that implements that interface implements that type, and an object instantiated from that class is an instance of that type.
If not can someone explain interfaces in
laymen's terms and the what is the usefulness of
implementing Interfaces?The utility of interfaces is that, without being tied down to particular implementations, allows types of objects to be defined.
For example....is there a doctor in the house? Say:
1) a Doctor is:
- human
- has gone to medical school
- heals the sick
2) a Norwegian is:
- a person from Norway (assume we've defined "person" at this point)
3) a Canadian is:
- a person from Canada
So:
4) a Norwegian Doctor is a person from Norway who fulfills the qualifications to be a doctor
5) a Canadian Doctor is a person from Canada who fulfills the qualifications to be a doctor
6) a Canadian who is not a Canadian Doctor, is a person from Canada who does not fulfill the qualifications to be a doctor.
If your appendix is about to burst, you need a doctor. You don't care if the doctor is Canadian or Norwegian (assume everybody including yourself speaks Esperonto so there aren't any language issues). You do care deeply if the person trying to remove your appendix is a Canadian but not a Canadian doctor.
If you were establishing a hospital, you'd want doctors. If you were to write this in code, you'd have something like:
Hospital.hireDoctor(Doctor doc);meaning, that method in the Hospital class will hire a doctor, which is the important part. So this would work:
Doctor doc = NorwegianMedicalSchool.createDoctor();
Doctor doc_eh = CanadianMedicalSchool.createDoctor();
yourHospital.hireDoctor(doc);
yourHospital.hireDoctor(doc_eh);The point is, you can write your Hospital code without knowing precisely how "Doctor" is implemented -- whether the doctors come from Canada or Norway. You just write it as appropriate given that you have a Doctor. And then when a Doctor is created by a Japanese medical school, it doesn't break your code. This is why interfaces are said to define "contracts" -- an interface defines what something is (i.e., what it does), and you can make your code insist on getting something that does that (fulfills the contract).
Does this make any sense? Have I offended the Canadians? Probably.