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Finishing the Card Class4:08 with Ben Deitch
In this video we'll see how to add a constructor to our class and initialize our properties!
Right now, we've got our value and our suit declared as immutable. 0:00 Which means they can never be changed from zero and an empty string. 0:04 Let's see how to add a constructor and use it to initialize our properties. 0:09 In Kotlin, 0:13 the constructor follows immediately after the class name, check it out. 0:14 Up here after the word Card, let's add some parentheses, 0:18 and then let's declare the parameters of our constructor inside the parentheses. 0:23 First we have value, which is an Int. 0:28 And then we have suit, which is a String. 0:33 And finally we have faceUp, which is a Boolean. 0:36 Once we've taken care of all the parameters to our constructor, 0:41 we can add this optional init block below our properties. 0:44 Whenever the constructor is called, this init block will be called too. 0:50 Essentially, Kotlin just separates the constructor into two pieces. 0:55 The parameters and the initialization code. 1:00 An important thing to note about the parameters of the constructor is that they 1:04 have a pretty limited scope. 1:08 We can only use these parameters in initializing a property or 1:10 inside the init block. 1:15 Anywhere else is off limits. 1:17 So now, instead of initializing our properties to potentially wrong values. 1:19 Let's initialize them to the values passed into the constructor. 1:24 So value will equal value. 1:28 Suit will equal suit. 1:32 And faceUp can equal faceUp. 1:36 Now, we don't actually need the init block for our card class. 1:39 But before we get rid of it, let me show you something kind of cool. 1:43 We know that if we don't initialize our value property, we'll get an error. 1:46 But we are allowed to initialize it in the init block instead. 1:52 This.value = value. 1:56 All right, let's undo that and as promised, let's delete the init block. 2:00 Then, to get this down to one line, we just add value or 2:08 var in front of the constructor parameters. 2:12 In Kotlin, adding val or 2:16 var in front of a constructor parameter upgrades it into a property. 2:17 So we can massively simplify our code by adding val in front of 2:23 this value parameter, val in front of the suit parameter and 2:28 var in front of the faceUp parameter. 2:32 Then since we no longer need our initial properties, let's delete them. 2:36 And since there's nothing inside our class, 2:41 we can even delete the brackets too. 2:44 While we're at it, let's also delete the JavaCard class. 2:49 Okay, just one more cool thing and then I promise we'll move on from the Card class. 3:09 So each card in the deck starts face down. 3:14 And wouldn't it be nice if we could have an alternate version of this 3:17 constructor that didn't require us to pass in the faceUp parameter? 3:21 Well, with Kotlin, we can specify default values for 3:25 our parameters just by adding an equal sign. 3:28 So let's add =false. 3:31 And now, we can create new facedown card instances with just a value and a suit. 3:34 Kotlin is an awesome language that gives us a ton of new tools to play with. 3:40 But that also means things can be a bit confusing at times. 3:45 If you're ever feeling stuck, 3:49 there's tons of excellent documentation at kotlinlang.org. 3:50 Seriously, the Kotlin docs are way better than the Android docs. 3:55 And don't forget about your friends at Treehouse. 3:59 We're here to help too. 4:01 Coming up, we're going to expand from this lonely card class into an entire deck. 4:03
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