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--- a/content/courses/prog-intro/lectures/intro.md
+++ b/content/courses/prog-intro/lectures/intro.md
@@ -88,4 +88,58 @@ Java comes in two parts: **JDK - Java Development Kit** and **JVM - Java Virtual
There is also a **JRE - Java Runtime Environment**. For example if we want to run our code somewhere on the server we don't need to compile it there because we have our byte code and we just need JRE to run it.
+Some of the most popular JVMs right now are:
+
+- OpenJDK
+- Eclipse
+- Azul Systems
+- Excelsior JET
+
+The disadvantage of such system is in the connection between JVM and a native processing unit. In case of C++ compiler that we reviewed earlier the source code is compiled directly into machine-code but in case with Java it is compiled into byte-code. And so the problem is to develop such a JVM that would quickly turn our byte-code into machine-code. Anyway it takes extra time. That's why it mostly will be slower than direct compilation into machine-code. So ultimately while we have the advantage of compiling out code only once, we have the disadvantage of turning byte-code into machine-code slower.
+
+None the less, there is a way to speed up this process which is called JIT - Just In Time compilation. How does it work? While our program is running some of the instructions or functions turns directly into processor commands.
+
+# What is garbage collection?
+
+For example we have `int` which is represented with 4 bytes of data which is directly stored in memory.
+
+
+*Img. X -- `int` in memory*
+
+But what if we have a `String`. How many memory cells does this string take? We don't know. We will say that our `String` that has length of 5 symbols is stored at `0x12347865`. We defined an address where this string is located in memory. And somewhere in the memory of our programm will be a large buffer where the 5 cells will be stored.
+
+
+*Img. X -- `String` in memory*
+
+But what do we do with that buffer? We won't need it forever and so sometimes we need to clear that buffer. In case with C/C++ whoever created the memory for that string is in charge of clearing it. There are also languages with garbage collection such as Java and Python. The purpose of the **Garbage Collector** is to automatically find variables that we no longer need and clear their memory.
+
+Suppose we have some code like this.
+
+```java
+if (someCondition) {
+ x = [1, 3, 7] // first link
+ // some code here
+ y = x // second link
+ // some code here
+} // no links
+```
+
+After the `if`-statement we no longer need `x` or `y`. Every variable in this case `x` and `y` is the link to our array (`[1, 3, 7]`). After we left the `if`-statement the amount of links to the array is zero, so we can safely delete the array. This approach is implemented in Python. The problem is that if we have to objects linked to one another and there are no external links, they will not be deleted by garbage collector since there link counter is 1.
+
+
+*Img. X -- Edge case for link counting*
+
+The advanced way of implementing the garbage collection is *traversing the graph of links* which is implemented in Java, C# and Go.
+
+# What other advantages does Java have?
+
+- It's easy (in terms of syntax)
+- It's secure and stable
+- It supports Unicode
+- It supports multithreading
+- It has backwards compatibility
+
+# How should Java code look like?
+
+You can go and learn about it [here](https://google.github.io/styleguide/javaguide.html).
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diff --git a/public/courses/prog-intro/lectures/intro/index.html b/public/courses/prog-intro/lectures/intro/index.html
index 730fb2e..3ee8d7a 100644
--- a/public/courses/prog-intro/lectures/intro/index.html
+++ b/public/courses/prog-intro/lectures/intro/index.html
@@ -53,7 +53,7 @@ What does Java consist of?Java consists of multiple main components. The first o
Img. 1 - TIOBE Programming Community Index
Also, Java has a lot of advantages for beginners such as:
It’s fairly easy It has a broad spectre of usage Server Desktop Mobile devices Smart-cards What does Java consist of?Java consists of multiple main components. The first one being the Java compiler. The process of converting human-readable text to machine code is called compilation.">
-
+
-
On this page
+
On this page
diff --git a/public/en.search-data.json b/public/en.search-data.json
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-{"/courses/prog-intro/lectures/intro/":{"data":{"what-does-java-consist-of#What does Java consist of?":"Java consists of multiple main components. The first one being the Java compiler. The process of converting human-readable text to machine code is called compilation.\nImg.2 - Simplified Compilation Process","why-do-we-choose-java#Why do we choose Java?":"Why do we choose Java?According to TIOBE Programming Community Index Java is one of the most demanded languages in the programming field.\nImg. 1 - TIOBE Programming Community Index\nAlso, Java has a lot of advantages for beginners such as:\nIt’s fairly easy It has a broad spectre of usage Server Desktop Mobile devices Smart-cards"},"title":"Lecture 1. Introduction"},"/courses/spring-boot/":{"data":{"configuring-application-properties#Configuring Application Properties":"Let’s take a look at src/main/resources/application.properties:\nspring.application.name=store To use this property in our code, we can use the @Value annotation. Let’s update HomeController to print the application name:\npackage tech.codejava.store; import org.springframework.beans.factory.annotation.Value; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.RequestMapping; @Controller public class HomeController { @Value(\"${spring.application.name}\") private String appName; @RequestMapping(\"/\") // this represents the root of our website public String index() { System.out.println(\"application name = \" + appName); return \"index.html\"; // this returns the view } } After running the application, we can see store printed in the terminal:\n... 2026-02-19T15:32:37.507+03:00 INFO 41536 --- [store] [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Initializing Servlet 'dispatcherServlet' 2026-02-19T15:32:37.509+03:00 INFO 41536 --- [store] [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Completed initialization in 2 ms application name = store ...","controllers#Controllers":"Spring MVC stands for Model View Controller.\nModel is where our application’s data lives. It represents the business logic and is usually connected to a database or other data sources. In Spring Boot, the model can be a simple Java class. View is what the user sees. It’s the HTML, CSS or JavaScript rendered in the browser. In Spring MVC, views can be static files or dynamically generated. Controller is like a traffic controller. It handles incoming requests from the user, interacts with the model to get data and then tells the view what to display. Let’s add a new Java class called HomeController at src/main/java/tech/codejava/store/HomeController.java:\npackage tech.codejava.store; public class HomeController {} To make this a controller, decorate it with the @Controller annotation:\npackage tech.codejava.store; import org.springframework.stereotype.Controller; @Controller public class HomeController {} Now let’s add an index method. When we send a request to the root of our website, we want this method to be called:\npackage tech.codejava.store; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.RequestMapping; @Controller public class HomeController { @RequestMapping(\"/\") // this represents the root of our website public String index() { return \"index.html\"; // this returns the view } } Now we need to create the view. Add index.html at src/main/resources/static/index.html:\n\u003c!doctype html\u003e \u003chtml lang=\"en\"\u003e \u003chead\u003e \u003cmeta charset=\"UTF-8\" /\u003e \u003cmeta name=\"viewport\" content=\"width=device-width, initial-scale=1.0\" /\u003e \u003ctitle\u003eView\u003c/title\u003e \u003c/head\u003e \u003cbody\u003e \u003ch1\u003eHello world!\u003c/h1\u003e \u003c/body\u003e \u003c/html\u003e Let’s build and run our application using mvn spring-boot:run. From the logs:\n2026-02-19T14:55:23.948+03:00 INFO 36752 --- [store] [ main] o.s.boot.tomcat.TomcatWebServer : Tomcat initialized with port 8080 (http) Our app is up and running at localhost:8080.\nImg. 7 — Our app is up and running!","dependency-injection#Dependency Injection":"Imagine we’re building an E-Commerce application that handles placing orders. When an order is placed, the customer’s payment needs to be processed — so OrderService depends on a payment service like StripePaymentService. We can say that OrderService is dependent on (or coupled to) StripePaymentService.\nImg. 8 — Depends On/Coupled To relation\nLet’s talk about the issues that arise when one class is tightly coupled to another.\nInflexibility — OrderService can only use StripePaymentService. If tomorrow we decide to switch to a different payment provider like PayPal, we would have to modify OrderService. Once we change it, it has to be recompiled and retested, which could impact other classes that depend on it. Untestability — We cannot test OrderService in isolation, because OrderService is tightly coupled with StripePaymentService and we can’t test its logic separately from it. Note\nThe problem here isn’t that OrderService depends on StripePaymentService — dependencies are normal in any application. The issue is about how the dependency is created and managed.\nAnalogy: Think of a restaurant. A restaurant needs a chef — that’s a perfectly normal dependency. If the current chef becomes unavailable, the restaurant can hire another one.\nImg. X — Restaurant — Chef dependency (Normal)\nNow what if we replace “chef” with a specific person: John? Our restaurant is now dependent on John specifically. If John becomes unavailable, we can’t replace him — the restaurant is in trouble. This is an example of tight or bad coupling.\nImg. X — Restaurant — John dependency (Bad coupling)\nWe don’t want OrderService to be tightly coupled to a specific payment service like Stripe. Instead, we want it to depend on a PaymentService interface, which could be Stripe, PayPal, or any other provider. To achieve this we can use the interface to decouple OrderService from StripePaymentService.\nImg. X — PaymentService as interface\nIf OrderService depends on a PaymentService interface, it doesn’t know anything about Stripe, PayPal, or any other payment provider. As long as these providers implement PaymentService, they can be used to handle payments — and OrderService won’t care which one is being used.\nBenefits:\nIf we replace StripePaymentService with PayPalPaymentService, the OrderService class is not affected. We don’t need to modify or recompile OrderService. We can test OrderService in isolation, without relying on the specific payment provider like Stripe. With this setup, we simply give OrderService a particular implementation of PaymentService. This is called dependency injection — we inject the dependency into a class.\nImg. X — Dependency Injection example\nLet’s see how it works in our project. Create OrderService at src/main/java/tech/codejava/store/OrderService.java:\npackage tech.codejava.store; public class OrderService { public void placeOrder() {} } Note\nIn a real project we would need to provide something like Order order to this method, but for teaching purposes we won’t do that.\nNow create StripePaymentService in the same directory:\npackage tech.codejava.store; public class StripePaymentService { public void processPayment(double amount) { System.out.println(\"=== STRIPE ===\"); System.out.println(\"amount: \" + amount); } } Let’s implement placeOrder in OrderService using StripePaymentService:\npackage tech.codejava.store; public class OrderService { public void placeOrder() { var paymentService = new StripePaymentService(); paymentService.processPayment(10); } } Important\nThis is our before setup — before we introduced the interface. In this implementation, OrderService is tightly coupled to StripePaymentService. We cannot test OrderService in isolation, and switching to another payment provider would require modifying OrderService.\nLet’s fix this. Create a PaymentService interface in the same directory:\npackage tech.codejava.store; public interface PaymentService { void processPayment(double amount); } Modify StripePaymentService to implement PaymentService:\npackage tech.codejava.store; public class StripePaymentService implements PaymentService { @Override public void processPayment(double amount) { System.out.println(\"=== STRIPE ===\"); System.out.println(\"amount: \" + amount); } } The recommended way to inject a dependency into a class is via its constructor. Let’s define one in OrderService:\npackage tech.codejava.store; public class OrderService { private PaymentService paymentService; public OrderService(PaymentService paymentService) { this.paymentService = paymentService; } public void placeOrder() { paymentService.processPayment(10); } } Now let’s see this in action. Modify StoreApplication:\npackage tech.codejava.store; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class StoreApplication { public static void main(String[] args) { // SpringApplication.run(StoreApplication.class, args); var orderService = new OrderService(new StripePaymentService()); orderService.placeOrder(); } } Running the application (output intentionally reduced):\n... === STRIPE === amount: 10.0 ... Now let’s create a PayPalPaymentService in the same directory:\npackage tech.codejava.store; public class PayPalPaymentService implements PaymentService { @Override public void processPayment(double amount) { System.out.println(\"=== PayPal ===\"); System.out.println(\"amount: \" + amount); } } Now we can switch from StripePaymentService to PayPalPaymentService in StoreApplication — without touching OrderService at all:\npackage tech.codejava.store; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class StoreApplication { public static void main(String[] args) { // SpringApplication.run(StoreApplication.class, args); // var orderService = new OrderService(new StripePaymentService()); var orderService = new OrderService(new PayPalPaymentService()); orderService.placeOrder(); } } ... === PayPal === amount: 10.0 ... Notice that we didn’t change OrderService. In object-oriented programming this is known as the Open/Closed Principle:\nA class should be open for extension and closed for modification.\nIn other words: we should be able to add new functionality to a class without changing its existing code. This reduces the risk of introducing bugs and breaking other parts of the application.","dependency-management#Dependency Management":"Dependencies are third-party libraries or frameworks we use in our application. For example, to build a web application we need an embedded web server like Tomcat, libraries for handling web requests, building APIs, processing JSON data, logging and so on.\nIn Spring Boot applications, instead of adding multiple individual libraries, we can use a starter dependency.\nImg. 5 — Spring Boot Starter Web\nTo use this dependency, copy the following into your pom.xml:\norg.springframework.boot spring-boot-starter-web 4.1.0-M1 So the dependencies section would look like this:\norg.springframework.boot spring-boot-starter org.springframework.boot spring-boot-starter-test test org.springframework.boot spring-boot-starter-web Important\nNotice that the version is commented out. It’s a better practice to let Spring Boot decide what version of the dependency to use, as it ensures compatibility across your project.","initialize-spring-boot-project#Initialize Spring Boot Project":"To initialize a new Spring Boot project, go to start.spring.io and select the options that suit you.\nImg. 3 — Spring Boot options\nAfter unpacking the zip archive, you’ll have this template project:\n. ├── HELP.md ├── mvnw ├── mvnw.cmd ├── pom.xml ├── src │ ├── main │ │ ├── java │ │ │ └── tech │ │ │ └── codejava │ │ │ └── store │ │ │ └── StoreApplication.java │ │ └── resources │ │ └── application.properties │ └── test │ └── java │ └── tech │ └── codejava │ └── store │ └── StoreApplicationTests.java └── target ├── classes │ ├── application.properties │ └── tech │ └── codejava │ └── store │ └── StoreApplication.class ├── generated-sources │ └── annotations ├── generated-test-sources │ └── test-annotations ├── maven-status │ └── maven-compiler-plugin │ ├── compile │ │ └── default-compile │ │ ├── createdFiles.lst │ │ └── inputFiles.lst │ └── testCompile │ └── default-testCompile │ ├── createdFiles.lst │ └── inputFiles.lst ├── surefire-reports │ ├── TEST-tech.codejava.store.StoreApplicationTests.xml │ └── tech.codejava.store.StoreApplicationTests.txt └── test-classes └── tech └── codejava └── store └── StoreApplicationTests.class The “heart” of our project is pom.xml:\n\u003c?xml version=\"1.0\" encoding=\"UTF-8\" ?\u003e","prerequisites#Prerequisites":"PrerequisitesBefore diving in, make sure you’re comfortable with the following:\nJava — solid understanding of the language Object-oriented programming — classes, methods and interfaces Databases — tables, primary keys, foreign keys, relationships, etc. SQL — ability to write basic SQL statements","setter-injection#Setter Injection":"Another way to inject a dependency is via a setter. In OrderService, let’s define one:\npackage tech.codejava.store; public class OrderService { private PaymentService paymentService; public void setPaymentService(PaymentService paymentService) { this.paymentService = paymentService; } public OrderService(PaymentService paymentService) { this.paymentService = paymentService; } public void placeOrder() { paymentService.processPayment(10); } } We can use it like this in StoreApplication:\npackage tech.codejava.store; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class StoreApplication { public static void main(String[] args) { // SpringApplication.run(StoreApplication.class, args); // var orderService = new OrderService(new StripePaymentService()); var orderService = new OrderService(new PayPalPaymentService()); orderService.setPaymentService(new PayPalPaymentService()); orderService.placeOrder(); } } Important\nIf you remove the constructor from OrderService and forget to call the setter, the application will crash with a NullPointerException. Use setter injection only for optional dependencies — ones that OrderService can function without.","what-is-a-spring-framework#What is a Spring Framework?":"Spring is a popular framework for building Java applications. It has a lot of modules, each designed to handle a specific task. They are combined into a few different layers.\nImg. 1 — Spring layers\nLayer Purpose Core Handling dependency injection, managing objects Web Building web applications Data Working with databases AOP Aspect oriented programming Test Testing spring components While the Spring Framework is powerful, using it often involves a lot of configuration. For example, if you want to build a web app you might need to setup a web server, configure routing and manage dependencies manually. That’s when Spring Boot comes in.\nNote\nYou can think of Spring Boot as a layer on top of the Spring Framework that takes care of all of the setup. Spring Boot simplifies Spring development by providing sensible defaults and ready-to-use features.\nBy the way, the Spring Framework is just one part of a larger family of projects in the Spring ecosystem.\nImg. 2 — Spring ecosystem\nModule Name Purpose Spring Data Simplifying database access Spring Security Adding authentication and authorization Spring Batch Batch processing Spring Cloud Building microservices and distributed systems Spring Integration Simplifying messaging and integration between systems"},"title":"_index"}}
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+{"/courses/prog-intro/lectures/intro/":{"data":{"what-does-java-consist-of#What does Java consist of?":"Java consists of multiple main components. The first one being the Java compiler. The process of converting human-readable text to machine code is called compilation.\nImg.2 - Simplified Compilation Process\nThe behaviour of the Java compiler is described by the Java Language Specification or JLS.\nLet’s talk about compilers a little bit. For example if we will take C++. If we take C++ compiler for x86-64 platform and Windows operating system, and launch the compiler on source code it will turn it directly into assembly code.\nImg. X – C++ code compilation process under x86-64 architecture.\nBut what if I want to run my program on Linux instead of Windows. I will need to take a different compiler under different operating system and recompile my code using a new compiler. It’s not very convenient.\nJava tries to protect us from that. By converting the Java source code into Java byte code. And then the byte code will be ran on the Java virtual machine which will run our program on the native processor.\nImg. X – Java code compilation\nThis approach allows to change only JVM according to our platform (x86_64, ARM, …) and operating system (Windows, MacOS, GNU-Linux, …) while byte code stays the same. We can only write our code once, than compile it and run under everywhere.\nAs the motto says:\nWrite once – debug run everywhere.\nThe third component of Java is the standart library which is included in the JVM.\nThere are multiple redactions of Java-platforms:\nStandart edition For regular aplications Enterprise edition For server aplications Micro-edition For mobile aplications Isn’t in use nowadays 🪦 Java Card Sim- and smart-cards There also were multiple versions of Java throughout its history.\nJDK 1.0 (Jan 1996) J2SE 1.2 (Dec 1998) Collections Framework J2SE 5.0 (Sep 2004) Generics Java SE 8 (Mar 2014) Streams and Lambdas Java SE 9 (Sep 2017) Modules Java SE 10 (Mar 2018) var Java 11 (Sep 2018) jshell Java 17 [LTS-old] (Sep 2021) Previous stable version Many little changes Java 21 [LTS] (Sep 2023) Current stable version Java 25 (Sep 2025) Next version Java comes in two parts: JDK - Java Development Kit and JVM - Java Virtual Machine.\nThere is also a JRE - Java Runtime Environment. For example if we want to run our code somewhere on the server we don’t need to compile it there because we have our byte code and we just need JRE to run it.","why-do-we-choose-java#Why do we choose Java?":"Why do we choose Java?According to TIOBE Programming Community Index Java is one of the most demanded languages in the programming field.\nImg. 1 - TIOBE Programming Community Index\nAlso, Java has a lot of advantages for beginners such as:\nIt’s fairly easy It has a broad spectre of usage Server Desktop Mobile devices Smart-cards"},"title":"Lecture 1. Introduction"},"/courses/spring-boot/":{"data":{"configuring-application-properties#Configuring Application Properties":"Let’s take a look at src/main/resources/application.properties:\nspring.application.name=store To use this property in our code, we can use the @Value annotation. Let’s update HomeController to print the application name:\npackage tech.codejava.store; import org.springframework.beans.factory.annotation.Value; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.RequestMapping; @Controller public class HomeController { @Value(\"${spring.application.name}\") private String appName; @RequestMapping(\"/\") // this represents the root of our website public String index() { System.out.println(\"application name = \" + appName); return \"index.html\"; // this returns the view } } After running the application, we can see store printed in the terminal:\n... 2026-02-19T15:32:37.507+03:00 INFO 41536 --- [store] [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Initializing Servlet 'dispatcherServlet' 2026-02-19T15:32:37.509+03:00 INFO 41536 --- [store] [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Completed initialization in 2 ms application name = store ...","controllers#Controllers":"Spring MVC stands for Model View Controller.\nModel is where our application’s data lives. It represents the business logic and is usually connected to a database or other data sources. In Spring Boot, the model can be a simple Java class. View is what the user sees. It’s the HTML, CSS or JavaScript rendered in the browser. In Spring MVC, views can be static files or dynamically generated. Controller is like a traffic controller. It handles incoming requests from the user, interacts with the model to get data and then tells the view what to display. Let’s add a new Java class called HomeController at src/main/java/tech/codejava/store/HomeController.java:\npackage tech.codejava.store; public class HomeController {} To make this a controller, decorate it with the @Controller annotation:\npackage tech.codejava.store; import org.springframework.stereotype.Controller; @Controller public class HomeController {} Now let’s add an index method. When we send a request to the root of our website, we want this method to be called:\npackage tech.codejava.store; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.RequestMapping; @Controller public class HomeController { @RequestMapping(\"/\") // this represents the root of our website public String index() { return \"index.html\"; // this returns the view } } Now we need to create the view. Add index.html at src/main/resources/static/index.html:\n\u003c!doctype html\u003e \u003chtml lang=\"en\"\u003e \u003chead\u003e \u003cmeta charset=\"UTF-8\" /\u003e \u003cmeta name=\"viewport\" content=\"width=device-width, initial-scale=1.0\" /\u003e \u003ctitle\u003eView\u003c/title\u003e \u003c/head\u003e \u003cbody\u003e \u003ch1\u003eHello world!\u003c/h1\u003e \u003c/body\u003e \u003c/html\u003e Let’s build and run our application using mvn spring-boot:run. From the logs:\n2026-02-19T14:55:23.948+03:00 INFO 36752 --- [store] [ main] o.s.boot.tomcat.TomcatWebServer : Tomcat initialized with port 8080 (http) Our app is up and running at localhost:8080.\nImg. 7 — Our app is up and running!","dependency-injection#Dependency Injection":"Imagine we’re building an E-Commerce application that handles placing orders. When an order is placed, the customer’s payment needs to be processed — so OrderService depends on a payment service like StripePaymentService. We can say that OrderService is dependent on (or coupled to) StripePaymentService.\nImg. 8 — Depends On/Coupled To relation\nLet’s talk about the issues that arise when one class is tightly coupled to another.\nInflexibility — OrderService can only use StripePaymentService. If tomorrow we decide to switch to a different payment provider like PayPal, we would have to modify OrderService. Once we change it, it has to be recompiled and retested, which could impact other classes that depend on it. Untestability — We cannot test OrderService in isolation, because OrderService is tightly coupled with StripePaymentService and we can’t test its logic separately from it. Note\nThe problem here isn’t that OrderService depends on StripePaymentService — dependencies are normal in any application. The issue is about how the dependency is created and managed.\nAnalogy: Think of a restaurant. A restaurant needs a chef — that’s a perfectly normal dependency. If the current chef becomes unavailable, the restaurant can hire another one.\nImg. X — Restaurant — Chef dependency (Normal)\nNow what if we replace “chef” with a specific person: John? Our restaurant is now dependent on John specifically. If John becomes unavailable, we can’t replace him — the restaurant is in trouble. This is an example of tight or bad coupling.\nImg. X — Restaurant — John dependency (Bad coupling)\nWe don’t want OrderService to be tightly coupled to a specific payment service like Stripe. Instead, we want it to depend on a PaymentService interface, which could be Stripe, PayPal, or any other provider. To achieve this we can use the interface to decouple OrderService from StripePaymentService.\nImg. X — PaymentService as interface\nIf OrderService depends on a PaymentService interface, it doesn’t know anything about Stripe, PayPal, or any other payment provider. As long as these providers implement PaymentService, they can be used to handle payments — and OrderService won’t care which one is being used.\nBenefits:\nIf we replace StripePaymentService with PayPalPaymentService, the OrderService class is not affected. We don’t need to modify or recompile OrderService. We can test OrderService in isolation, without relying on the specific payment provider like Stripe. With this setup, we simply give OrderService a particular implementation of PaymentService. This is called dependency injection — we inject the dependency into a class.\nImg. X — Dependency Injection example\nLet’s see how it works in our project. Create OrderService at src/main/java/tech/codejava/store/OrderService.java:\npackage tech.codejava.store; public class OrderService { public void placeOrder() {} } Note\nIn a real project we would need to provide something like Order order to this method, but for teaching purposes we won’t do that.\nNow create StripePaymentService in the same directory:\npackage tech.codejava.store; public class StripePaymentService { public void processPayment(double amount) { System.out.println(\"=== STRIPE ===\"); System.out.println(\"amount: \" + amount); } } Let’s implement placeOrder in OrderService using StripePaymentService:\npackage tech.codejava.store; public class OrderService { public void placeOrder() { var paymentService = new StripePaymentService(); paymentService.processPayment(10); } } Important\nThis is our before setup — before we introduced the interface. In this implementation, OrderService is tightly coupled to StripePaymentService. We cannot test OrderService in isolation, and switching to another payment provider would require modifying OrderService.\nLet’s fix this. Create a PaymentService interface in the same directory:\npackage tech.codejava.store; public interface PaymentService { void processPayment(double amount); } Modify StripePaymentService to implement PaymentService:\npackage tech.codejava.store; public class StripePaymentService implements PaymentService { @Override public void processPayment(double amount) { System.out.println(\"=== STRIPE ===\"); System.out.println(\"amount: \" + amount); } } The recommended way to inject a dependency into a class is via its constructor. Let’s define one in OrderService:\npackage tech.codejava.store; public class OrderService { private PaymentService paymentService; public OrderService(PaymentService paymentService) { this.paymentService = paymentService; } public void placeOrder() { paymentService.processPayment(10); } } Now let’s see this in action. Modify StoreApplication:\npackage tech.codejava.store; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class StoreApplication { public static void main(String[] args) { // SpringApplication.run(StoreApplication.class, args); var orderService = new OrderService(new StripePaymentService()); orderService.placeOrder(); } } Running the application (output intentionally reduced):\n... === STRIPE === amount: 10.0 ... Now let’s create a PayPalPaymentService in the same directory:\npackage tech.codejava.store; public class PayPalPaymentService implements PaymentService { @Override public void processPayment(double amount) { System.out.println(\"=== PayPal ===\"); System.out.println(\"amount: \" + amount); } } Now we can switch from StripePaymentService to PayPalPaymentService in StoreApplication — without touching OrderService at all:\npackage tech.codejava.store; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class StoreApplication { public static void main(String[] args) { // SpringApplication.run(StoreApplication.class, args); // var orderService = new OrderService(new StripePaymentService()); var orderService = new OrderService(new PayPalPaymentService()); orderService.placeOrder(); } } ... === PayPal === amount: 10.0 ... Notice that we didn’t change OrderService. In object-oriented programming this is known as the Open/Closed Principle:\nA class should be open for extension and closed for modification.\nIn other words: we should be able to add new functionality to a class without changing its existing code. This reduces the risk of introducing bugs and breaking other parts of the application.","dependency-management#Dependency Management":"Dependencies are third-party libraries or frameworks we use in our application. For example, to build a web application we need an embedded web server like Tomcat, libraries for handling web requests, building APIs, processing JSON data, logging and so on.\nIn Spring Boot applications, instead of adding multiple individual libraries, we can use a starter dependency.\nImg. 5 — Spring Boot Starter Web\nTo use this dependency, copy the following into your pom.xml:\norg.springframework.boot spring-boot-starter-web 4.1.0-M1 So the dependencies section would look like this:\norg.springframework.boot spring-boot-starter org.springframework.boot spring-boot-starter-test test org.springframework.boot spring-boot-starter-web Important\nNotice that the version is commented out. It’s a better practice to let Spring Boot decide what version of the dependency to use, as it ensures compatibility across your project.","initialize-spring-boot-project#Initialize Spring Boot Project":"To initialize a new Spring Boot project, go to start.spring.io and select the options that suit you.\nImg. 3 — Spring Boot options\nAfter unpacking the zip archive, you’ll have this template project:\n. ├── HELP.md ├── mvnw ├── mvnw.cmd ├── pom.xml ├── src │ ├── main │ │ ├── java │ │ │ └── tech │ │ │ └── codejava │ │ │ └── store │ │ │ └── StoreApplication.java │ │ └── resources │ │ └── application.properties │ └── test │ └── java │ └── tech │ └── codejava │ └── store │ └── StoreApplicationTests.java └── target ├── classes │ ├── application.properties │ └── tech │ └── codejava │ └── store │ └── StoreApplication.class ├── generated-sources │ └── annotations ├── generated-test-sources │ └── test-annotations ├── maven-status │ └── maven-compiler-plugin │ ├── compile │ │ └── default-compile │ │ ├── createdFiles.lst │ │ └── inputFiles.lst │ └── testCompile │ └── default-testCompile │ ├── createdFiles.lst │ └── inputFiles.lst ├── surefire-reports │ ├── TEST-tech.codejava.store.StoreApplicationTests.xml │ └── tech.codejava.store.StoreApplicationTests.txt └── test-classes └── tech └── codejava └── store └── StoreApplicationTests.class The “heart” of our project is pom.xml:\n\u003c?xml version=\"1.0\" encoding=\"UTF-8\" ?\u003e","prerequisites#Prerequisites":"PrerequisitesBefore diving in, make sure you’re comfortable with the following:\nJava — solid understanding of the language Object-oriented programming — classes, methods and interfaces Databases — tables, primary keys, foreign keys, relationships, etc. SQL — ability to write basic SQL statements","setter-injection#Setter Injection":"Another way to inject a dependency is via a setter. In OrderService, let’s define one:\npackage tech.codejava.store; public class OrderService { private PaymentService paymentService; public void setPaymentService(PaymentService paymentService) { this.paymentService = paymentService; } public OrderService(PaymentService paymentService) { this.paymentService = paymentService; } public void placeOrder() { paymentService.processPayment(10); } } We can use it like this in StoreApplication:\npackage tech.codejava.store; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class StoreApplication { public static void main(String[] args) { // SpringApplication.run(StoreApplication.class, args); // var orderService = new OrderService(new StripePaymentService()); var orderService = new OrderService(new PayPalPaymentService()); orderService.setPaymentService(new PayPalPaymentService()); orderService.placeOrder(); } } Important\nIf you remove the constructor from OrderService and forget to call the setter, the application will crash with a NullPointerException. Use setter injection only for optional dependencies — ones that OrderService can function without.","what-is-a-spring-framework#What is a Spring Framework?":"Spring is a popular framework for building Java applications. It has a lot of modules, each designed to handle a specific task. They are combined into a few different layers.\nImg. 1 — Spring layers\nLayer Purpose Core Handling dependency injection, managing objects Web Building web applications Data Working with databases AOP Aspect oriented programming Test Testing spring components While the Spring Framework is powerful, using it often involves a lot of configuration. For example, if you want to build a web app you might need to setup a web server, configure routing and manage dependencies manually. That’s when Spring Boot comes in.\nNote\nYou can think of Spring Boot as a layer on top of the Spring Framework that takes care of all of the setup. Spring Boot simplifies Spring development by providing sensible defaults and ready-to-use features.\nBy the way, the Spring Framework is just one part of a larger family of projects in the Spring ecosystem.\nImg. 2 — Spring ecosystem\nModule Name Purpose Spring Data Simplifying database access Spring Security Adding authentication and authorization Spring Batch Batch processing Spring Cloud Building microservices and distributed systems Spring Integration Simplifying messaging and integration between systems"},"title":"_index"}}
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