Generics In Java - Full Simple Tutorial

in this video we're going to talk all about generics in java generics can be super confusing the first time you see them with all the t's and question marks and angle brackets and k's and v's but i promise by the end of this video you'll know exactly what generics are why they exist and why they're useful and how you can use them in your own programs my name is john i'm a lead java software engineer and i love sharing what i've learned with you in a clear and understandable way i also have a full job of course available and a link down in the description if you're interested first to have a good understanding of what generics do and why they exist it helps to know the kinds of problems that java developers were running into before generics existed let's say i wanted to create a class where all it would do is hold an integer value that i give it and then print out that integer whenever i wanted to so we might call something like that integer printer you can go ahead and click finish and create that so this class would be pretty simple we would have an integer field for the thing that we want to print so we can call it thing to print and we can have a constructor for this class that takes in this thing to print so that would be public integer printer then it takes in an integer for the thing to print and that would just set this dot thing to print equal to the thing to print that was passed in and we can have one method to actually print this out whenever we want to so that would be public void print and all that does is just print out the thing to print to the console so system.out.printline thing to print so if we wanted to use this integer printer class we would just create an integer printer printer equals new integer printer and pass in the integer that we want to print here in the constructor so let's say we want to print out the number 23 now of course we can call printer.print and when we run our program of course it prints out 23. but what if we wanted to do exactly the same thing for a double instead of an integer well we couldn't use this class right because it only holds an integer it doesn't hold a double so what can we do well basically what we have to do is make a copy of this entire class and we can call it double printer and then inside that double printer we would basically have all of the exact same code except instead of integer we would have double so this new class will work great for holding and printing doubles but what if we now want a class that'll do exactly the same thing but work for strings well of course you have to do the exact same thing again you have to copy that class make a new class called string printer and change all these doubles to strings if you want your class to be able to print strings so now we have three completely separate versions of essentially the same kind of class and you can start to see the problem we would need a whole other class if we wanted to do this with floats or dogs or cats or cars or trucks or whatever type of object that we wanted to print so we'd end up with a ton of code duplication and this is with a really simple class if you had a more complicated class the code duplication would be even worse well that is where generics come in generics offer you the ability to have one class like this that is flexible for many many different types so let's go back to our original integer printer and make it generic so first instead of calling it integer printer let's just call it printer because it should be able to print anything now when you want to use generics in a class like this in java right here after the class name in the declaration but before the curly braces you have to define what is called a type parameter and you'll put your type parameter in angle brackets like this by convention in java typically you'll see this type parameter just called t but technically you can call this type parameter whatever you want but i'm going to just be using t so you can kind of get used to seeing it because that's the convention that you'll see elsewhere you can think of it as standing for type in this class what this t represents is the type of thing that this printer is going to be able to hold and print so instead of just having an integer field here it's going to be of type t right now as we're coding this class we don't know what types of things this printer is going to be asked to print but this gives the ability to create printers for any types that you want so of course we also have to change the name of our constructor to just printer to match the class name and that constructor also instead of taking in an integer is going to take in something of type t so now this class is a generic printer for whatever type we want but now how do we actually use this class if we want to for example create an integer printer like we had before well back here in our main method instead of an integer printer now of course we're going to just create a printer but now you can see that we have a warning here that printer is a raw type and references to generic type printer should be parameterized it wants to know the type of thing that we want to be able to print and in this example we want to be able to print integers so how we specify that is in angle brackets here after printer we type in integer that tells java i want a printer for integers and we have a similar warning here to get rid of that you just need to put in angle brackets after the class name printer in older versions of java you used to have to put the type in there again but you don't have to do that anymore this integer here that we're passing in in the angle brackets is what's going to be used as this t-type in this printer object that we are creating now we can run our program as we did before and it still prints out 23. but now what's cool is we can go in and get rid of those two extra classes that we made the double printer and the string printer we can just delete those because now we can use our generic printer class to be able to create printers for doubles and strings also so now all we have to do if we want to create a printer for doubles we just say printer and pass in double or t the type that we want this printer to be able to print we can call it double printer equals new printer and give it a value like 33.5 and we can call doubleprinter.print and of course it prints out 33.5 so now we've created one printer class that can print any type that we get it doubles ants longs floats pigs monkeys cars trucks whatever one thing to note though is that generics don't work with primitive types like lowercase int and lowercase long but all you have to do is just use the wrapper classes like integer and everything should just work the same way one place that you've probably already used generics all the time is with java's collections framework for example if you wanted to create an arraylist you've probably done this where you've specified in angle brackets the type of thing that you want to have in your list so if you want to create an arraylist of cats you say arraylist cat we'll call it cats equals new array list this gives you an arraylist that you can only add cats to so you can say cats.add new cat and that works fine but if you try to do cats.add and pass in a new dog instead you get an error because we specified that cat was the type of thing that this list was going to hold and if we try to give it a dog we get an error in the same way up here where we created a printer for doubles if we try to pass in a string hey there we get a similar error because we told java that this was going to be a printer for doubles and here we're trying to give it a string but you might be thinking well hey john if we want to be able to create a list of anything we want why do we have to deal with all these generics why can't we just create like an array of just objects and then we can put whatever we want in it well it is true that you can do that and your code will work but it won't be type safe at all let me show you what i mean let's say that our arraylist instead of holding cats just holds objects this is the kind of thing you'd have to do if generics didn't exist you just have to create an arraylist that can hold whatever so in this imaginary world without generics we still want to create a list of cats so we've added our new cat to our list but let's say sometime later on in the code we want to be able to get stuff off our list and use it so if we want to get this first cat off the list and put it in a variable we would want to create a cat call it my cat equals cats.get the first element of the list at index 0. but here we get an error that we have a tight mismatch you can't convert from object to cat java doesn't know that this is supposed to be a list of cats it just knows it's a list of objects so we have to tell java yes i know this is a cat so you can go ahead and cast it to a cat so i can store it in this variable but what if instead of adding a cat to the list somebody adds a dog well the code all looks the same there's no compilation errors or anything but of course if you try to run it you're going to get a classcast exception right here on this line because you're trying to cast this element in the list as a cat but it's actually a dog so just having an arraylist that can hold whatever type of object causes all types of these type safety issues and generics solves that problem for us so now we can say yes i want to create a list of cats so then in the code if somebody tries to add a dog to that list they get an error and it won't even compile and also since java knows that this is a list that can only contain cats whenever you get a thing off of that list it is 100 guaranteed to be a cat and you don't have to do any special casting that's how generics help us they offer the ability to create classes that can accommodate tons of different types but also the structure and the type safety of knowing exactly which type you're using that class with at the moment but there are even more cool advanced things that you can do with generics let's go back to our generic printer class so right now we can create a printer of whatever class that we want to right but what if instead of being a printer for any type we want this to be a more specialized printer for animals for example i have this cat class here that extends the animal class and i also have a dog class here that extends the animal class so if we want this to just be a printer for any type of animal instead of just saying t we can say t extends animal now back here in our main method we now get an error when we try to create a printer for integers or doubles because now this printer only works for animals so we can create a cat printer and in the constructor we can pass in a new cat or we can create a printer for a dog and pass in a new dog but what's cool about that is now in our printer class since we know that whatever is passed in as this type extends animal any method that is defined in the animal class is available to us in our thing to print variable because we know that this thing to print this t is going to be some type of an animal so now because we know this thing to print is an animal down here in our print method we can call thing to print dot eat because this eat method is available in the animal class so if we didn't have this extends animal if we just took in any type at all we can't call the eat method because this tea type isn't guaranteed to be an animal at all so there's no way to know whether it will have an eat method available on it or not when you do this type of thing it's called a bounded generic because you're giving a certain bound a limit on the type that's able to be passed in you can also use bounds with interfaces let's say you wanted to guarantee that this type implemented the serializable interface you just put in extends serializable you might think that because this is an interface you should put implements here instead of extends but no it's just not the proper syntax it still has to be extends even though it's a little bit weird another cool thing you can do is have multiple bounds let's say you wanted to make sure that it was a subclass of animal and also implemented the serializable interface to do that you can just say extends animal and serializable here with an ampersand and you can list as many things as you want here all separated by ampersands there's just a couple restrictions you can only have one class which makes sense because java doesn't support multiple inheritance and you always have to list the class name first with any interfaces after that if you try to switch it around and put the interface first you'll get an error in addition to having generic classes you can also have generic methods let's say we wanted to make a method that can take in any type of object and print it out with exclamation points private static void let's call it shout since it'll print things out with exclamation points it'll take in some kind of type that we're going to call t we'll call it thing to shout and inside that method all we're going to do is print out that thing to shout with a bunch of exclamation points after it so right now we're getting an error here because java is saying hey t isn't the type what do you want me to do with this to tell java that this t is supposed to be a generic type right here in the method signature right before the return type you put that generic type in angle brackets again you can call this whatever as long as it matches here and in the parameters but by convention you're still just going to see t most of the time so now here in our main method we can call our shout method with whatever we want you can send in a string like john you can send in an int 57 or you can even send in a cat so this can take in any type of parameter that we want to send and when we run it it'll print them out with exclamation points java also supports the ability to have multiple different generic types here so in addition to taking in this t thing to shout we can also take in say the other thing to shout all we have to do is add this v to our angle brackets here and separate them with a comma and now this method will take in two of any sort of type so we can pass in john and 74 and also go ahead and print out other thing to shout and that works and is now flexible with any two types at all if you want you can also have multiple generic types like this in your generic classes so over here in our printer class if we wanted to have another field here let's call it v other thing and all we have to do is specify that up here in our angle brackets as well separated by a comma if you happen to want to return one of these generic values that are being passed into your generic methods all you have to do is specify that return type as the return type here so instead of void if you want to return something of type t you just put t here as the return type and then in here of course you just have to return something of type t which can just be this thing to shout the other advanced generics topic i want to talk about is wild cards let's talk about a situation where you might want to use a wild card and then we'll show how you can do it let's say we wanted to be able to create a method that can take in a list that holds any type of objects and we want to be able to print out that list so we might call that print list so how do we specify in the parameters here that we want to be able to take in a list that contains any type of thing you might think that we can just say okay i want to take in a list of objects we can call it my list and all we'll do is just print out that list to the console so now up here we can create a list of integers call it in list equals new array list we can take our int list and add the number three and then we can try and call our print list method with our int list as the parameter but if we try and do that we get an error here that says we can't pass in this list of integers as this parameter that's supposed to be a list of objects but it might feel like you should be able to do that right of course an integer is a subclass of object so shouldn't we be able to pass in just a list of integers well even though integer is a subclass of object a list of integers is not a subclass of a list of objects so this doesn't work this is where a wild card will work to solve your problem so instead of saying that this is a list of objects we say that this is going to be a list of unknown by passing in a question mark this question mark is the wild card you use a wild card a question mark as the type parameter when you're using a generic when you don't know what exactly that generic type is going to be so it's saying hey java i've got this method here and i want it to be able to take in a list of anything but i don't know what it's going to be a list of so now you can call this method with a list of whatever you want in this case we're doing it with a list of integers but you can also do it with a list of anything else let's say we had a list of cats add a new cat to that list change this to catlist here and pass the catlist into our print list method and it now works with lists of both types with no errors you can also have bounded wild cards similarly to how we did in our generic class so instead of being a list of anything at all we can say this has to be a list of something that extends the animal class but now you can see up here that we're getting an error when we're trying to call this method with a list of integers and that's because integer doesn't extend animal we don't get that same error here when we're calling it with a list of cats because cat does extend animal if you enjoyed this video or learned something please let me know by leaving a like and be sure to subscribe so you don't miss each new java tutorial and of course don't stop now keep learning by watching one of the other videos below thank you so much for watching i really appreciate you being here with me i'll see you next time

Channel: Coding with John

Views: 620

Keywords: java, codingwithjohn, coding with john, java beginner lesson, generics in java, java generics, java generics interview questions, generics java, java generics wildcards, java wildcards, java generic, generics and collections in java, generics and collections, java generics tutorial, java generics explained

Id: K1iu1kXkVoA

Length: 17min 34sec (1054 seconds)

Published: Mon Dec 20 2021