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Adding and Removing Items Means Copying7:57 with Craig Dennis
An arrays length is immutable. If you want to add or remove, you need to declare a new array and copy into it. Don't worry you won't be doing this too much.
Example ArrayList code
// This is coming from the Java Collection Framework import java.util.List; import java.util.ArrayList; // This is using Generic syntax, we'll get to it... List<String> friends = new ArrayList<>(); friends.add("Ben"); // adds "Ben" to the friends list friends.size(); // returns 1 friends.contains("Ben"); // returns true friends.remove("Ben"); friends.size() // returns 0
[MUSIC] 0:00 [SOUND] Now that we've seen the power of arrays, 0:04 it's time to look at some of their limitations. 0:06 Now, we've been dealing with arrays where we either know all of the items at 0:08 declaration time or at least we know how many there will be eventually. 0:12 In reality, that's not always the case, is it? 0:17 I'm sure you can imagine programs where we're taking input from a user, and 0:20 they want to either add or remove elements from the array. 0:23 Now, if you recall when we first started talking about this 0:26 fancy new data structure, we talked about how it's length is immutable. 0:29 Now remember, that means it can't be changed. 0:33 And the reason for this immutability was because the declaration required 0:36 all of the elements to be one after the other, contiguous. 0:40 So that brings up quite a big gotcha. 0:45 In order to add an element, 0:48 you would in fact have to change the length of the array, and you can't. 0:49 So, what do you do? 0:54 Well, the answer is you copy that array into a brand new one, 0:56 that includes the needed space for you new element. 1:00 In fact, I have a pretty good example of how this can creep up. 1:03 In our list of friends, I forgot to add myself. 1:07 Let's take a look at how to handle this common gotcha. 1:10 Okay, so I'm gonna do this is JShell. 1:13 So I'm gonna use the scratch file. 1:15 What I'm gonna do, is I'm going to grab this string friends array here. 1:17 I'm gonna copy that, and I'm gonna pop it over into the scratch file. 1:21 I'm gonna get JShell up and running. 1:25 While we do this, Jshell will be running. 1:27 And then scratch, I'm gonna just erase what we have here. 1:29 I'm gonna drop our front array in there, cuz it's a scratch file, right? 1:31 It doesn't really matter. 1:35 You can put whatever you want in here, and then we can open it up and use it later. 1:36 That's the idea, and we explore around. 1:40 Okay, let's do it. 1:43 Let's get that file in there. 1:45 We'll say open scratch.java. 1:46 And then we'll type friends. 1:50 Awesome, okay. 1:54 So the goal is to add me to the end of this list. 1:55 We wanna attend me. 1:59 So there are couple of approaches. 2:01 Now the first thing you do is create your array to what size you want. 2:03 And let's do that, so let's see. 2:09 It's gonna be a string array. 2:13 It's gonna be a new string array. 2:14 We'll call it friendsAndMe, great. 2:17 And then we'll do = new string. 2:21 And we want one more than's there, cuz I'm gonna come to this party, now so 2:25 there's . 2:29 There we go. 2:30 So you'll notice that it defaulted all here to null, 2:32 which is the default for objects. 2:35 Okay, and then what we wanna do is we wanna copy all of the elements from our 2:37 original array into this array. 2:41 So the method itself is a bit on the older side of Java. 2:44 It's been around forever, and 2:49 it's dangling off of a system class. 2:53 It's called array copy. 2:59 So if I just start typing arr and I press tab, it will go ahead and fill it out. 3:00 Now, one thing that you might not know about JShell, and I really like this about 3:06 JShell is if I press tab again, it will actually show the documentation. 3:09 The first tab, it shows just the signature. 3:16 And the second time, it's gonna show the documentation for that, and 3:18 press tab again. 3:22 So it shows the method declaration which is important, 3:24 cuz we need to know these signatures are. 3:27 We need to know about this here, what the names of these are, what they mean? 3:29 They're not the greatest, but okay, so source is the source. 3:33 It's what we're trying to copy, so that's our original array. 3:37 Original array is friends. 3:41 All right, source pause. 3:44 [LAUGH] That's the position, right? 3:46 So that's the starting index of where we wanna start copying from. 3:48 So the position of that is 0, so we wanna start the very first element. 3:52 Okay, and then the next parameter here is the destination array. 3:58 So the destination array that we want is the one we just created. 4:02 So we wanna copy that to friendsAndMe. 4:05 And the next is the desk post, so the destination position, 4:10 where do you wanna place that two? 4:13 And since we're just copying over, we want me at the end, we'll start at 0. 4:15 And finally, the link. 4:20 How many of those items from the first one do you wanna copy over? 4:22 And I'm just gonna use friends.length, saying that we wanna copy them all. 4:25 Okay, so now, if we take a look at friendsAndme, 4:32 we'll see that we have an extra space at the end here, where we can put me. 4:37 So I wanna put myself in that last one there. 4:44 So let's say friendsAndMe = "Graig". 4:46 I'll go ahead and pop that to the top here. 4:52 And there we go. 4:56 So that's quite a bit to remember. 4:56 And I'm gonna bring this back, so we can take a look at it. 4:58 That's quite a bit to remember. 5:01 Now, there's actually another approach, that's a little bit better. 5:03 And it's actually really similar to what we just did. 5:07 There's a helper class, which provides a lot of helpful methods. 5:10 However, it's not readily available like our system one was. 5:13 We need to actually import the class to use it. 5:17 Now, if importing is new to you, check the teacher's notes. 5:19 For right now, though, let's just do this line. 5:22 Let's do an import here. 5:25 Erase this line. 5:27 I'm gonna say, import java.util.Arrays. 5:30 So that's gonna bring this class into our scope here. 5:36 And there's a static helper method off of this class called copy of, and 5:41 it makes copy of your Array. 5:45 So let's just use it. 5:47 So a String array called friendsAndMe2, 5:48 And we'll access this Arrays. 5:57 And off of that, we will access copyOf. 6:01 And copyOf this time is camel case. 6:04 And what that takes is, it takes the source array, which is friends. 6:07 And you tell it how long you want the new array to be. 6:14 This is a little bit different. 6:19 So we're gonna say that we want a new array to be friends.length. 6:21 And we want it to be one more, cuz that's what we're gonna add, so + 1. 6:24 So there you go. 6:30 It's basically the same thing, it's a little more succinct. 6:31 Now, the important think to recognize here is that in order to make a change 6:34 to an array, you need to manage the copying of it. 6:38 Now, the arrays of value, 6:42 you basically just don't copy it over to your new copy of the array. 6:44 Essentially removing it, it's a little gross. 6:47 Check the teacher's notes for more. 6:51 So how'd that feel? 6:53 A little gross, right? 6:55 Well, the good news is there's a solution to this. 6:57 As the old saying goes, if it hurts when you do that, don't do that. 7:00 When using arrays, you actually very rarely ever do any adding or 7:04 removing of elements. 7:09 This is because in practice, 7:11 there is another data structure that is known as a list. 7:12 It is intended for dynamically adding and removing values. 7:16 Now, list are out of the scope at this course. 7:19 But we'll get to them very shortly in an upcoming one, so hold tight. 7:21 Now, don't stress too much on the mechanics of adding and removing. 7:25 It's super easy to do with a list. 7:28 Side note, the most common type of list is called an array list. 7:31 And what you just learned how to do, 7:35 is exactly what is abstracted away from you for adding and removing. 7:37 It manages the array growing and shrinking, and it does all the copying. 7:41 If you just can't wait, check the teacher's notes for more information. 7:44 Now that I bummed you out a bit about arrays, let me bring back your 7:48 appreciation for them by showing off how you can control their order by sorting. 7:52
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