diff --git a/content/courses/prog-intro/homeworks/sum/ParseIntExample.class b/content/courses/prog-intro/homeworks/sum/ParseIntExample.class index 44335b9..7ae4c4c 100644 Binary files a/content/courses/prog-intro/homeworks/sum/ParseIntExample.class and b/content/courses/prog-intro/homeworks/sum/ParseIntExample.class differ diff --git a/content/courses/prog-intro/homeworks/sum/ParseIntExample.java b/content/courses/prog-intro/homeworks/sum/ParseIntExample.java index 33a275d..e66010b 100644 --- a/content/courses/prog-intro/homeworks/sum/ParseIntExample.java +++ b/content/courses/prog-intro/homeworks/sum/ParseIntExample.java @@ -5,8 +5,7 @@ public class ParseIntExample { public static void main(String[] args) { String s = "123"; int sum = 321; - //! sum = sum + s; - sum = sum + Integer.parseInt(s); - System.out.println(sum); + String result = s + sum; + System.out.println(result); } } diff --git a/content/courses/prog-intro/homeworks/sum/Sum.class b/content/courses/prog-intro/homeworks/sum/Sum.class new file mode 100644 index 0000000..d5ff717 Binary files /dev/null and b/content/courses/prog-intro/homeworks/sum/Sum.class differ diff --git a/content/courses/prog-intro/homeworks/sum/Sum.java b/content/courses/prog-intro/homeworks/sum/Sum.java index 27a3033..9225eda 100644 --- a/content/courses/prog-intro/homeworks/sum/Sum.java +++ b/content/courses/prog-intro/homeworks/sum/Sum.java @@ -1,17 +1,32 @@ // Sum.java public class Sum { + public static void main(String[] args) { - System.out.println("number of arguments: " + args.length); - int sum = 0; for (String argument : args) { - sum = sum + argument; - System.out.println(argument); - System.out.println(sum); + StringBuilder number = new StringBuilder(); // we will form numbers here using StringBuilder + for (char c : argument.toCharArray()) { + if (!Character.isWhitespace(c)) { + // if character is not whitespace + number.append(c); // concatenate new digit + } else { + // if character is whitespace + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number.toString()); // add number to sum + } + number = new StringBuilder(); // empty the number by creating new empty StringBuilder + } + } + + // check for any remaining digits in number + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number.toString()); // add number to sum + } } - + System.out.println(sum); } } - diff --git a/content/courses/prog-intro/homeworks/sum/_index.md b/content/courses/prog-intro/homeworks/sum/_index.md index 49cf3e2..5b756b1 100644 --- a/content/courses/prog-intro/homeworks/sum/_index.md +++ b/content/courses/prog-intro/homeworks/sum/_index.md @@ -235,11 +235,226 @@ public class ParseIntExample { public static void main(String[] args) { String s = "123"; int sum = 321; - //! sum = sum + s; - sum = sum + Integer.parseInt(s); - System.out.println(sum); + //! int result = s + sum; } } ``` The commented out line (`//!`) would have given us an exception. + +```bash +$ javac ParseIntExample.java +ParseIntExample.java:8: error: incompatible types: String cannot be converted to int + int result = s + sum; + ^ +1 error +``` + +> [!IMPORTANT] +> Please note that if we change `result` data type to `String`, the code will work just fine. +> ```java +> // ParseIntExample.java +> public class ParseIntExample { +> +> public static void main(String[] args) { +> String s = "123"; +> int sum = 321; +> String result = s + sum; +> System.out.println(result); +> } +> } +> ``` +> ```bash +> $ javac ParseIntExample.java && java ParseIntExample +> 123321 +> ``` +> In this case `sum` will be ***automatically casted*** to `String` and two strings will just concatenate. + +So we figured out that we need to cast `argument` to integer type. Let's change our code and test it. + +```java +// Sum.java + +public class Sum { + public static void main(String[] args) { + System.out.println("number of arguments: " + args.length); + + int sum = 0; + for (String argument : args) { + sum = sum + Ineteger.parseInt(argument); + System.out.println(argument); + System.out.println(sum); + } + + System.out.println(sum); + } +} +``` + +```java +// Sum.java + +public class Sum { + public static void main(String[] args) { + System.out.println("number of arguments: " + args.length); + + int sum = 0; + for (String argument : args) { + sum = sum + Integer.parseInt(argument); + System.out.println( + "current argument: " + argument + ", current sum: " + sum + ); + } + + System.out.println("final sum: " + sum); + } +} +``` +```bash +$ java Sum 1 2 3 +number of arguments: 3 +current argument: 1, current sum: 1 +current argument: 2, current sum: 3 +current argument: 3, current sum: 6 +final sum: 6 +``` +```bash +$ java Sum 1 2 -3 +number of arguments: 3 +current argument: 1, current sum: 1 +current argument: 2, current sum: 3 +current argument: -3, current sum: 0 +final sum: 0 +``` +```bash +$ java Sum "1 2 3" +number of arguments: 1 +Exception in thread "main" java.lang.NumberFormatException: For input string: "1 2 3" + at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67) + at java.base/java.lang.Integer.parseInt(Integer.java:662) + at java.base/java.lang.Integer.parseInt(Integer.java:778) + at Sum.main(Sum.java:9) +``` + +As we can see because we have only 1 argument (`"1 2 3"`) we try to parse it as an integer but `"1 2 3"` contains empty spaces, so it can't be parsed as an integer. + +We need to manually extract numeric values from the `argument` string. + +In order to do that we will iterate over each symbol of the `argument` string and extract numeric values. + +We will have another variable `number` of `String` type in which we will be storing extracted numeric values. + +For every character or symbol in the string, we will check it to be an empty space or a digit. If it is a digit, we will add/concatenate with `number`, and if it is an empty space, we will add the numeric value of `number` to `sum` and then assign empty string to `number` because we reached the end of the current number and are ready to move to the next one. + +> [!IMPORTANT] +> It is important to add numeric value of `number` (if it's not empty) to `sum` after we iterated over the entire string because if it ends like this `"1 2 3"`, we don't nessesarily have an empty space at the end so `"3"` (in this case) will be stored in `number` but NOT added to `sum`. + +Let's implement those ideas into solution. We will get rid of all the `println` messages so it's not drawing our attention. + +> [!NOTE] +> Also I want to point out that we can't just iterate over `String`. First we need to turn it into an ***array of characters*** using [`String.toCharArray()`](https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#toCharArray--) method. + +> [!NOTE] +> We will be checking for an empty space using [`Character.isWhitespace(char ch)`](https://docs.oracle.com/javase/8/docs/api/java/lang/Character.html#isWhitespace-char-) which outputs `true` or `false` accordingly. + +```java +// Sum.java + +public class Sum { + + public static void main(String[] args) { + int sum = 0; + for (String argument : args) { + String number = ""; // we will form numbers here + for (char c : argument.toCharArray()) { + if (!Character.isWhitespace(c)) { + // if character is not whitespace + number = number + c; // concatenate new digit + } else { + // if character is whitespace + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number); // add number to sum + } + number = ""; // empty the number + } + } + + // check for any remaining digits in number + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number); // add number to sum + } + } + + System.out.println(sum); + } +} +``` + +```bash +$ java Sum 1 2 3 +6 +``` +```bash +$ java Sum 1 2 -3 +0 +``` +```bash +$ java Sum "1 2 3" +6 +``` +```bash +$ java Sum "1 2" " 3" +6 +``` +```bash +$ java Sum " " +0 +``` + +This code works and passes all the tests! + +But can we improve it? One efficiency improvement we can make is to get rid of concatenating strings. You can read why it's bad [here](https://stackoverflow.com/questions/18561424/using-for-strings-in-a-loop-is-it-bad-practice) or [here](https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.reddit.com/r/javahelp/comments/19e9jic/question_about_dont_concat_string_in_a_loop/&ved=2ahUKEwiCpva0y-GTAxUeB9sEHbvzHj4QFnoECCAQAQ&usg=AOvVaw3gk39Xk7Jz9_PmKVIO81nK). + +We can use [StringBuilder](https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html) because it can ***build*** strings more efficiently. + +So here's the updated code + +```java +// Sum.java + +public class Sum { + + public static void main(String[] args) { + int sum = 0; + for (String argument : args) { + StringBuilder number = new StringBuilder(); // we will form numbers here using StringBuilder + for (char c : argument.toCharArray()) { + if (!Character.isWhitespace(c)) { + // if character is not whitespace + number.append(c); // concatenate new digit + } else { + // if character is whitespace + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number.toString()); // add number to sum + } + number = new StringBuilder(); // empty the number by creating new empty StringBuilder + } + } + + // check for any remaining digits in number + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number.toString()); // add number to sum + } + } + + System.out.println(sum); + } +} +``` + +> [!IMPORTANT] +> Note, that we can't just do `Integer.parseInt()` diff --git a/public/courses/prog-intro/homeworks/sum/ParseIntExample.class b/public/courses/prog-intro/homeworks/sum/ParseIntExample.class index 44335b9..7ae4c4c 100644 Binary files a/public/courses/prog-intro/homeworks/sum/ParseIntExample.class and b/public/courses/prog-intro/homeworks/sum/ParseIntExample.class differ diff --git a/public/courses/prog-intro/homeworks/sum/ParseIntExample.java b/public/courses/prog-intro/homeworks/sum/ParseIntExample.java index 33a275d..e66010b 100644 --- a/public/courses/prog-intro/homeworks/sum/ParseIntExample.java +++ b/public/courses/prog-intro/homeworks/sum/ParseIntExample.java @@ -5,8 +5,7 @@ public class ParseIntExample { public static void main(String[] args) { String s = "123"; int sum = 321; - //! sum = sum + s; - sum = sum + Integer.parseInt(s); - System.out.println(sum); + String result = s + sum; + System.out.println(result); } } diff --git a/public/courses/prog-intro/homeworks/sum/Sum.class b/public/courses/prog-intro/homeworks/sum/Sum.class new file mode 100644 index 0000000..d5ff717 Binary files /dev/null and b/public/courses/prog-intro/homeworks/sum/Sum.class differ diff --git a/public/courses/prog-intro/homeworks/sum/Sum.java b/public/courses/prog-intro/homeworks/sum/Sum.java index 27a3033..9225eda 100644 --- a/public/courses/prog-intro/homeworks/sum/Sum.java +++ b/public/courses/prog-intro/homeworks/sum/Sum.java @@ -1,17 +1,32 @@ // Sum.java public class Sum { + public static void main(String[] args) { - System.out.println("number of arguments: " + args.length); - int sum = 0; for (String argument : args) { - sum = sum + argument; - System.out.println(argument); - System.out.println(sum); + StringBuilder number = new StringBuilder(); // we will form numbers here using StringBuilder + for (char c : argument.toCharArray()) { + if (!Character.isWhitespace(c)) { + // if character is not whitespace + number.append(c); // concatenate new digit + } else { + // if character is whitespace + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number.toString()); // add number to sum + } + number = new StringBuilder(); // empty the number by creating new empty StringBuilder + } + } + + // check for any remaining digits in number + if (!number.isEmpty()) { + // if number is not empty + sum = sum + Integer.parseInt(number.toString()); // add number to sum + } } - + System.out.println(sum); } } - diff --git a/public/courses/prog-intro/homeworks/sum/index.html b/public/courses/prog-intro/homeworks/sum/index.html index 7e452a9..bc2e315 100644 --- a/public/courses/prog-intro/homeworks/sum/index.html +++ b/public/courses/prog-intro/homeworks/sum/index.html @@ -35,7 +35,7 @@ java Sum 1 2 -3 Expected output: 0." /> - + public static void main(String[] args) { String s = "123"; int sum = 321; - //! sum = sum + s; - sum = sum + Integer.parseInt(s); - System.out.println(sum); + //! int result = s + sum; } }

The commented out line (//!) would have given us an exception.

+
+ +
$ javac ParseIntExample.java
+ParseIntExample.java:8: error: incompatible types: String cannot be converted to int
+        int result = s + sum;
+                       ^
+1 error
+ +
+
+
+

Important

+ +
+

Please note that if we change result data type to String, the code will work just fine.

+
+ +
// ParseIntExample.java
+public class ParseIntExample {
+
+    public static void main(String[] args) {
+        String s = "123";
+        int sum = 321;
+        String result = s + sum;
+        System.out.println(result);
+    }
+}
+ +
+
+
+ +
$ javac ParseIntExample.java && java ParseIntExample 
+123321
+ +
+
+

In this case sum will be automatically casted to String and two strings will just concatenate.

+
+
+

So we figured out that we need to cast argument to integer type. Let’s change our code and test it.

+
+ +
// Sum.java
+
+public class Sum {
+    public static void main(String[] args) {
+        System.out.println("number of arguments: " + args.length);
+        
+        int sum = 0;
+        for (String argument : args) {
+            sum = sum + Ineteger.parseInt(argument);
+            System.out.println(argument);
+            System.out.println(sum);
+        }
+        
+        System.out.println(sum);
+    }
+}
+ +
+
+
+ +
// Sum.java
+
+public class Sum {
+    public static void main(String[] args) {
+        System.out.println("number of arguments: " + args.length);
+
+        int sum = 0;
+        for (String argument : args) {
+            sum = sum + Integer.parseInt(argument);
+            System.out.println(
+                "current argument: " + argument + ", current sum: " + sum
+            );
+        }
+
+        System.out.println("final sum: " + sum);
+    }
+}
+ +
+
+
+ +
$ java Sum 1 2 3
+number of arguments: 3
+current argument: 1, current sum: 1
+current argument: 2, current sum: 3
+current argument: 3, current sum: 6
+final sum: 6
+ +
+
+
+ +
$ java Sum 1 2 -3
+number of arguments: 3
+current argument: 1, current sum: 1
+current argument: 2, current sum: 3
+current argument: -3, current sum: 0
+final sum: 0
+ +
+
+
+ +
$ java Sum "1 2 3"
+number of arguments: 1
+Exception in thread "main" java.lang.NumberFormatException: For input string: "1 2 3"
+	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
+	at java.base/java.lang.Integer.parseInt(Integer.java:662)
+	at java.base/java.lang.Integer.parseInt(Integer.java:778)
+	at Sum.main(Sum.java:9)
+ +
+
+

As we can see because we have only 1 argument ("1 2 3") we try to parse it as an integer but "1 2 3" contains empty spaces, so it can’t be parsed as an integer.

+

We need to manually extract numeric values from the argument string.

+

In order to do that we will iterate over each symbol of the argument string and extract numeric values.

+

We will have another variable number of String type in which we will be storing extracted numeric values.

+

For every character or symbol in the string, we will check it to be an empty space or a digit. If it is a digit, we will add/concatenate with number, and if it is an empty space, we will add the numeric value of number to sum and then assign empty string to number because we reached the end of the current number and are ready to move to the next one.

+
+

Important

+ +
+

It is important to add numeric value of number (if it’s not empty) to sum after we iterated over the entire string because if it ends like this "1 2 3", we don’t nessesarily have an empty space at the end so "3" (in this case) will be stored in number but NOT added to sum.

+
+
+

Let’s implement those ideas into solution. We will get rid of all the println messages so it’s not drawing our attention.

+
+

Note

+ +
+

Also I want to point out that we can’t just iterate over String. First we need to turn it into an array of characters using String.toCharArray() method.

+
+
+
+

Note

+ +
+

We will be checking for an empty space using Character.isWhitespace(char ch) which outputs true or false accordingly.

+
+
+
+ +
// Sum.java
+
+public class Sum {
+
+    public static void main(String[] args) {
+        int sum = 0;
+        for (String argument : args) {
+            String number = ""; // we will form numbers here
+            for (char c : argument.toCharArray()) {
+                if (!Character.isWhitespace(c)) {
+                    // if character is not whitespace
+                    number = number + c; // concatenate new digit
+                } else {
+                    // if character is whitespace
+                    if (!number.isEmpty()) {
+                        // if number is not empty
+                        sum = sum + Integer.parseInt(number); // add number to sum
+                    }
+                    number = ""; // empty the number
+                }
+            }
+
+            // check for any remaining digits in number
+            if (!number.isEmpty()) {
+                // if number is not empty
+                sum = sum + Integer.parseInt(number); // add number to sum
+            }
+        }
+
+        System.out.println(sum);
+    }
+}
+ +
+
+
+ +
$ java Sum 1 2 3
+6
+ +
+
+
+ +
$ java Sum 1 2 -3
+0
+ +
+
+
+ +
$ java Sum "1 2 3"
+6
+ +
+
+
+ +
$ java Sum "1 2" " 3"
+6
+ +
+
+
+ +
$ java Sum " "
+0
+ +
+
+

This code works and passes all the tests!

+

But can we improve it? One efficiency improvement we can make is to get rid of concatenating strings. You can read why it’s bad here or here.

+

We can use StringBuilder because it can build strings more efficiently.

+

So here’s the updated code

+
+ +
// Sum.java
+
+public class Sum {
+
+    public static void main(String[] args) {
+        int sum = 0;
+        for (String argument : args) {
+            StringBuilder number = new StringBuilder(); // we will form numbers here using StringBuilder
+            for (char c : argument.toCharArray()) {
+                if (!Character.isWhitespace(c)) {
+                    // if character is not whitespace
+                    number.append(c); // concatenate new digit
+                } else {
+                    // if character is whitespace
+                    if (!number.isEmpty()) {
+                        // if number is not empty
+                        sum = sum + Integer.parseInt(number.toString()); // add number to sum
+                    }
+                    number = new StringBuilder(); // empty the number by creating new empty StringBuilder
+                }
+            }
+
+            // check for any remaining digits in number
+            if (!number.isEmpty()) {
+                // if number is not empty
+                sum = sum + Integer.parseInt(number.toString()); // add number to sum
+            }
+        }
+
+        System.out.println(sum);
+    }
+}
+ +
+
+
+

Important

+ +
+

Note, that we can’t just do Integer.parseInt()

+
+
diff --git a/public/en.search-data.json b/public/en.search-data.json index e57d07c..585033f 100644 --- a/public/en.search-data.json +++ b/public/en.search-data.json @@ -1 +1 @@ -{"/courses/prog-intro/homeworks/sum/":{"data":{"solution#Solution":"After reading about String, Integer, System.err we now know about some usefull methods:\nInteger.parseInt(String s, int radix) which parses the string argument as a signed integer in the radix specified by the second argument. So it basically converts a number from the String data type to int. Character.isWhitespace(char ch) which checks if a character ch is a space symbol or not. Now let’s start coding. Firstly let’s define the structure of our program. I will be putting the name of the file at the first line comment in a file and its path if its nessesary.\n// Sum.java public class Sum { public static void main(String[] args) { // ... } } Okay that’s done. What do we do next? Let’s look at String[] args argument to our main method. It represents an array of command line arguments which we need to sum. So know we made our task a little bit easier. Now we can just say that we need to find sum of elements of array args.\nHow are we going to do it though? First let’s understand what we can do with this array. Let’s modify our class a little bit.\n// Sum.java public class Sum { public static void main(String[] args) { System.out.println(args.length); } } We’ve added System.out.println(args.length) which takes the field length from our args object and prints it to the console.\nLet’s try it. First compile our class using\n$ javac Sum.java And then we can do some manual testing.\n$ java Sum 1 2 3 3 We got 3 as an output as expected. We gave our program 3 command line arguments: 1, 2 and 3.\nHere are all of the examples\n$ java Sum 1 2 -3 3 $ java Sum \"1 2 3\" 1 Note\nNotice, that we got 1 instead of 3. That’s because we put our arguments in \"\" so this becomes a single string argument.\n$ java Sum \"1 2\" \" 3\" 2 $ java Sum \" \" 1 Note\nHere program gives us 1 instead of 0, because despite not having any numbers in the arguments a single whitespace is still an argument.\nNow let’s try not obly to count our arguments but to list them as well. Let’s modify our program a little bit more.\n// Sum.java public class Sum { public static void main(String[] args) { System.out.println(\"number of arguments: \" + args.length); for (String argument : args) { System.out.println(argument); } } } Here I used for loop to do printing for every String element in args.\nLet’s try this with our examples. And don’t forget to recompile using javac Sum.java.\n$ java Sum 1 2 3 number of arguments: 3 1 2 3 $ java Sum 1 2 -3 number of arguments: 3 1 2 -3 $ java Sum \"1 2 3\" number of arguments: 1 1 2 3 Note\nAgain, notice only one string argument.\n$ java Sum \"1 2\" \" 3\" number of arguments: 2 1 2 3 $ java Sum \" \" number of arguments: 1 Okay, now that we know how to iterate (or do something for every element), we can calculate the sum of all the numbers in the args array. To do that we need to create a new variable sum of integer type and assign it the value of 0. Then for every number in args we will add it to sum, and so by the end of array we will have the sum of all numbers stored in variable sum. This is what the code will look like\n// Sum.java public class Sum { public static void main(String[] args) { System.out.println(\"number of arguments: \" + args.length); int sum = 0; for (String argument : args) { sum = sum + argument; System.out.println(argument); System.out.println(sum); } System.out.println(sum); } } Seems ok. But let’s test it.\n$ javac Sum.java Sum.java:9: error: incompatible types: String cannot be converted to int sum = sum + argument; ^ 1 error We got a compilation error that says String cannot be converted to int. It means that when we try to add 2 variables sum and argument their types don’t match. The type of sum is integer and string for argument. It seems pretty logical because what do we expect when adding for example 1 and apple?..\nImportant\nSo we need to cast argument to integer data type so that we can add it to sum.\nWe can do so using Integer.parseInt() method.\nIt takes a String s and returns an int, which a string s represents. For example this code will work as expected\n// ParseIntExample.java public class ParseIntExample { public static void main(String[] args) { String s = \"123\"; int sum = 321; //! sum = sum + s; sum = sum + Integer.parseInt(s); System.out.println(sum); } } The commented out line (//!) would have given us an exception.","task#Task":"Task You need to create a Sum class which will sum integers from command line arguments and output the sum to console.\nExamples:\njava Sum 1 2 3 Expected output: 6.\njava Sum 1 2 -3 Expected output: 0.\njava Sum \"1 2 3\" Expected output: 6.\njava Sum \"1 2\" \" 3\" Expected output: 6.\njava Sum \" \" Expected output: 0.\nArguments can be:\ndigits, signes + and -, space symbols You can assume that int type is sufficient for in-between calculations and result.\nBefore doing the task make sure to read docs for classes String and Integer.\nFor debugging use System.err, because it will be ingnored by the testing program."},"title":"Sum"},"/courses/prog-intro/lectures/intro/":{"data":{"how-should-java-code-look-like#How should Java code look like?":"You can go and learn about it here.","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.\nSome of the most popular JVMs right now are:\nOpenJDK 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.\nNone 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#What is garbage collection?":"For example we have int which is represented with 4 bytes of data which is directly stored in memory.\nImg. X – int in memory\nBut 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.\nImg. X – String in memory\nBut 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.\nSuppose we have some code like this.\nif (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.\nImg. X – Edge case for link counting\nThe 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#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","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"}} \ No newline at end of file +{"/courses/prog-intro/homeworks/sum/":{"data":{"solution#Solution":"After reading about String, Integer, System.err we now know about some usefull methods:\nInteger.parseInt(String s, int radix) which parses the string argument as a signed integer in the radix specified by the second argument. So it basically converts a number from the String data type to int. Character.isWhitespace(char ch) which checks if a character ch is a space symbol or not. Now let’s start coding. Firstly let’s define the structure of our program. I will be putting the name of the file at the first line comment in a file and its path if its nessesary.\n// Sum.java public class Sum { public static void main(String[] args) { // ... } } Okay that’s done. What do we do next? Let’s look at String[] args argument to our main method. It represents an array of command line arguments which we need to sum. So know we made our task a little bit easier. Now we can just say that we need to find sum of elements of array args.\nHow are we going to do it though? First let’s understand what we can do with this array. Let’s modify our class a little bit.\n// Sum.java public class Sum { public static void main(String[] args) { System.out.println(args.length); } } We’ve added System.out.println(args.length) which takes the field length from our args object and prints it to the console.\nLet’s try it. First compile our class using\n$ javac Sum.java And then we can do some manual testing.\n$ java Sum 1 2 3 3 We got 3 as an output as expected. We gave our program 3 command line arguments: 1, 2 and 3.\nHere are all of the examples\n$ java Sum 1 2 -3 3 $ java Sum \"1 2 3\" 1 Note\nNotice, that we got 1 instead of 3. That’s because we put our arguments in \"\" so this becomes a single string argument.\n$ java Sum \"1 2\" \" 3\" 2 $ java Sum \" \" 1 Note\nHere program gives us 1 instead of 0, because despite not having any numbers in the arguments a single whitespace is still an argument.\nNow let’s try not obly to count our arguments but to list them as well. Let’s modify our program a little bit more.\n// Sum.java public class Sum { public static void main(String[] args) { System.out.println(\"number of arguments: \" + args.length); for (String argument : args) { System.out.println(argument); } } } Here I used for loop to do printing for every String element in args.\nLet’s try this with our examples. And don’t forget to recompile using javac Sum.java.\n$ java Sum 1 2 3 number of arguments: 3 1 2 3 $ java Sum 1 2 -3 number of arguments: 3 1 2 -3 $ java Sum \"1 2 3\" number of arguments: 1 1 2 3 Note\nAgain, notice only one string argument.\n$ java Sum \"1 2\" \" 3\" number of arguments: 2 1 2 3 $ java Sum \" \" number of arguments: 1 Okay, now that we know how to iterate (or do something for every element), we can calculate the sum of all the numbers in the args array. To do that we need to create a new variable sum of integer type and assign it the value of 0. Then for every number in args we will add it to sum, and so by the end of array we will have the sum of all numbers stored in variable sum. This is what the code will look like\n// Sum.java public class Sum { public static void main(String[] args) { System.out.println(\"number of arguments: \" + args.length); int sum = 0; for (String argument : args) { sum = sum + argument; System.out.println(argument); System.out.println(sum); } System.out.println(sum); } } Seems ok. But let’s test it.\n$ javac Sum.java Sum.java:9: error: incompatible types: String cannot be converted to int sum = sum + argument; ^ 1 error We got a compilation error that says String cannot be converted to int. It means that when we try to add 2 variables sum and argument their types don’t match. The type of sum is integer and string for argument. It seems pretty logical because what do we expect when adding for example 1 and apple?..\nImportant\nSo we need to cast argument to integer data type so that we can add it to sum.\nWe can do so using Integer.parseInt() method.\nIt takes a String s and returns an int, which a string s represents. For example this code will work as expected\n// ParseIntExample.java public class ParseIntExample { public static void main(String[] args) { String s = \"123\"; int sum = 321; //! int result = s + sum; } } The commented out line (//!) would have given us an exception.\n$ javac ParseIntExample.java ParseIntExample.java:8: error: incompatible types: String cannot be converted to int int result = s + sum; ^ 1 error Important\nPlease note that if we change result data type to String, the code will work just fine.\n// ParseIntExample.java public class ParseIntExample { public static void main(String[] args) { String s = \"123\"; int sum = 321; String result = s + sum; System.out.println(result); } } $ javac ParseIntExample.java \u0026\u0026 java ParseIntExample 123321 In this case sum will be automatically casted to String and two strings will just concatenate.\nSo we figured out that we need to cast argument to integer type. Let’s change our code and test it.\n// Sum.java public class Sum { public static void main(String[] args) { System.out.println(\"number of arguments: \" + args.length); int sum = 0; for (String argument : args) { sum = sum + Ineteger.parseInt(argument); System.out.println(argument); System.out.println(sum); } System.out.println(sum); } }","task#Task":"Task You need to create a Sum class which will sum integers from command line arguments and output the sum to console.\nExamples:\njava Sum 1 2 3 Expected output: 6.\njava Sum 1 2 -3 Expected output: 0.\njava Sum \"1 2 3\" Expected output: 6.\njava Sum \"1 2\" \" 3\" Expected output: 6.\njava Sum \" \" Expected output: 0.\nArguments can be:\ndigits, signes + and -, space symbols You can assume that int type is sufficient for in-between calculations and result.\nBefore doing the task make sure to read docs for classes String and Integer.\nFor debugging use System.err, because it will be ingnored by the testing program."},"title":"Sum"},"/courses/prog-intro/lectures/intro/":{"data":{"how-should-java-code-look-like#How should Java code look like?":"You can go and learn about it here.","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.\nSome of the most popular JVMs right now are:\nOpenJDK 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.\nNone 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#What is garbage collection?":"For example we have int which is represented with 4 bytes of data which is directly stored in memory.\nImg. X – int in memory\nBut 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.\nImg. X – String in memory\nBut 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.\nSuppose we have some code like this.\nif (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.\nImg. X – Edge case for link counting\nThe 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#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","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"}} \ No newline at end of file