Navigating the Class Hierarchy 2

Student: One thing I find really confusing is when we store a subclass object in a superclass variable. Is there any way to tell which type the object actually is?

Prof: There is in fact! Check it out, we can use it on our old friend, Pict:

Pict c = new Circle(20, "red");
boolean isCircle = c instanceof Circle;
boolean isRectangle = c instanceof Rect;

Prof: So it tells us that c is in fact a Circle, and is not a Rect, from our program.

Student: Huh, that’s interesting. What would c instanceof Pict say?

Prof: Let’s try a few more:

Pict r = new Rect(20, 40, "red");
boolean isCircle = r instanceof Circle;
boolean isRectangle = r instanceof Rect;
boolean isPict = r instanceof Pict;

Student: Huh, interesting. So does it work for extends, too?

Prof: Indeed!

interface Place {}
class Location implements Place {}
class BusStop extends Location {}
class Restaurant extends Location {}
Place b = new BusStop();
boolean isPlace = b instanceof Place;
boolean isLocation = b instanceof Location;

Student: OK, so how does that work? Does it like… search the class diagram?

Prof: Exactly. a instanceof C starts at the class of the object in a. If that isn’t C, it checks that class’s superclass and anything it implements, all the way up the diagram, looking for C. If a instanceof C, then the value stored in a could be stored in a variable of type C.

Student: Cool, where do I use that?

Prof: Honestly? Mostly you don’t.

Student: Mostly I don’t? What do you mean?

Prof: Well, most of the time we try to make our programs work out using the general interface types, like Pict or Place, for most variables. Then we just rely on the methods defined on them being present. Field lookups and class-specific work happen inside the class, and calling a method will select the class for the right object. So instanceof is honestly not needed much in well-written code.

Student: Hm… then why’d you show me?

Prof: Well, you’ll still see it in some places. And we need it to wrap up some of our discussion of Object and equals.

Practice

Using these classes, is each instanceof true or false?

interface Place {}
class Location implements Place {}
class Park extends Location {}
class DogPark extends Park {}
class BusStop extends Location {}
Place p = new Park();
Location loc = new DogPark();

Object

Location

Park

DogPark

BusStop

interface

Place

p instanceof Place is
p instanceof Location is
p instanceof DogPark is
p instanceof BusStop is
loc instanceof Park is

With the same classes, what is new Location() instanceof Park?

false
The search starts at the object’s own class, Location, and only goes up from there: to Place and Object. Park is below Location, so it never comes up.
true, because Park extends Location
The arrow from Park to Location says every Park is a Location. It doesn’t say every Location is a Park.
It doesn’t compile
A Location variable could hold a Park, so Java lets you ask. It refuses only when the answer could never be true.