[NOISE] All right, you've learned a little bit about inheritance. And you've played that matching game with Leo where you learned what kinds of references can refer to what kinds of objects. It turns out that is a really, really tricky idea to get. So, we've got a concept challenge for you that's gonna exercise your ability to make those matches. Don't worry if this is a little bit tricky. It takes a while to get it. So work through it, you know the drill. Work through it on your own, answer the end video quiz, discuss with others, watch our learner videos, and then watch our explanation. Here's your challenge. You've got three classes. There's a Person class, it has a name, member variable. And it has a getName, method. There's a Student class which extends person. It has an ID member variable that's an integer, and it has a getID method, it returns an integer, and there's a Faculty class which also extends Person. It also has an ID variable, but this time it's a string, and it has a getID method that returns a string. We have five statements over there on the right that create new objects and assign references to refer to those new objects. All of those statements compile and run just fine. Your question is going to be to answer questions about which of an additional series of statements work, and which cause errors at either compile time or run time. >> Okay, my name is Linda. >> My name's Mustafa. >> My name is Jason. So how did you guys answer question one. >> I seem to think that question one didn't have any issues. S is a student, and they're trying to get the name from the student, and- >> Well I thought there was a problem with that because when I look at the class of Student I realize there is no method called getName. >> That's true, but Student extends from Person. >> So it will inherit all the parent class methods. >> Yeah, it's a public method. >> Oh so, it will inherit the Persons getName method. >> Right, yeah. >> Oh okay, I see. What about the second p = s? >> For the second one, I also thought it was fine. >> That's fine? >> Cuz Person p is a new Person. And if you change the reference of p to become the object created from Student s equals new Student, then every Student is a Person. >> Okay, so that's the part that I missed. I was thinking it would be a problem cuz Student and Person are not the same type, so I wasn't sure if you could cast it that way. >> Oh, right. >> So too much p instead of referencing to Person, I was referencing to Students. >> Mm-hm. >> Okay. >> If that makes sense? >> Yeah, okay. >> Now look at the next line of- >> I don't think that would work because Person doesn't have a getID method- >> Right. >> So where would it be looking to get that? >> I thought that during compile time error since p is still a Person, it'll go into the Person class and then look at, oh, where's the getID method? >> But didn't we just updated a Person class with pointing to a Student class? >> Mm-hm. >> The compiler doesn't know that. The compiler still sees it as a Person object because it was originally set up that way. >> It's only at run time that the compiler will see that p is actually a Student. >> Oh, okay. Even that I believe we have to cast it I guess. >> Right. >> Mm-hm. >> How about the third line of code, faculty equals Person? >> Person? >> Well person isn't necessarily a Faculty member. So like he was saying that if you go around and ask a random person on the street, they might not have the information for faculty. So I'm not sure if you can cast the faculty as a broad term person. >> Right, not every person is faculty. >> Right. >> Yeah that makes sense. And then the last one, o equals a Student, or object class equals Student class. >> I think we've previously learned that every object inherits from the object class, right? >> Yeah. >> Oh, I see. >> So s is a Student. >> Okay, I like, every class would inherit object meaning. There's implicit extent. >> Yeah, yeah exactly. >> Perfect. >> All right let's step through these lines one at a time to figure out which ones work and which ones cause errors. This first line here, it's pretty straightforward. It creates a new variable of type String called n, and it's gonna set it equal to s.getName(). Well first we need to figure out what is s. Well s is a Student reference, it's a reference to a Student. And it actually does refer to a Student object. And you can see that right here, when we created that Student object and assigned the reference s to point to. So now we can look in the Student class and see if there's a getName method. We look here, no getName method, but that doesn't concern us too much, because Student extends Person. So now we can go up in person and look at those getName method there. And sure enough, there is. It returns a string. So this will run just fine, again, because this getName method is public. So everything is good. This line will work just fine. The next line says that the reference p should be set to refer to the same location as the reference s refers to. Now s refers to a Student object. What we're saying is that now p should refer to a Student object, and we have to figure out if that's okay. Is it okay for a Person reference to refer to an object that's actually a student? So you ask the question are all students people? Is a student always a person? And I hope the answer is always yes. So that should be just fine. That line works as well. Next line here says that int m gets the value of p.getID. Now, we have to think about things a little more carefully. So what we're asking, assuming that this line has already completely, and p now refers to a Student object, we wanna know is it okay to call p a getID. There is a getID method inside the student class, but the problem is that p is a Person reference. So the compiler doesn't know that p actually refers to a Student object. The compiler is gonna complain about this statement here. It's gonna say it's a piece of reference to a Person, I don't know about this getID method, there's no getID method in Person class, and so I'm gonna cause an error. We can fix this with a cast. So if we wanted to fix this problem, we would just insert a cast here. We could cast p, so instead of just p, we'd say Student, and then p right there, and then .getID. Because we know that object is actually a Student. So that's fine to do, but again, be a little careful with your cast because your compiler will trust you no matter what you say. We can do this in this case cuz we know that's the term. All right, so the next statement there, so going back actually to the last statement, this statement right here. This statement right there does not work as written, but it will work if we add that cast. Let's consider the next line, where it says f gets the value of q. So what is f? F is a Faculty reference. It's a reference to some sort of Faculty object. And what is q? Well, q is a reference to a Person. So now we need to figure out if a Faculty reference can point to a person object. And again we will ask the question, are all people faculty? And the answer to that question is of course no. So we can't set a Faculty reference equal to a Person object. That will cause an error. And there's nothing that we can do to make that work. If we try to cast q to a faculty, compiler might accept it, but at run time we're going to get what is called a class cast exception, because it's illegal to cast a Person object to a Faculty type. That line will not work. What about this next line where we say o = s. So o is an Object type reference, and s is a Student object, up here. Well, that should work, right? Because everything in Java is an Object. The Object text references are allowed to point to any kind of Object that they want. So that one's pretty straightforward. That works. So now we're done. And to summarize what we found was this first line here was just fine. The next line here was also just fine, because all students are people. The next line is not okay. But we could make it okay with a cast. The next line was there not okay and there is nothing we could do to make that line work. And finally this last line here was just fine.
