Spring Framework Crash Course 🚀 Dependency Injection, @Autowired & @Qualifier Explained

Key Takeaways

Anybody any doubts? Sir please clarify what kind purpose to use classes interface and also abstract class. So abstraction is the reason why we have uh interfaces and abstract classes in Java. They both have a very unique property called as abstract functions. So when you want to make a mandate or when you want to make sure that something must know how to do ABC then in that situation you will want to use interface or a abstract class for example if let's say you are creating a vehicle then that vehicle no matter of what type it must always know how to move forward how to move back how to move left how to move right whether it's a rocket or whether it's a cycle Right. So just like that you can create an abstract class vehicle and create whatever children you create of that class or vehicle whatever type of vehicle you create. You can always ask these classes to have the function of start, stop, left, right something like that. So that is where we will use abstract classes or interfaces. If your abstract class does not have any uh does not have any non-abstract members and does not have any data members, you can convert it into an interface also. I would really prefer if you can unmute yourself so that we can talk rather than writing the questions on the chat box. Hello sir. >> Yes. Hello >> sir. I have confused the singleton that topic is a singleton that I have confused that you that you can clear my >> okay so singleton is basically a type of class. I mean singleton is a design pattern actually which means that at any point in time only one object of that class can exist. That is what it means. Are you understanding what I'm saying? That is what it means. A singleton is a class whose only one object can exist at one point in time. That is the meaning of singleton. Let me show it to you. Let's say I create a class called as my singleton. Okay, this is my so I want to make this class a singleton class. So what I will do is I will write >> so that coding we can't we can't see that. >> H oh sorry sorry I was trying to code and I did not share my screen. Thanks for pointing that out. Can you see my screen now? >> Yes sir. So this could have written right now. So this is a singleton class. Now if I create a different class and if I write main function, if I create first object and if I write the second object M2 then you will see that they both are pointing towards the same location. You will see it will say true. Why does it say false? Uh yeah, I know it's always true. Then why is it saying false? M1 equals to M2. Oh, I'm running a different file, I guess. Oh, that's strange. One second. Let me check it. H Oh, I think I know the mistake. I know my problem. Hm. Now it's the same. So this is an example of a single class. At any point in time, there's going to be only one object of this class. >> Thanks sir. >> No problem. predicate function. >> Yes. >> Okay. For those who do not speak in Hindi, uh Prince was asking what is a you what is a predicate function in Java? Okay. So, Prince is English okay for you or you want me to explain it to you in Hindi? I can do that. >> No sir, both. Okay, sir. I can. >> Okay, fine. So, let's look at the predicate function. So, predicate first of all is a functional interface in Java. That means it can be written as a lambda. It can be written as a lambda as well. So let's try to understand this functional interface. It has a it has it is a generic interface. It takes a type t as a parameterized uh type and it has a only one function called as test whose return type is boolean. So in short what predicate does is that if you give it something let's say you give predicate information of a user okay and you want to see whether this user can vote or not then you can say if the user age is greater than 18 then they can vote right can we say that prince? >> Yes sir. >> If the user age is greater than equal to 18 then they can vote. If lower than equal to 18 if lower than 18 then they cannot vote. Right? Simple logic. So that is how we use uh that is how we use the predicate function. So we give it something. Let's say I write predicate of let's say string. Predicate of string uh predicate is equals to whatever string I'll pass str whatever string I'll pass. I want to see whether the string is total length is greater than greater than 10 suppose. So here what I'm doing is I'm taking a predicate of type string and I am checking if the string's length is greater than equal to 10 or not. Now let's say create a string string my string hello I am something like this. Okay then I can say predicate dot test my string. Then what will this do? Please do not draw on my screen h okay so I've written str.length uh greater than equal to 10. Are you able to understand what I've written? >> Yes sir. >> Okay. So for any string I can always write a test function and I can see whether this value is greater than 10 or not. That is what I can do. Correct. So if I print this, what should it print? True or false? It should print true. >> Clear. >> We can use without lambda function also. So >> yeah, we can use without lambda functions also. But we we can use without lambda functions also if we had to write code straightforward. But sometimes we want to pass a predicate function where uh we want some logic to happen. For example, the filter operation of the of the stream of the streams. So that takes a predicate for the with the help of which you have to filter out something from a collection. If you want to make this work without a lambda function, you have to create a class that implements this predicate interface and then you have to create an object of that class and then call the test function. But why to do it when it is a function interface and we can make use of lambda. >> Okay sir. >> Hello sir. >> Hello. >> I'm audible. >> Yes >> sir. In Java why we can't create object for abstract classes >> sorry why why we cannot create object >> object for abstract classes >> that's how Java is so you cannot create an object of an abstract class you have to create an child of that abstract class and create an object like that Java does not allow creation of object from of an abstract Yes. Any more questions? >> Okay sir. In stream API that we are using that collector that concept you can clarify. >> Can you repeat your question? >> Stream you are using that con uh contract uh sorry >> collectors. >> Yes sir. H so it is a so when you write dot collect it is a terminating operation of your stream. So you can have an option you have an option of collecting all the elements in your stream and converting it into a collection. That's where you use collectors.colct or stream.colct. So if you have let's say stream of integers and you want to make a list out of it then you can use dot collect and then inside it you can write collectors dot to list. >> Okay. So when we uh collector in that time we are adding making one uh new array list on that time what we can do s you are indeed making a new array right. So for example, let's say you have a list of list of integers. Okay. So you have a list of numbers, right? So you can let's say filter out only even numbers. So here you have filtered it and now you are putting it into a list. So you're going to say list of integer even numbers. So here you see this filter call it takes a predicate which is what we studied above. >> Yes sir. >> So collector will basically collect everything that is in the stream and then put it in this variable return it. Okay. Any more questions? >> Hello sir. >> Did you say yes or no? I didn't didn't catch that. >> Hello sir. >> Yes. Yes. Tell me. >> Sir. Uh I'm talking about the application context uh in the so >> in spring. >> Yeah. >> So in spring the uh in application context it allows us to internationalization for internationalization. >> Uh what is it? Can you please explain with an example? I couldn't figure out. >> You want to understand what is internationalization? >> Yes. Internationalization or INTL is basically a technique that is used for front- end engineering a little bit in back end engineering where you want to present your data in an international format according to where people are living. For example, in India, if you have to if you have to show this number let's say one. How will you show this number? It is one lakh. So you will show it like this. >> Yes sir. >> Right. So this is one lakh >> in India. But if you have to do this in USA, you'll say >> 10 100,000. >> Yeah. So this is equal to 100,000. So using internationalization, we do these kind of things. You you change you translate error messages, you translate responses, you translate websites. That's what happens in internationalization. You also take care of time zones and time local time etc. >> Okay. That is what is a that is the main fund of internationalization. >> So please explain internal working of map. That's okay. It will take a long time but okay I can I can explain internal working of a hashmap. One second. >> H. So, do you know what is a hashmap? It's a key value pair. >> Correct? >> Yes. Do you know what is hash hashing? >> No sir. >> Okay fine. So there is a class in Java called as object. This class is the parent of all the classes of Java whether created by you or already present in Java. By default the object class is the parent of all classes in Java. And this class has a function uh this class has a function called as hash code. This has a function called as hash code. So now hash code can convert any object to hash. Now what is hash? Hash is equal to integer value. Okay. So, hashmap depends on the logic of hashing and it m and what is hashing? Hashing makes use of the technology of hash code which can convert any object to a hash which means any integer value. Okay, are you able to understand this much at least? Are you able to understand this much that there is something called as hash code? >> Yes. >> Okay. Okay. So now we know that a hashmap is a key value pair. So let's create our own key. So you can so your key has to be a class and value also has to be a class. So you can I can I can maybe name it as my key for better understanding. My key. Okay. Now let's say I create something like string key. Then I create a constructor. Then I override the hashmap. Oh sorry. Then I write equals and hash code functions. Okay. Now look at this equals and hash code function. So what I want to do in hash code, I want to write int this dot e do.carat zero. What am I trying to do over here? I am trying to tell Java how to convert this value into a hash code by overriding that function. Are you able to understand this? >> Yes sir. >> So you what am I doing over here? Whatever is the key I have I'm taking the ask key value of the first character to understand what is the hash code. So ask key value is what? Integer only right? Yes. >> So I'm trying to get the first character of this key to understand what is going to be my hash code. Okay. So I'll also have to override the equal. So how do I understand if two values of two my keys are equal? If this is equal if two objects of this class have this have same value for this then we can say they both are equal. >> Not clear. >> So please explain once again. If I'm I'm saying that if let's say you have two objects of this class my key >> object my key A and my key B right and my A dot key is also equal to test and B dot key is also equal to test then we can say that A is equal to B >> yes >> right so the same thing we'll do over here so I will write return this dot key dot equals equals O dot uh one second. Oh, one second. I think I'm making one small mistake. H is this making sense? The same thing I'm doing converting the key value and saying they both are equal or not. Clear or not clear this part? >> Yes. clear right? >> Yes. >> Okay. Now let's move ahead. Okay. So now let's say I write something like this. So when I create a hashmap in a different class demo, what happened to this? Is there already a demo class somewhere? Oh yes, remove it. H. Okay. Now here if I create a hashmap of type my key and value as let's say string or age integer map is equals to new hashmap. The moment I do this, the moment I do this, what Java will do is that Java will create something that looks like an array. of size 16. Are you understanding what I'm saying? >> Yes. >> The moment I do this, Java will create something like this inside the memory whose whose size is going to be 16 by default. Okay, this is what is going to happen. So now when you create or when you let's say try to add a new value map.put put map.put put and let's say you add a value as new my key and you add a value visual and let's say you add age like 20 then what is going to happen what is going to happen so look at this so vishal so my key this is the object this is the key that we have created >> yes >> so what internally hashmap will do is that it will find the hash code of this so for calculating the hash code it will come here yes or no it will come here because it will go to the my key dohashcode function >> to get the hash code so when it comes here it will find that the hash code is equal to 118 because hash code of capital V is equal to 118 sorry lower case V is equal to 118 it is equal to 118 okay so now we know that hash code is this okay but What are these boxes that we have created? Is this an array? In reality, yes, but also no. Okay. So, what are these things called? So, each of these things is called a bucket. Each of these things is called a bucket. What is it called a bucket? Okay. And what is a bucket? A bucket is a a bucket is a link list of nodes. And the node looks like this. I can write it over here. Class node. The node looks like this. So it looks like class node. And then inside it you will see three to four things. It contains int hash. It contains your key k key. It contains v value and it contains node. Next. This is how your node looks like. So now you know that a bucket is what? It is a link list of nodes. So do you see the behavior of link list over here by looking at this or do you understand link list? That's that should be my first question. Do you understand link list? >> Yes sir. >> Okay. So can you see that this is like a link list? because of this part line number 40. >> Yes. >> Okay. So what is a bucket? A bucket is a link list of nodes. Right? A bucket is a link list of node. Now we have this hash code as 118. So now in order to place this we have to find a node inside this. Okay. Now first of all to find a node we have to understand under which bucket it will go. So there are so many 16 buckets are there. Right? There are 16 buckets. Let me write it for you. So it's 0 1 2 3 4 5 6 7 8 9 let's say 10. So 0 11 12 13 14 15. Correct. This from here it assume it's 10 11 12 13 14 15. Okay. So there are 16 buckets that you have index is 0 to 15. Okay. So on the basis of this value you will understand under which bucket it will go. Okay. On on the basis of this hash value you will understand under which bucket it will go. So the calculation is something like this. This is a formula. So what is the hash code? Hash code is equal to 118. Then there is a bitwise and with n minus one. So what is n? n is equal to 16. So if you do a bitwise and bitwise and of8 and 15 you will get a value which is equal to something. Now what is this value? I think as far as I know it should be equal to I guess six. It is going to be equal to six. That means this information will be going into node number six. That means somewhere over here. How to bind six? >> This look at this uh function. This this is the logic hash code of your key logical and sorry bitwise and with n minus one where n is the capacity of your buckets or the number of buckets you have. So n is equal to 16. So n minus 1 is equals to 15 logic bitwise and with 118. So 118 and percent 15 is equal to six. So it will go into the bucket number six over here. Are you understanding what I said? >> Okay. >> So this is a formula internal formula. We don't we don't create it. >> Six means 1180 divid by 15 like >> this is not this is logical. This is bitwise and and operation you have studied right? 1 and 1 is equals to 1. >> Okay. >> Semiconductors if you remember logic gates. >> Yes. Yes. >> 1 and 0 is equals to 0. 0 and 0 is equals to 0. Like that. You must have studied this in school or college. >> Okay. >> This one this is called logical and not divide. It is not division. So 118 amp 15 is equal to 6. So it will go into bucket number six over here. So over here you will see a node because what is a bucket? A bucket is a link list of nodes, right? So bucket number six will start to look like this. Bucket number six, it will have a value like this. It will have one node inside which we have hash as 118. Then key is whatever key we have passed that is this thing. I'll copy it. This is my key. Okay. And then my value is 20. And then the node next is equal to null. So it will start to look like this may technically in bucket number six we have a link list which has 118 as hash key is equal to vishall and value is equal to 20 and then there is no next node so it will look like this are you understanding what I'm This is how bucket number six will look. Are you understanding what I'm doing? >> Yes sir. >> Okay. Now let's say you want to add a new value. You added map.put new key such and let's say the value is equal to 30. Then what will happen? So what is the hash of suchin? The hash of suchin is 115. Okay. Then again we will have to follow this formula. So 115 amperand 15 is equal to 3. So now in bucket number three it will look like this 115 suchin, 30 with no node. After this this is how it's going to look. Now let's say you want to add a third node map.put new my key and you add web of equal to 40. Now again the hash code will be 118 because the logic of hash code is first character is aski. So the this is also vishal and first character is v and this is webhub and the first character is again v. So hash code will become same. Are you understanding this much? Why hash code became same? >> Yes sir. >> Yes, you're able to understand right. So now again we have hash code is equals to same. So if we do 118 ampers 15 again we will get the value of six. So now six bucket is not empty. It has some value. So this is called as hash collision because h because two entries have the same hash. So it is called as hash collision. >> Yes. Yes. >> It is called as hash collision. Now what will happen? This is not free. This bucket is not empty. Then what to do? So now this function will come into picture. Equals. Now this function will do the logic. Now the moment you come to you understand it's bucket number six. Then you will come to the sixth bucket and you see it is not empty. But you have to add in bucket number six only. So what you will do is that you will check for your key. So in this case your key is webhove and in this case your key is visual. You will see are they same using the equals function. You will check are they same. If your answer is yes let's assume if your answer is yes then what it will do is that it will replace this key with with with this one. If the answer was yes if they both were equal. If they both were equal for example if I was adding vishal only vishal vishal. Okay. Then what it would have done is that if the key was the same then what it would have done is that it would have replaced the value with a new with the value whatever you're trying to enter over here. But in this case the key is not the same then what it will do is that it will go and check first because it is not the same it will go and append a new node in the link list by looking at it something like this. And this is how an hashmap will work. So now when you try to fetch a value of web hub over here, whenever you try to fetch webhub, it will find the hash. It will see 118. It will try to see which bucket it is. It is in six number bucket. Then it will do a linear search on all the nodes to see which one is web hub. Then first it will come to this node. It says okay this is not web then it will go to the next note okay this is web then it will return the value 40. This is how an inter this is how hashmaps work internally. >> This is ask value right now. >> Which one? >> Vishall ask value 118. >> That is ask value of v. Yeah. Yeah. Yeah. H So can concurrent hashmap be >> concurrent hashmap is the same but it follows concurrency principles that means it is thread safe. So it uses a synchronized keyword synchronization safe that is what it is. >> Concurrent hashmap is also a hashmap but thread safe that's it. Any other questions? Yes. >> How does uh hash tree works internally? >> Hash >> and what is the difference between linked hashmap and hashmap? >> There is nothing called as hash tree by the way. There's nothing like a hash tree. So I'm not sure what what you're trying to talk about with a hash tree. I can tell you what is the difference between a linked hashmap and a hashmap. >> Okay. >> So let's try to tell you what is a linked hashmap and a hashmap. So you already know what is a hashmap in a linked hashmap. In a linked hashmap this much this much activity does not happen. So it does not follow the hashing principle. A linked mapmap hashmap does not follow the hashing principle. What it does is it just stores the key and values in the same order as your entry. So in this case you can see that because in bucket number three such is there and in bucket number six Vishal and web are there. So when I print this map you will see that suchin will be printed first. Oh, sorry. I need to have a two string. See such gets printed first, right? So here, even though I entered suchin in the second number, but it is printing first. So the order of insertion is not maintained in a hashmap. But in a linked hashmap, the order of insertion is maintained. But the time complexity of fetching an element in a hashmap is faster than it is in a linked hashmap. In a linked hashmap, the time complexity of fetching an element is O of N. But in linked in a hashmap, it is O of 1. any more questions? We have 10 minutes last. sir. >> H >> actually I was asking hash table. >> Hash table. Okay. So you want to understand the difference between hash table and hashmap, right? >> Yes. >> Okay. Let's try to look. Hash table. This one. This one. Right. So, what is a hash table? First of all, hasht is synchronized. Uh, it is it has multi-threaded support. Okay. Uh, hasht is technically everything that hashmap can do but synchronized. That is one thing. Second thing is that uh in hash tables uh okay now one second. The only difference is uh one second let me let me try to write it for you in a way that that will help you So first of all hashmap is not thread safe. Hasht is thread safe. Okay then it is faster because not thread safe slower. >> Sir I have doubt what is actual means of thread sir. For that you need to know multi- threading. Do you know multi- threading? >> No sir, I have doubt in thread. >> Yeah. So if you don't know multi- threading, it will become difficult to explain thread safety. >> Can you explain shortcut sir? >> Uh it will take a lot of time. Multi-threading is one of the most difficult topics to understand. I think in seven minutes we'll not be able to cover multi- threading. Okay. Yeah, I think that's that's it basically. H this is the difference nobody uses hash table now Any more questions? Anyone I am not sure about that. Is it possible to map another key new key vague 10? Uh I'm not able to understand your question Mr. T SN the other one which is about keys. >> Sorry. Yeah, this is the illustrated the shine here. >> Okay. Any more questions? We have 2 minutes. while doing uh this week's uh uh quiz uh there was a question about the be uh in the beans. So on which of the annotations are used for for beans? There were options uh at the rate component at the rate beans. >> I I gave at the rate beans why it is uh >> incorrect it is incorrect. So it was showing at the rate component. >> Yeah that's so so it's a spring boot logic. So add the component is used for creating beans. Add the bean is used for telling how to create a bean. >> Oh >> yeah. Add the rate bean is used on a function that tells how to create a bean of a certain type. So the real answer is add the rate component. That is what is used to make a bean. >> Add the component is for making the bean and the bean is for specifying which type of bean it is. >> No, at the rate bean is to tell how to make a bean. >> Okay. >> For example, you want a bean of type A. Then on class A you will write at the rate component but how to make a for that you will have to use at the rate beam. >> Okay sir. >> Okay. >> Any more questions? We have one last the last one minute. Okay, if there are no more questions then I can take a hard stop here. Thank you everybody for joining the class. I'll see you next time. Bye-bye. And