Accessor Methods

[MUSIC] 0:00 So far, the invader class is completely empty. 0:04 We can start to code it up now. 0:08 One attribute the invader should have is its location on the map. 0:10 We need this in order to determine how close they are to towers, 0:14 to see if they're in range of each other. 0:17 Let's add a MapLocation field to the invader class and call it Location. 0:20 It'll need to be public so the other classes can get the invader's location. 0:26 And it can't be read only because the location changes throughout the game. 0:30 Generally it's a bad idea to make fields public 0:34 because this exposes too much of the internal workings of the class. 0:37 In other words, this is poor encapsulation. 0:41 A better way to go is to make the Location field private and 0:44 then write public methods to access the field. 0:48 We'll need two methods. 0:51 One that gets the location and one that sets the location. 0:53 This pattern is so common in programming that there are special terms for 0:57 these kinds of methods. 1:01 They're called accessor methods. 1:03 The method that's used to get the value is called a getter. 1:05 And the method that's used to set the value is called a setter. 1:09 We'll call the getter GetLocation. 1:12 So I'll say public and 1:15 it will return our MapLocation cuz that's the type of the field. 1:17 And we'll call it GetLocation. 1:22 And it won't take any parameters because all it's doing is getting the value 1:27 of the location field and inside here we'll just return location. 1:33 And I need to change this to underscore 1:40 lowercase l because now this is a private field. 1:43 For the setter, we'll say public void because it won't return anything. 1:47 And we'll call it SetLocation and 1:53 it will take a MapLocation as its parameter. 1:57 And we'll call that value. 2:01 And it will set the value of _location to the value passed in. 2:07 Now we have a getter and a setter for the location field. 2:13 At this point you're probably wondering why we wrote these two methods 2:16 instead of just making the location field public. 2:19 There are a number of reasons for using getters and setters. 2:22 For one, it makes it possible to add code associated with reading and 2:25 writing this field. 2:30 We can now add code inside of these methods that will be executed 2:31 every time another class attempts to read or write the location. 2:35 Say for example that we wanted to make sure that the value parameter 2:39 meets certain conditions before we change the value of location. 2:43 Or we might want to return something other than what's stored in location 2:47 under certain conditions. 2:52 Having these methods also makes debugging much easier. 2:54 There are special programs called debuggers which we'll learn about 2:57 more in future courses. 3:01 Debuggers can be used to pause the program before a line is executed. 3:02 The catch is that the line must be executable. 3:07 Not all lines of code are considered executable. 3:11 Returning location and assigning location are both executable lines of code. 3:15 The location field all by itself is not executable. 3:21 It just represents a place to store information. 3:26 The line of code we tell the debugger to stop at is called a breakpoint. 3:29 We can't set a breakpoint at the location field. 3:33 But, having these two methods gives us two convenient breakpoint opportunities 3:36 that we wouldn't have if we had made location a public field. 3:41 A third reason for using getters and setters is to future-proof the code. 3:45 I know it's hard to imagine right now, but 3:49 someday we might need to change something about the location field. 3:52 For example, maybe we don't want the location to be stored in this class. 3:56 We might move it to another class that this class in turn uses. 4:00 If we were to make a change like that, 4:05 we'd have to fix every other class that was directly accessing the location field. 4:07 That we wouldn't even have the ability to fix those other classes 4:12 because they're maintained by other people who we don't even know. 4:15 There are a few really good reasons for using getters and 4:20 setters instead of public fields. 4:22 We'll learn more about these reasons only after we've learned the other two 4:25 principles of object oriented programming, abstraction and polymorphism. 4:28