Virtual Methods

In C# objects, we learned that inheritance can be used to create an is 0:00 a relationship between two types. 0:05 For example, map location is a type of point. 0:08 We subclass the point class to create a new type of 0:12 point which we called map location. 0:15 Map location, by virtue that it inherits from point, also has fields for 0:18 X and Y coordinates. 0:23 Objects often share many of the same attributes and 0:26 behaviors as other types of objects. 0:29 A classic example is an animal. 0:32 All animals have similar characteristics and behaviors. 0:35 For example, a defining characteristic of all animals is the ability to move. 0:38 There are two types of animals, vertebrates and invertebrates. 0:44 Vertebrates have backbones and 0:48 invertebrates don't, but they're both still animals. 0:50 So, they can both move. 0:53 But, not all animals move in the same way. 0:55 Mammals and 0:58 birds are both types of vertebrates, but they move quite differently. 0:59 In object oriented programming, we also need the ability for 1:03 objects to behave differently based on the type of object that they are. 1:07 This is called polymorphism. 1:11 And it's one of the core principles of object oriented programming. 1:13 Let's see what this looks like in code. 1:17 Let's create a new type of invader. 1:21 First let's create a new file. 1:24 We will call our new invader type shielded invader. 1:27 So we'll name this file shielded invader.cs. 1:30 We'll copy the namespace and class name from invader paste 1:35 it in shielded invader and add the closing curly braces. 1:39 Instead of invader, we want our class to be named shielded invader. 1:43 And we want it to inherit from invader. 1:47 We'll add a constructor that takes a path parameter and all this constructor will do 1:51 is pass the path parameter up to the constructor of the base class like so. 1:55 Now we have a new type of invader. 2:02 As the code stands right now, 2:04 shielded invader behaves exactly like the original invader class. 2:06 That's because it inherits all of the attributes and behaviors from invader. 2:11 Let's make it so that when a shielded invader is hit, 2:16 there's a chance that it won't sustain any damage. 2:20 For that we need to change the way decrease health method works. 2:23 But wait, the decrease health method is in the invader class definition. 2:27 We don't have it here in ShieldedInvader. 2:32 Let's take a look at the decrease health method in invader. 2:34 When the decrease health method is called, 2:38 it will decrease the health of the invader by the factor passed in. 2:40 This is exactly how we want it to work for most invaders. 2:44 But if the invader is a shielded invader, 2:48 we don't always want to decrease the health every time the method is called. 2:51 One way we could implement this is to check if the current object 2:55 is an instance of shielded invader. 2:59 We can type, if(this is ShieldedInvader), 3:01 then do something here. 3:08 Else decrease health by the factor passed in. 3:14 There are many problems with writing code like this. 3:19 For one, it requires the invader class to know about the other classes that 3:22 inherit from it. 3:26 This is not an extendable way to add new types of invaders because 3:27 we have to change the invader class each time we want to add a new type of invader. 3:31 Also, our elegant single line method would end up being polluted 3:36 by code that doesn't directly have to do with this specific type of invader. 3:41 We could eventually end up with a lot of these if this is statements here. 3:45 The is operator is C#'s way of checking to see if an object is of a certain type. 3:50 When I find myself tempted to write a bunch of type checks like this, 3:56 it's a tell tale sign that I should make the class polymorphic. 4:00 This means, making it so that subclasses of this class can provide 4:04 their own implementations of decreased health. 4:08 The word polymorphic means to have many forms. 4:11 Just like how birds move differently, 4:15 shielded invaders should have a different way to decrease help. 4:17 We can make it possible for subclasses to provide their own implementation of 4:22 decreased health by adding the keyword virtual to the method here. 4:26 Adding virtual is telling C# that this is just one possible implementation of 4:31 the decrease health method and 4:35 subclasses can provide their own implementations of this method. 4:37 Let's copy the declaration and paste it in ShieldedInvader. 4:42 And we'll add the curly braces here. 4:46 Because this is a subclass, we need to change the virtual here to override to 4:49 show that ShieldedInvader is overriding the virtual method of the invader class. 4:53 In C# objects, we learned about the random class and it can help us out here as well. 4:58 Let's create a static random instance in the shielded invader and call it random. 5:04 Then in DecreaseHealth, we'll say, if(random.Nextdouble 5:13 Is less than .5, Then call 5:22 the base classes DecreaseHealthMethod and pass in the factor. 5:27 Next double returns a random number between zero and one. 5:33 Because it is uniformly random, half of the time, 5:36 this number will be less than 0.5. 5:40 So the body of this if statement will only be executed 50% of the time. 5:42 Here we just call the base classes, 5:48 that is the normal invaders, decrease health method. 5:50 We do this by typing base. 5:53 It's not enough to just type decrease health, 5:56 because now there are two decrease health methods that can be called. 5:59 We need to specify which one we want called. 6:03 You should know that overridden methods don't have to call the base classes 6:06 method. 6:10 We could have done something other than call base.decrease health. 6:10 It just so happens that what we want to do here is what is already done 6:14 in the base classes decrease health method. 6:18 Now when towers attempt to decrease the health of a shielded invader, 6:21 they will be successful only half of the time on average. 6:25 Other than its implementation of decrease health, 6:28 a shielded invader is exactly like a normal invader. 6:31 We would not have wanted to write an entirely new shielded invader class that 6:35 was just a copy of most of the code of the invader class. 6:40 Instead we're reusing most of the functionality of the Invader class and 6:43 making one small alteration to make the shielded Invader class. 6:48 The really neat thing about this is that all of the code that is 6:53 already designed to work with the based Invader type 6:56 will also work with the shielded Invader type without making any changes. 6:59 A shielded invader is still an invader, it's just been enhanced a bit. 7:04 So we can go to Game.cs and change one of these invaders here to ShieldedInvader. 7:09 Notice that this is still an array of invaders. 7:15 We don't have to change anything else. 7:19 These invaders get past the level which knows about the base invader type. 7:21 There is no reference to ShieldedInvader in here. 7:26 In the play method, these invaders get passed through the towers 7:30 FireOnInvaders method. 7:33 See here again, there is no reference to a shielded invader yet, but 7:37 all this code will still work. 7:41 When decrease health is called on an invader, 7:43 that is a normal invader, its health will be decreased. 7:46 However, if decreased health is called on an invader that is actually 7:50 of type shielded invader, it may or may not suffer damage. 7:54 To really see what's going on, let's move this console output. 7:58 We'll reprint shot at and hit an invader to the invaders decrease health method. 8:02 We could add a using system directive to the top of the file, but 8:08 instead we'll just add the namespace name here. 8:12 Now in ShieldedInvader's DecreaseHealthMethod, 8:15 let's add some output here too. 8:19 If the shield block the shot, 8:21 we'll say System.Console.Writeline and 8:25 print (Shot at a shielded invader but 8:30 it sustained no damage.") Let's compile this 8:34 to make sure that we didn't introduce any compilation errors. 8:39 We'll say msc- out:TreeHouseDefense, *.cs. 8:48 Now let's run it to see what we get. 8:57 As you can see, 9:02 even though fire on invaders accepts an array of the base invader type, we still 9:03 get different behavior when decreased health is called on a shielded invader. 9:08 That's because the DecreaseHealth method is 9:13 overridden in the shielded invader class. 9:16