Eboard 13: Inheritance

You are probably being recorded, perhaps even transcribed.

Approximate overview

• Administrivia
• Questions
• Our call model, revisited
• Lab

Preliminaries

• I think today is Junior Visit Day. We’ll give our visitors some chance to answer and ask questions.
• Sam broke his hearing aids and hears even less well than normal.
• Please DM me your GitHub username if you haven’t already. (I’ll start adding you during the beginning of lab.)
• The first set of LAs have been released. I may add a few more.

Upcoming work

• No readings for Tuesday.
• Wednesday, 2024-02-21, 1pm: Submit Today’s Lab

Tokens

Academic/Scholarly

• Tuesday, 2024-02-20, noon–1:00pm, Some PDR. _CS Table: ???.
• Tuesday, 2024-02-20, 4:15-5:30pm, HSSC A1231 (the Kernel). The Meskwaki and the Amana Colonies.
• Tuesday, 2024-02-20, 8:00–9:00pm, Science 3821. CSC-207 Mentor Session.
• Thursday, 2024-02-22, 11:00-noon, JRC 101. Scholars’ Convocation: Luis Fabiano De Assis on Human Trafficking.
• Thursday, 2024-02-22, 4:00-5:00pm, Science ????. CSC Major Declaration Session

Cultural

• Friday, 2024-02-23, 4:00–5:00pm, HSSC N1170 (Global Living Room). Middle of Everywhere.
• Friday, 2024-02-23, 7:00–9:00pm, Sebring-Lewis. Squatters on Red Earth.

Peer

Wellness

• Tuesday, 2024-02-20, noon-1pm, BRAC P103. HIIT and Strength Fitness Class.
• Tuesday, 2024-02-20, 12:15–12:50, Bucksbaum 131. Yoga in the Museum. (I think)
• Tuesday, 2024-02-20, 4pm, BRAC P103 (Multipurpose Dance Studio): Yoga.

Misc

• GrinTech Alumni Meetup 4:00-5:00 pm in HSSC N2116.

Other good things to do (no tokens)

• Monday, 2024-02-20, 9-11pm, Bob’s Underground. Bob’s undergound open mic night 9-11pm

Revisiting our mental model

Note: You can find code in examples/inheritance.

We’ll address some of the questions from the readings by revisiting our mental model.

Questions

Inheritance

Do superclasses include info about their subclasses?

No

Can you cast from a subclass to a superclass?

Yes, but there’s rarely a reason to so.

Can you explain protection for fields?

Sure. Remember that we use protection to achieve one of the goals of encapsulation: We are free to make changes to our code without significantly affecting our clients. Different situations require different choices.

There are four levels: private, “package” (write nothing), protected and public.

private means that the field or method is only available to the class or other objects in the class.

“package” expands that availability to other classes in the same package as well as objects in those classes.

protected further expands it to subclasses in other packages.

public expands it to everyone.

Can you explain how we use super in methods?

You will most frequently use super in constructors to make a call to the appropriate constructor in the superclass.

If you don’t do so, Java will look for a zero-ary constructor in the superclass and call that.

At times, when we want to use a method from the superclass that has been overridden in the subclass, we can write super.methodName.

How do we know when to use interfaces and when to use inheritance?

Practice.

In general, we use inheritance when we have fields and methods we want to reuse from another class.

We use interfaces when we don’t have that kind of reuse.

We use either when we want to write functions that work with multiple similar objects.

Do you need to specify public or private for all fields?

Nope. There’s also “package” (writing nothing) and protected.

I prefer package protection because it often achieves the right level of protection (available to other classes in the same package, but not classes outside the package).

What’s wrong with these as solutions to the self checks?

public class StaffMember extends Person {
  String department;
}

public class Administrator extends person {
}

MP4

Other topics

Lab