19 - Java Collections Framework/019 Set Implementations (HashSet practice) & java.util.Colelctions class_en.srt

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Hello.

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Yes, students, today we're going to learn implementations of set interface and talk about utility

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methods from Collection's class on how that works inside when we review hashad source code.

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After this lesson, you'll understand how set are different from list will answer what mechanisms is

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used to keep only unique elements inside the set.

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I'm going to show you straight safe implementation of that interface and we'll have practice today.

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And at the end of the lesson, we'll review with your class, with utility methods.

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That is collections class.

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Let's start, as you remember, said Interface is located in the collection Iraqi and does not introduce

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any new methods.

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Do not forget that that is not a list and said by default does not guarantee any order of elements and

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index access to the elements.

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The main feature of that interface and all of its implementations is that it contains only unique elements

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that various business cases when you need to use set, for example, you are retrieving data from different

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data sources and you know that data might be duplicated, but you don't need any duplication in this

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case.

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You just add elements to the set that takes control over all duplicates, or in the case you interact

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with user and want to avoid any duplication in information that was provided by user.

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You you said there are really a lot of cases where you can understand that you need set.

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And now imagine that if you would be asked to implement something that keeps only unique elements and

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contain it, how would you do that?

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Would you use any other containers that you already know something that's already stores?

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Only unique elements?

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Try to think for a second before I would give my answer on map implementation store set of keys.

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And each key is unique.

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There is no sense to have duplicated keys.

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That's why in the case we would need to implement set interface.

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Most likely we would use any map implementation and delegate method invocations to the map.

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But from the side perspective, it would look like we interact only with keys and we have regular collection,

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but not the map.

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Let's review the most popular implementations of that interface.

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I would say that one of the most popular set implementations is a hash set based on my experience and

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most of the cases when I need that implementation, I use hash set.

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It extends abstract set that gives implementation to some basic operations, namely two equals hash

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code and remove all methods and implements.

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Set interface hash that is based on the hash map hash that aggregates map inside.

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And we'll see this today when we investigate the source code and other popular implementation of that

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interface.

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Israel hash set this one user linked hash map inside.

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It extends hash said main feature of link hash said is that it provides predictable iteration order.

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The next set implementation is called Resat.

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Can you guess what it is based on?

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You are right.

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We said use a stream mapping site to keep all elements sorted according to natural ordering, what with

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the help of comparator that was provided during the construction of the object.

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It also extends Absaroka set and implements navigable set interface that in turn extend sort of set

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interface.

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And I really believe there will be no need to investigate these interfaces from scratch.

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Take into account we reviewed in detail, sorted map and navigable map interfaces.

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One will look at the source code.

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You understand that implementation of set interface is pretty straightforward for you in case you learned

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maps.

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Well, that's why I always, along with my students, map interfaces.

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First, if you need thread safe implementation of set, you have to choose among different options.

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Some of them are Kapyong Right array set the said that is based on compound right or at least that keep

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on the unique elements in container concurrence.

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Keep list set that is based on concurrence.

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Keep list map where elements are sorted according to natural ordering or according to the comparator.

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In other words, this is thread safe implementation of navigable set.

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And there's also one more option is to use utility methods from collections glass that will churn any

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map to set.

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There's also one more implementation of set interface that is usually mentioned in books that is and

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set.

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This type exists specifically to store only enum elements.

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To be honest, I didn't use it very often, but it worth to match it.

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OK, it looks like now you are familiar with the set Iraqi and now you're ready for the demo, and we'll

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start our demo today from overview of hash that Scott.

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Here you can see the property of Type Hashmat Parametrized by any type four keys and object for values.

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Also, you can see here a constant object that is damage value that will be used as a value that will

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be associated with all keys in this map.

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Take into account and have said we are not bothering about the values in the internal map.

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There is a way to save some memory by putting always the same object as a value into the internal map.

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Does it make sense?

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You can find different variations of constructors here, but also considering the fact that Hashad is

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based on the Hashmat, that in turn is based on the hash table.

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We can specify initial capacity and load factor in case we want to change performance a bit.

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And after that you can see that implementation of all methods just delegated the course to the map and

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adopt interface when it is needed.

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Here is a size method that just calls map size.

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Inside here is is empty masset that just cause map is empty inside and so on.

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Let's look at at method here.

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You can see that we call Puth Masset from MAP.

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We put the element that was passed as an argument to this set into its map and associate it with the

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constant dummy object that we saw at the beginning of our view here.

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According to collection interface at Mass, it should return true in the case element was inserted and

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force in case element was not added to collection for some reasons.

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I believe you remember that put method in map returns the last value associated with this key.

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And in case this is in Uki, it returns now.

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So in the case this statement is true, that means an element was added to set.

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Does it make sense how this makes things clearer?

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Let's quickly review Sodded set a navigable set interfaces to make sure that these interfaces are pretty

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similar to sorted map and navigable map.

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Here is the source code of sorted set interface.

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You can see pretty familiar to you interface that has methods for retrieving first and last element

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from container and creating different variations of a subset.

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Now let's open navigable set interface.

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If you remember navigable map interface, you already can see that massas name are familiar to you.

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The only difference is there are no methods that returns entry type, but the rest is really similar.

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Now let's practice a bit to see how that actually works.

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I created this demo file to show you how you can interact with anisette implementation.

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Here you can see that I declare the variable of type set parametrized by integer.

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That means calculator will watch for me that I would add only integers in this set.

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OK, let me add a few elements to the set here and print the output of each at Abberation.

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After that, I will print all set to console, let me run this program and you can see that when I added

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one the first time it was successfully inserted, but when I tried to insert one, the second time,

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it wasn't inserted and we got false.

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Two and three were added successfully.

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And here you can see my set of numbers.

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Only three unique numbers are here without any duplication, just one common here.

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I believe you understand that uniqueness of elements is determined based on the hash code and equals

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matters, because these two methods help us to place element in hash table.

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Let me bring you this great new set parametrized by user type in case you missed homework about online

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store.

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That is our type from the online store application.

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I create two objects of default user type that are the same, but the default user class does not override

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hash code and equals masses.

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That's why, even in case logically these two objects are equal, set will store both of these elements

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and here print it all set elements to console and you can see that both users are in the container even

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despised.

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They seem to be logical the same.

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Now, let me show you another example.

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I agree to objects that overwrites equals and hash code matter.

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If you remember, during one of the lessons we created the user for hash tables, and this is exactly

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the object from our lesson about hash tables.

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If you remember here, you can see equals and hash code overridden.

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In case I would put these two objects in the set, only one will be added.

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Here, I change the first name to William and set a different idea to distinguish users without hash

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code and equals methods, and we use these methods of freedom here.

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When I printed all users to console one by one, you can see that only one of them was added here.

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Also, the things that you have already noticed is that you can use Iterator and foreach loop to iterate

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over all elements in set take into account.

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It is also implements iterable interface and has its iterator.

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Now, you know how to use sets in your work, and the last thing that I wanted to share with you today

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is collections glass.

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Why do we need it?

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Sometimes we need utility mass.

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It's like sort of a collection.

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For example, let me open the source code of this glass and explore it together with you.

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It has really a lot of the masses.

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I will not provide my comments for all the masses listed here, but I will tell Fumaroles about only

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the most popular ones in my opinion.

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And I encourage you to use a source code of this class after the lesson and practice and meet with this

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class by calling different methods.

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Here we can see Masset.

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So sometimes instead of calling sort method on the least, we can call source method of collections

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class, you can reverse order of elements in your list if you wish.

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You can call reverse Masset and pass reference to any object there if you wish.

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Here is a shuffle method that you saw we used for demo purposes in one of the lessons and past.

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Also, you can see some of Masset to swap elements by specifying position indexes or field method to

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feel content with the specific elements.

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He is very useful copying method that allows you to copy list methods to find the least.

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And the greatest elements in collection are also here and here.

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And one of the coolest methods you can take is a list or said unnavigable, said unnavigable map or

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anything other to get unverifiable object.

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In other words, the methods that can rub your objects with unquantifiable ropa when in you just imagine

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that you implement a method for cars or objects in online store and you don't want to return products

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of cards and to let anyone else in other parts of the program to add products directly to the list.

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Does it make sense?

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What you want instead is that in case somebody wants to add product to the cart, they have to use that

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product that contains additional business logic.

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It can be tax free calculation or discount of card calculation.

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And now we can say that we return on modifiable list because of clients of our cars.

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Want to investigate what products in it they can look at them.

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There is no harm in this.

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But in case they would like to modify the cards, they will catch and unsupported separation exception,

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the sad creation of wrappers for one modifiable collections.

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We also can create wrappers to turn our regular containers to thread safe containers.

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You can find a bunch of methods here that can return to synchronized list or map or set or anything

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else.

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If you wish, let me open another my file to show you a few more examples.

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Collections.

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Glass also has Masset that can turn any map implementation to set.

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For example, I can create that safe set on the basis of concurrent Hashmat just like this.

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Take into account en methods are static and collections class.

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I write collections class name after that I call method new said from map and parse the concurrent hash

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map object where keys are parametrized with the same type as my set.

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According to interface, the values should be parametrized by boolean type and assign this object to

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my set variable.

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When you might want to use this, for example, you're extremely satisfied with the performance of concurrent

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Hashmat and you don't want to use generalized wrapper suggested by collections class.

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That's why you might want to use this method to create set implementation exactly on the basis of specific

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map implementation.

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Does it make sense?

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And the last but not the least super useful feature of collections.

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Glass is empty constants.

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To make you understand what the benefit we can get, I prepared example here for you.

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Imagine that you have some in a program that is supposed to return to your list of products that are

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associated with a specific course, i.e. let's call this method get products by Karzai.

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Awesome.

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Now we are trying to retrieve products from database by card idea and imagine we got now in response

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because there are no products associated with this card.

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Remember, as a rule, my return now from any method you increase the risk of facing with no point the

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exception in case somebody would interact with the result of this method.

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In other parts of the program, what is recommended?

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It is recommended to return empty lists instead to avoid Northpoint exception.

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OK, let's look at this option now.

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Imagine that you have huge online store and this method is invoked very often.

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This will lead you to creation of thousands of objects of our least the need to load your memory even

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with empty lists.

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I don't think so.

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But what is the best option?

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In my opinion, the best option in this case is to use predefined constants from collections.

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Glass Empty List.

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Masset will return you parametrized collection based on the empty list constant if you need you.

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And also use empty, sad or empty map, for example, hope this information will help you a lot in the

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future.

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OK, now I believe we can recap the things that we have learned today.

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Today, we learned that Iraqi you learned the main interfaces and the most popular implementations of

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that interface after that review, the source code of Hashmat.

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And so how set implementations use map implementations inside.

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We also had the practice with Hash said, and you saw how hash that keeps only unique elements.

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At the end of the lesson, we learned a lot of utility methods from Collection's class.

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You also learn best practices of using unverifiable and synchronized robbers.

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You also know when and why you have to use empty containers.

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Thanks a lot for your attention.

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Have a great day and see you in the next lesson.