Maps

Complete Step By Step Java for Testers Java Advanced - Collections Framework
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Transcript

Hello, guys, welcome back to the next tutorial. So going forward in the Java collections framework, today we're going to take a look at the maps. So what I've been focusing This is because it is really one of the most used data structures and the collections framework after maybe list and the ArrayList. So just pay attention. It's really common and very popular interview question as well, because a lot of in a lot of interviews, these days, they kind of focus on how you can use the maps and how you can use this data structure to utilize in your, in your programming. So let's, without wasting much time, let's quickly bring up our Eclipse.

And let's see how we can use the maps. So I just got a class called maps demo, and I'm just going to use it right now. So it's a little different than the list and the ArrayList. And the sets. what it's gonna do is it just stores the elements in pairs. So I'm just gonna mention that stores in pairs and the pairs are key value pair.

So the weights Define is the key value pair. And that's why they are known as maps because they have a mapping of key and value. So if there's a key call, maybe, let's say one, there's a value called maybe by our key called tool and this value call men or something like that. So it's stored in the way of key value pair. And that one thing is known as entry. So this one key value pair is one entry.

So that's always a map declaration. So the way the way we define is a key value pair. And one key value pair is one entry. So a map can have many entries as much as we want to define, right. So, again, similar to the ArrayList, linkless sets we're going to have is we wanna tell the data structure what kind of objects we want to store in the map. So we're just gonna use the map interface because all the kind of maps there are like multiple kinds of maps like hash map link, hash map or tree map exactly the way we looked in sets, but they are Implement the map interface.

So we can just use map and we can say, this is the key. So right now I'm using integer for my key and strength for the value. So that's how we define a map. So the first one is the key, and the second one is the value. And we're just defining the type for those things. But I can name it, let's say, map, just just name it a map, a new hash map.

And again, let's define what kind of objects they are and integer string in the parenthesis and semicolon. So then I can either import like that, or I can just say, say Command Shift O. And from the Java dot util, it's gonna import everything for me so important the switch statements for me map and hash map are in the Windows you can say Ctrl, Shift O. So how we can add values to map in the list and all those things to use the name of the variable dot add right? In the maps, we use map Put, and it accepts the key value pair, right? It's the entry.

So we have to first define the key. So let's give a key called maybe one. And then give it a value, let's say BMW, right? So this is how we define or enter values in a map. So let's say map dot put. And, again, we have to see some other value.

So maybe let's see six, and let's give it another. So I'm not defining in any particular order, no one or two or something like that. So I'm just using map dot code. And the key value pair, I'm just giving something so let's give it maybe a four. And let's give it on that side. So we have defined a map and we have added three entries into a map.

So three key value pairs, right. So one thing to note here is the keys are unique and this is a very important question as well. Keys are always unique values can be complicated, right? So what I mean by that is, let's say, I want to have, Okay, you know what let's first try to see how we can get the values back. So when I define a map, what I can say is because my values are of type string, and I want to get a value back, I want to store it in the variable which is of type string, right? So I'm just going to define a variable called maybe value one, something like that.

And map dot get is the method provided by map, which accepts a key if you guys can see it accepts a key. And if we provided a key, let's say let's give one the time, it says one of the dummy value, I can just use system out print statement, and I can see what's my value one. And if I just print it out, it's just going to give me BMW because BMW is mapped against the key one, and we learned the value of the key one, right? So this is the way that we Define a map, we store values into a map and we retrieve the three values from the map. And if the key does not exist, let's say I just give any arbitrary number, which is not even in the, in my map entry, entry. So let's say if I give to a pair, and just say I want to see the value, so it's going to give me now why because it cannot even find the key.

So if there is no key as to and there is no value related to the key number two, right, so it's gonna give me the value as well. So that's what it returns. And that's what we get. So always remember this, if we don't have the key in the map, it's just going to give us null, right? So as we were talking about the keys are unique, but the values are not. So what we can do is we can just try to see that map dot put something like that.

And we can use the same key again, let's say for, right, let's let's define another key tool, and let's give it different Car maybe? I don't know. Okay, let's let's define BMW. Let's let's use the BMW again. Right? So what I'm doing right now is I am using a new key, right?

Key number two, but I'm using the same value BMW, right? So in this way, what I can do is I can again define value. And let's say map dot get, right? And I can just say key number two or something like that. Yeah, that should be it right? And let's print this one out.

So let's copy this caveman, copy and pair. And so if I say this first one is now forget about it, because their toy is not defined yet, when we use the map dot get to, it's not yet defined. We defined the key up here after this statement. So it's going to print the null before it right and once we define the entry For the key number two, and again, give it a value BMW, and then we fetch it, then it's gonna give me the value, right? So then I just gave me the value BMW. So what we have shown here is we can duplicate the values now I have the, so let me just give it one here.

So both the things are gonna retrieve BMW for me because both the keys mapped to BMW C, but also maps to BMW and to also maps to BMW, as you guys can see, this is one and this is two. So but we can not actually duplicate the case. So if I again say here, let's say I against a map dot put, and I use the key, let's say for again, and I use another Gar, you know what, let's let's maybe put mark or something like that. So what's gonna happen right now is, it might be seen that I'm duplicating the keys, but it's Not. And I'm just going to show you what's happening. So let's retrieve the value for key four here.

And let's see what's gonna happen. So for the key for what value am retrieving is mark this one, right. And this is completely gone. Why? Because whenever we say the key again and put some other value, it just overrides. So we are basically not duplicating the keys here, we are just overriding the value for the old key.

This is what happens when we have the values repeated. So that's how we repeat the values, but we cannot repeat the keys. And this is really important to know. So guys, this is pretty much it for the maps. And I mean, not completely, this is not completed. Input maps.

I'm just going to go with the next tutorials. I'm going to show you how we can iterate over the maps. And I'm going to talk about the different types of maps like linked hash map and the tree map other than the hash map and I'm going to talk Talk about the differences between them, which is really a very important interview question. So a lot of times they ask, what are the different kinds of maps? And when do you want to use which map? So these kind of things and what is the difference between hash map and the tree map and all those sort of things.

So I'm going to talk in the next lecture. So for this one, guys, this is pretty much it, I will just say practice a little bit, download this code, and see how you can define a map and then put some values and retrieve the values. So thanks for learning this one guys, and stay tuned for the next one.

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