19 - Java Collections Framework/014 Map Hierarchy_en.srt

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Hello, dear students, today we have important topic we are going to review another Iraq of Java collections

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framework that is MOP Iraqi today.

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We are going to run map interface.

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We are also going to review such interfaces like sorted map and navigable map.

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I'm going to hold overview of implementations of map interface and what features they have.

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Also, we'll talk about what the dictionary is and how hash tablecloth is different from hash map.

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After we learn, Ceri will focus on map interface and its methods.

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Your review of map interface.

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We also will learn what entry type is.

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As you can see that we have a lot of things to learn in our plans.

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Let's start and I'd like to start with you from the overview of Map Iraqi first and the key features

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of MAP.

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And after we learn this, we are going to dive deeper in the details to investigate each interface in

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particular on the top of our Iraqi, we have map interface.

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This type exists to map keys to values.

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All keys are unique, various may be duplicated.

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And there are a lot of places and our programs where we can apply map implementations.

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If you remember on one of the lessons I showed you example of mobile app for sports betting tips, for

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example, we can receive from server a lot of sports events.

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After that, we can group them by date and your keyword would be the specific date and the value would

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be a collection of sporting events on this day or another example, recently you have implemented Home

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Task Help Desk.

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In that task.

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You also could use map implementations to map support.

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Request type was its priority.

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In this case, support request type would be the key and it will be unique and priority would be the

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value and can be duplicated.

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And I keep creating examples on the fly because there really are a lot of cases where you need some

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data structure where you would store key value pass.

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OK, so we really understood why we need map.

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Now let's understand what specific different implementations of map interface have abstract map use

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implementations for some of the methods of map interface, hash map implementation of map interface.

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IT stores element using the mechanism of hash tables that guarantees constant amount of time for each

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region and place elements.

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That's why everyone loves hash map linked hash map.

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Besides storing elements also memorized the sequence of adding entries in the map.

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So the order of elements during the iteration is predictable because linked hash map maintains the list

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ran through all of its entries.

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If we need thread safe implementation of the map, we would opt for concurrent hash map that implements

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concurrent map.

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That in turn extends map interface.

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We have a few more interfaces that extend map interface.

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Zaya sorted map and navigable map sort of map provides ordering of a map based on its case.

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The map is ordered according to the natural order of its keys that is defined according to implementation

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of comparable interface or alternatively, Solin order may be defined by comparator navigable map extensors

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map and provides developers with possibility to return the closest matches for given such targets.

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The most popular implementations of this interface are three map and concurrence key place map for multithreaded

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environment.

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We also have APSA class dictionary.

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How it is different from the map interface you can reclass was created in Java version one and supposed

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to be a parent class for all types that map keys to values.

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Since multiple inheritance in Java is not supported by the map interface was introduced in Java version

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two.

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So I would say that dictionary is an old version of map interface and that is considered as an obsolete

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and use of map is preferred.

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But still, you might be asked about dictionary during the interview and for your common Java literacy,

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you have to be aware about it.

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Hash table is a concrete class that extends the dictionary type and internments map interface.

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It was considered as a thread safe version of hash map unless concurrent hash map was introduced in

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Java version five, the synchronization mechanism is not so efficient as it is implemented in concurrent

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Hashmat, in case you are not familiar with the trending topic.

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That would explain the difference in the implementation of low interest rate and support in these classes.

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In simple words, hash table closes the door in your home.

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Concurrent hash map closes a separate doors in your home to allow everyone to walk around the home until

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only one room is closed.

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Can you feel the difference in implementation of concurrency support in these two classes and meet later

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in the separate lessons?

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We are going to learn multithreaded together and we will get back to a thread safe map implementations.

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That's how MAP Iraqis look like.

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Before we proceed with the details, I want to finish the topic of differences between hash map and

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hash table.

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So the first thing that is different we already discussed on MassArt and hash table are synchronized.

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And that's why hash table is considered to be a thread safe map.

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Hash table also does not allow news, a new case nor new values.

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On the other hand, hash map allows us to store one key and multiple values.

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Hash table is a legacy class, whereas hash map is newer.

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And now I suggest opening the source code of map interface to learn that the communication and methods

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of this interface here is map interface.

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Pay attention to this interface doesn't extend iterable interface like collection while review and map

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interface.

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I'm going to use and work by saying entry, I would always mean the key value pair.

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Is it clear, for example, size returns, amount of entries in the map.

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In other words, a number of key value pairs is empty.

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This is self-described Macit.

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It checks whether the current map is empty or no, contains key verifies whether a map contains specific

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contains value, similar method to check value in map, yet method to get value from the map we have

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to use.

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So we ask you to get Masset to retrieve our value.

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There is no method in map.

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Instead, we have put method.

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We can put key value pair in map.

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That's why put method takes two arguments.

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Pay attention to the return tab here.

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Imagine that you put in map integer value one and some customer reference type user as a value to the

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map in this case, but will return now and now.

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Imagine that you put new value but with the same key you call put MassArt again was integer value one

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and new user object.

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In this case, both method will return the value that was previously associated with the key.

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That means previous user.

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Does it make sense?

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If you want to remove some entry from the map, you have to call remove Masset and Paskey as a method

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argument map interface, declare methods for bulk operations, for example.

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Put all method can add all entries from one object to another.

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Clear message.

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Remove all mappings from the current map object.

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He has said massive returns contain the offset type that contains only keys from this map object.

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Why set?

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Because all keys are unique and map.

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That's why I said type is returned.

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In case we want to retrieve all whaler's from the map, we have to call values method that returns collection

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of elements.

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Why collection?

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Because values are not unique and collection is returned.

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Here's one more very useful method and set this massive return set of entries of this map.

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Each key value pair is represented as a single object of type entry.

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You really understand why set returned?

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Because each key value pair is unique and three is in our interface in map.

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Let's look at it.

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Entry type has to match assets that are used more often than others.

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They are yet key and get value.

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For example, we can iterate over a set of entries in foreach loop and in the foreach loop body we can

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use reference to the entry object together is a key or value.

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It depends on what exactly we need.

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According to our logic.

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There are also other methods that data on my opinion are not used very often.

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For example, set value.

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It allows us to set value associated with the current key in the entry.

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In my opinion, if there would be such need, I mean to update the value associated with a specific

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key, most likely you want to do this with entry time, but instead we'll just call good method with

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the same key on map object to update the value.

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But probably one day you would face with the case that while iterating over entries, you need to implement

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logic that under some specific conditions you have to update value that is associated with a specific

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key.

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And for this case, you already know what method can help you with that.

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You can see equals and hash code.

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You already know these methods and also entry has a few interesting static methods.

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We are going to use them in our next lessons.

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Right now.

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Let me explain you and see what I say about we have comparing biochem assets at source entries by case

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in case there implement comparable interface and we compare the parameter in case keys don't implement

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comparable interface.

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These methods return compared to that we can use to search and now attention to search our entries set,

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but not elements in map bytes key.

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And we also have comparing by value method that returns comparisons that can compare entries set by

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values.

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Imagine that you want to search your map by.

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Well, to be honest, compared to that source, my case is not so cool.

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Why?

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Because we can create object of three map and put all elements from any map there and elements will

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be sorted automatically by case, but sorting by various.

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That is really helpful compared to now when we answer it, what entry type is.

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Let's investigate what matters are left here in the map interface equals and hash good methods and.

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The default message I described, for example, here is interesting and useful message.

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Imagine that you need to get away from that, but you don't want to get new in case Element is absent

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in map.

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Instead, you would like to get some default value we can call yet or default to retrieve element by

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key in case and market for such key present or to return default value in case there is no mapping for

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such key.

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Pay attention to the fact that in case we exist and values that is associated with this key is no value,

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then no will be returned.

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Default value will be returned only in case there is no such key in the map.

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For each matsutake takes function that should be applied for each and every one will return functional

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programming with Java.

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That will show you how you can apply this method.

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Replace all text functions that will be applied to each entry and each entry in turn will be substituted

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with the result that will be returned by this function.

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But if absent method should be used when you don't want to override the values that is already associated

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with a specific key.

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So new value will be put into the map only in case there is no entry with such key or keys associated

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with no value.

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The removal method can help you to remove entry by specifying the key value pair here.

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Also replace MassArt.

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They are different from Puth method because you can say that you want to replace Element only with a

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specific key and with a specific value, or in case of overall that method only in case such key exists.

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Comparative absence method allows you to compute value in case he is absent.

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We can pass the keys that doesn't exist in map and function that can compute new value.

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In case such care exists, the value won't be updated.

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Compute if present method takes key and functions that can compute the value based on the current key

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and previous value that is associated with it in case he exists and previous value is not.

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Now, the result of the function will be associated with this key compute method.

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Attempts to compute and mapping for the specified key and its current mapped value is there is no current

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mapping.

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Mersch method works in the following way.

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It is a specified key, is not already associated with a value or is associated with.

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Now associate it with the given no new value.

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Otherwise replace the associated value with the results of the given remapping function or removes if

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the result is now lost for assets.

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Require knowledge of functional programming in Java.

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That's why they will be reviewed in detail separately in the lessons about functional programming.

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Right now it is enough high level understanding what this message do, and now we are done with all

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default methods in my interface and it is time to review static methods in this interface.

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Here we see overloaded static method with the name of this method, say here since Java version nine

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and they can create immutable.

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That also means unverifiable map that contains a limited number of mappings.

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And you can see that the only difference between all of these methods is a number of mappings that it

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takes as a parameter.

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Here's a method that takes variable lengths, argument of type entry, and based on these creates and

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modifiable map and remasters returns and modifiable entry type based on the key value pair that is passed

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to the method, why this method is needed.

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This method is needed, for example, for the previous method that we have just discussed, because

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you want to eliminate the case when your entries changed because somebody got access to it by reference.

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That's why you might want to create an modifiable entry first.

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And this method will help you in that.

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And the last but not least method for today is copy of.

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If we want to create the copy of the map that nobody will be able to modify, we have to call this method

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and parse the map that we want to copy as an argument to this method.

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That's all method for map interface.

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I believe that right now, you know, map interface even better than some senior software engineers.

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Let's recap what we have learned today.

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Today, we learned Map Iraqi.

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We talked about features that are introduced in the implementations of map interface.

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We also talked about source, map and navigable map.

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We discussed what dictionary is.

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Now, you know what the difference is between Hash Table and Hashmat.

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After that, we learned methods of map interface.

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We also learned what entry type is.

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That's all what we have for today.

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Thanks a lot for your attention.

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See you in the next lesson.