Inheritance

[MUSIC] 0:00 In the first part of this course we wrote a class that models a point. 0:04 It has x and y fields and 0:08 even has a method that calculates how far it is from another point. 0:11 The Treehouse defense game uses this class to identify locations on the map. 0:16 But it could be used for a lot more than that. 0:21 Lots of software applications use x and y coordinates to identify a location. 0:24 Perhaps the next game we write will also need a point class. 0:29 Software that displays charts and graphs also needs points. 0:32 If we keep our implementation of the point class as general purpose as possible, 0:36 we can reuse it in all those different applications. 0:41 What if we need to add something to the point class that's specific 0:45 to Treehouse defense? 0:48 For example what if we wanted to make it impossible to create a point that 0:50 isn't on the map? 0:54 Making a change like that to the point class would make it unusable to 0:55 all other applications that don't have maps. 0:59 We don't want to add code to a general purpose class that we're 1:02 fairly certain won't be useful to other users of the class. 1:05 We also don't want to create a new class that's specific to Treehouse defense and 1:09 rewrite everything we wrote for the point class. 1:14 What we want is to create a new class that has our new functionality, but 1:17 also reuses the point class. 1:21 The principles of object-oriented programming 1:24 provide lots of ways to reuse and extend existing classes. 1:26 One of them is called inheritance. 1:31 There are four core principles of object-oriented programming. 1:34 Encapsulation, inheritance, polymorphism and abstraction. 1:38 By saying that C# is an object-oriented programming language, we're really saying 1:43 that C# has features built into the language to support these four principles. 1:48 As you can see, inheritance is one of the four core principles. 1:54 Don't worry about these other three principles right now. 1:59 We'll discuss them later. 2:02 The principle we want to focus on right now is inheritance. 2:04 You see, 2:08 many objects in the real world often share many of the same attributes and behaviors. 2:09 A classic example is an animal, all animals have similar characteristics. 2:15 For example, a defining characteristic of all animals is the ability to move. 2:20 There are two types of animals, vertebrates and invertebrates. 2:25 Vertebrates have backbones and invertebrates don't. 2:30 But they're both still animals, so they can both move. 2:34 Mammals and birds are types of vertebrates. 2:37 By saying that a creature is a mammal we're 2:40 also saying that it has a backbone and it can move. 2:43 You might say, 2:46 that it inherits these characteristics from the larger categories, 2:47 or classifications, of vertebrates and animals of which it's a member. 2:51 You can see now that the C# key word class is short for classification. 2:56 C# classes define what it means to be in a particular classification. 3:02 As we saw with the animal example, classes of animals 3:07 can have more refined classifications within them, we call these subclasses. 3:11 Subclasses inherit the attributes and behaviors of the more general classes. 3:16 So if we wanted a new type of point that inherited 3:22 all of the features of the point class, but 3:25 was more specific to our application, we just need to create a subclass of point. 3:28 Let's look at how we do that in code. 3:33