JAVA Challenge

<div style="color: white;">
<h3>Why Java?</h3>
The programming language known today as Java has been in development since 1990. James Gosling - considered to be the language creator - first called Oak to commemorate the tree he was seeing from his office window. It soon turned out that the name was in use and needed to be changed.
This time inspiration struck him in a coffee shop: Java is a variety of Arabica coffee.
Why Java? The answer is simple - Java is everywhere! Is there anyone who has not yet heard about it? Versatility was the creators' goal: the programs were to be portable and hardware-independent. And this was the key to success.
Nowadays Java is used across web applications, desktop projects, and Android-based mobile phone apps.
</div>

java icon.png

<h2 style="text-align: center; margin: 0 auto;">Java &ndash; an object-oriented language</h2>

What is the most characteristic feature of Java? A spiteful answer could be: the pervasive prevalence of objects. In a way it's true: objects are first-class citizens in Java. We assume that everything is an object: the world is built from them and each object has certain features and a state. Look around: what objects do you see? A window? Flower? Fish? What are their attributes (color, shape)? How do they behave (move, grow)?

<h2 style="text-align: center; margin: 0 auto;">Types and variables</h2>

We will store particular elements in variables. A variable can be imagined as a box with something inside. One box only has enough room for one element.
Java is a strongly typed language: when we put something in a box we need to at once declare what type it is. It's also important to remember that the type, once it is assigned, it can never change. The value of the variable has to match the declared type: otherwise the program will have an error.
Not all languages are so restrictive about typing; Java is uncompromising and catches every little inaccuracy. Thanks to it we can be sure that no unexpected conversions happen in the program.
We can differentiate three general data types:
<ul>
<li>string (text type: values are given in quotes),</li>
<li>int (numeric type: integer numbers from -2 147 483 648 to 2 147 483 647),</li>
<li>boolean (logical values: True or False).</li>
</ul>
We already know what a variable is and that it must have a type. Now let's learn how to declare variables: that is, how to assign values to them.
We can represent it like this: variable_type variable name; variable_name = variable_value.
There's a shorthand for this (it works the same): variable_type variable_name = variable_value;

Knowing that we need variables and that they need to have a proper type, let's try to describe a dog.

See examples of types and variables:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA1DeclaringvariablesEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

<h2 style="text-align: center; margin: 0 auto;">How to name variables?</h2>

The best name explains what the variable stores. We may remember for a while that the "abc" variable stores the number of parts used to repair a car. But if someone else sees the code they will have no idea what it means. That's why it is best for a box to have a label corresponding to what's inside it. Of course, it is up to the programmer to decide how to name the variables.
But there are some rules everyone needs to follow:
<ul>
<li>name of a variable starts with a lowercase letter, a dollar or underscore character;</li>
<li>name of a variable starts with a lowercase letter, a dollar or underscore character;</li>
<li>character case matters: variable 'abc' is not the same as 'ABC';</li>
<li>if the name has more than one word we separate them with underscores or start each of them with an uppercase letter.</li>
</ul>

<h2 style="text-align: center; margin: 0 auto;">Displaying data</h2>

Let's try and show the data on the screen. Once again let's use the structure:

We can show the values of variables on the screen using their names directly.

<div class="line_container">
<div class="line">&nbsp;</div>
Here's an example:
</div>

System.out.println(“The dog is called ” + name); // prints: “The dog is called Buddy”.

Note: if you want to enter the text to display, put it inside quotation marks; if you are referring to a variable, just write its name. If you want to connect them, you can do it using the plus "+" character.

See an example of displaying data in Java:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA1printStringsexampleEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

<h2 style="text-align: center; margin: 0 auto;">Comments</h2>

If you want to describe how the code works you can use comments:
<ul>
<li>single-line comment is preceded by two slashes (example: // this is a single-line comment)</li>
<li>multi-line comment is surrounded by slash and asterisk characters (example: /* this comment can span several lines */).</li>
</ul>

<h3>Login instructions</h3>
Before you start working on the tasks go to: <a href="https://repl.it/" target="_blank" rel="noopener">https://repl.it/</a> and create an account by clicking the "Sign Up" button in the upper right corner. Next, to verify your account, follow the instructions from the page.
Done? Great! Now you can start doing the tasks.
Remember to click the "Fork" button when you go to the page with each task. This will let you change the code.

dziewczyna z laptopem.jpg

Introduction

<h2 style="text-align: center; margin: 0 auto;">Arithmetic operators</h2>

We already know there is a numeric type of variables. Thanks to it, we can perform various mathematical calculations using the appropriate arithmetic operators:
<ul>
<li>addition (plus character: &bdquo;+&rdquo;)</li>
<li>subtraction (minus: &bdquo;-&rdquo;)</li>
<li>multiplication (asterisk: &bdquo;*&rdquo;)</li>
<li>division (slash: &bdquo;/&rdquo;)</li>
<li>modulo (percent: &bdquo;%&rdquo;)</li>
</ul>
The modulo operation, which returns the remainder of the division, may be new to you.
It's like performing a division, but we're not interested in its result just whether there's any remainder left.
&nbsp;
Such as: 7 % 2 (seven modulo divided by two) returns 1.
Let's explain one by one why that happens:
7%2 = 3 * 2 + 1;
There are three twos in a seven: and a one that remains. That remainder is the modulo division result.
&nbsp;
If we need to perform calculations, we don't need to do it ourselves - Java will do it for us. We only need to learn to give it the data on which we want to perform an operation.
So, if we want to perform an arithmetic operation on two numbers we declare two variables; we assign certain values to them, and assign the result of the operation to a third variable, which we can later display on the screen, in the way we already know.

See examples of arithmetic operations:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA2ArithmeticoperationsEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

<h2 style="text-align: center; margin: 0 auto;">Conditional statements</h2>

if (condition == true) {
// instructions
}

See an example of a conditional statement:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA2ifEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

That is, if the condition is met the program must perform certain actions. On the other hand, if the condition is not met, the program does nothing. We can make the example a bit more complex and specify what to do when the condition does not occur. It's easy to do with the else clause: the instructions to perform in case the condition is not met. We need to write:

if (condition == true) {
 // instructions
 } else {
 // instructions
 }

See the second example:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA2ifelseEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

We can also check more conditions with the if-else if-else structure.

if(condition == true) {
 // instructions
 } else if (condition == true) {
 // instructions
 } else {
 // instructions
 }

Let's see what's going on here:
<ul>
<li>
if (condition == true) {...} &ndash; we ask if the condition given in the parentheses is met. If so (the result is 'True'), the instructions given in the curly brackets execute. If it is not: the program omits the instructions and goes further on.
</li>
<li>
else if (condition == true) {...} &ndash; if the first condition result was 'False' we move on to the next one. Here we are checking another condition &ndash; and as before &ndash; if it is 'True' the instructions in the curly brackets execute; if it isn't the program moves on.
</li>
<li>
else {...} &ndash; and if none of the earlier conditions were met, the instructions written after 'else' execute.
</li>
</ul>
Let's see this in practice. Let's assume we want to show the user a message with their LOTTO results. To make it easier let's assume that only a 'five' and 'six' win.

See the 'if else' example:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA2IfelseEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

If we have several conditions, but they relate to one issue (for example, declaring a range of values, i.e.: the number is greater than 3 and at the same time less than 25), we do not need to use the if construction twice - we can combine both conditions in one set of parentheses.
There are two operators to combine conditions:
<ol>
<li>
AND, written as: &amp;&amp; - the value must meet both conditions: the one on the left and on the right.
</li>
<li>
OR, written as: || &ndash; the value can meet one of the conditions: doesn't matter which one.
</li>
</ol>
If we assume that a course group cannot be smaller than 7 people but at the same time not bigger than 15, we will use AND.

See the examples of combining conditions with AND:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA2AndEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

We can write the same in a different way: using the OR operator. Our group cannot be smaller than 7 and bigger than 15 so if only one of these conditions is not met the group will not be created.

See the example of combining conditions with OR:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA2OREN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

Arithmetic operators

<h2 style="text-align: center; margin: 0 auto;">Loops</h2>

We can use loops when we want to execute a part of code a certain number of times.

for (initializing_variable; checking_condition; changing_variable_value)
{
//instructions to perform
}

See a 'for' loop example:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA3ForloopEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

This example, for comparison purposes, shows two ways of printing ten numbers. It's obvious that loops greatly reduce the number of lines in the code: from ten we now have just two!
Look at the parenthesis in the first line of the loop:
<ul>
<li>int i = 1</li>
</ul>
A variable with the initial value of 1 was declared because we want to display the numbers from one (if you want to start from ten, it's enough to write int i = 10 in the same place).
You can name your variable any way you want but by convention it is 'i'&rdquo; : you will for sure keep seeing it while reading about loops.
<ul>
<li>i &lt;= 10</li>
</ul>
Next, there is a condition check: in our case we check if the &bdquo;i&rdquo; variable is less or equal 10 (because we want to print numbers including ten).
<ul>
<li>&nbsp;i++</li>
</ul>
We change the value of the variable. '++' is an increment operator: it makes the value bigger by one. In the same place we could write the full operation: i = i + 1; it would also be correct. The effect would remain the same. But usually the shorthand is used.
Next, between the curly brackets, we write the instruction to be executed: we will print the 'i' variable incremented by one up to 10.

Loops

<h2 style="text-align: center; margin: 0 auto;">Arrays</h2>

Until now we declared variables that stored a single value. But sometimes we have collections of data (e.g., days of the week, zodiac signs). Rather than create a variable for each of the elements we can create a single variable containing a set number of elements.
This is a characteristic feature of Java arrays: they have a predefined size that cannot be changed.
Another important thing is that an array can only contain elements of a single type.
We can imagine an array as a box with a set number of compartments.
Let's assume we have a box with seven compartments: one for each day of the week. The first compartment is Monday, second one: Tuesday, third: Wednesday and soon.
&nbsp;
NOTE: Remember one, very important thing about arrays: they are numbered from zero! This "compartment number" in Java is called 'index'.

String[] daysOfWeek = {"Monday",
 "Tuesday",
 "Wednesday",
 "Thursday",
 "Friday",
 "Saturday",
 "Sunday"};

Our Monday in the first compartment has an index of '0', Tuesday - in the second compartment - index '1'. That's why the number of the last index will always be smaller by one than the number of elements in the array.
In our case, the array has seven elements, so the last index (for the Sunday compartment) will be '6'. Thanks to this ordering of the values it is easy to reference each element of the array.

<h2 style="text-align: center; margin: 0 auto;">Array declaration</h2>

Let's try to declare an array. Declaring an array is similar to declaring a normal variable: we give its type and name. Another element is a set of square brackets that immediately inform us that we are dealing with an array.
Jeśli chcemy przypisać wartości, robimy to po znaku &bdquo;=&rdquo;, umieszczając w nawiasie klamrowym elementy rozdzielone przecinkiem.
If we want to assign values we do it by following the '=' character with elements divided by commas inside a square brackets.

array_type[] array_name = { element1, element2, element3, element 4, and so on... };

<div class="line_container">
<div class="line">&nbsp;</div>
Sample array declaration:
</div>

<div class="line_container">
<div class="line">&nbsp;</div>
Here's an example of referencing an element in the array:
</div>

array_name[index_number]; // we give the name of the array followed by the index number in square brackets.

See an example of declaring the table with a String type and displaying a specific element from it:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA4ArraysEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

<h2 style="text-align: center; margin: 0 auto;">Displaying elements with a loop</h2>

We can conveniently display an entire array or its specific part using the 'for' loop we have learned earlier. Thanks to this we will not need to reference each element separately.
We are going to use the array with days of week we declared earlier and display all of its elements using a loop.

See the example of displaying the elements with a loop:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA4ArraywithforloopEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="" /></a></div>

Arrays

<h2 style="text-align: center; margin: 0 auto;">Methods</h2>

Methods are defined sets of instructions. They have many advantages: one of the most important ones is that they in a clear way order the actions that the program performs. They allow certain functionalities to be used in different parts of the program effectively.
In fact we have been using one of them even before we started talking about methods. It was the 'main' method: a starting method of a program. Our programs can have many methods but without the 'main' one we will not be able to run them.
The syntax of the 'main' method is quite complex, it contains the words: 'public' and 'static' that we don't need to know yet. For now, let's assume that all of our methods are going to start from 'public' and 'static'.

<h2 style="text-align: center; margin: 0 auto;">Method declaration</h2>

If we want to write our own method we do it under the 'main' one. At the beginning let's learn basic declaration of methods. This is done according to the pattern:
returned_type method_name(arguments);
Now, what is this 'returned_type'?
Well, in this place we put the information on the type of value that the method must return after execution. If we want the execution results to be a number, we declare 'int'. If it is to be text: we write 'String', etc. And if we don't want the method to return anything we use the word 'void' in this place.

<h2 style="text-align: center; margin: 0 auto;">Method arguments</h2>

A method can take arguments: we write them inside parentheses, immediately after the name of the method.
But arguments are not a required element: if we don't want the method to take arguments, we leave the parentheses empty. There can be any number of arguments and they can have different types: we divide them with commas.

See an example of method creation:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA5MethodsEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

Notice that apart from declaring our methods we also need to call them in the starting method of our program, the 'main' method. Calling the method means writing its name and adding any arguments it has.
If we want to display the result of the method that returned something we do it with the instruction we already know System.out.println() and put the call to the method inside the parentheses.

<h2 style="text-align: center; margin: 0 auto;">Discussing and calling methods</h2>

&nbsp;
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA5methodsreturnintEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>
We declared a method that should return a number (int). The 'return' instruction tells us it is going to be 5:

public static int returnNumber(){
 return 5;
 } 

Call: System.out.println(returnNumber());

&nbsp;
<div class="baza_container"><a class="baza_link baza_btn" href="https://repl.it/@CodersLab/JAVA5methodreturnStringEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>
In this example we declare returning text (String type) and in the 'return' instruction we can see that it will be the sentence: "The method returns text".

public static String returnString() {
 return "The method returns text.";
 }

Call: System.out.println(returnString());

&nbsp;
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA5methodargumentsEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>
The following method, just like the previous one returns text but additionally takes a String type argument that we put inside parentheses behind the name of the method.
Here, the method result depends on the value of the argument that is passed when calling the method. In the example we have the name "Jola" so in this case the method is going to return "Hello Jola".

public static String sayHello(String name){
 return "Hello " + name;
 }

Call: System.out.println(sayHello("Jola"));

&nbsp;
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA5methodvoidEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>
This method has the word 'void' instead of the declared type. That's how we know that it will execute the instruction in the body but not return anything. Notice that there's no 'return' in this method.

public static void printText(){
 System.out.println("The method prints text,
but it does not return anything.");
 }

Call: printText();

&nbsp;
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA5methodtwoargsEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>
This method declares that it returns an int type value and takes two arguments. It performs addition in the body and returns the operation result.

public static int sum(int a, int b) {
 int sum = a + b;
 return sum;
 }

Call: System.out.println(sum(3,5));

Methods

<h2 style="text-align: center; margin: 0 auto;">Retrieving data from the user</h2>

We can keep on writing methods and displaying their results but it's getting rather boring. It would be much more interesting if the computer "talked" to us and reacted accordingly.
The next code snippet is going to let us read the data entered using the keyboard. We are going to ask the user about their name and then greet them.
We are going to use the Scanner class to do it: it is a standard Java class that has methods dedicated just for this.
To not muddle up how this works we are not going to discuss it in detail. Let's see the main ideas of such "communication":

See how the Scanner class works:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA6ScannerEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

What new do you see in the code?
<ul>
<li>
earlier there was not import instruction: thanks to we we will be able to use the method to read keyboard input:
</li>
</ul>

<ul>
<li>also the instruction below is new:</li>
</ul>

In general, we create a Scanner class object and use it to call a method reading keyboard input. Notice that declaring an object shares similarities with declaring variables. In this case the type of the object is Scanner; the name of the object: scan. There is, however, a keyword 'new' which is characteristic for structures creating new objects.
<ul>
<li>
scan.nextLine() - here the text entered by the user is acquired.
</li>
</ul>
The remaining parts of the code are already familiar. You can also check back in the previous lesson if you don't remember something.
Note: that the value we are taking has the type of String so if you want to assign it to a variable it also needs to be a String type.
We can create something resembling a dialogue with a user, taking turns asking questions and reading their answers.

See an example of the Scanner class:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA6ScannerexampleEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

If instead of String we wanted to get the numerical information to be able to perform calculations on it later, one of the ways to achieve it is by using the scan.nextInt instruction instead of scan.nextLine().
But while nextLine takes any values written by the user, nextInt only takes int type values. In case of a different type the program will throw an error.

Retrieving Data

<h2 style="text-align: center; margin: 0 auto;">Code your first game!</h2>

Let's try and code a simple game where the user plays against the computer. The user picks, and the computer randomizes one of the three values which are compared and the result is displayed on the console (that's how we sometimes call the screen) and written to an array.
Your job is to complete the code responsible for the game logic.
Some parts of the code were already written: they are necessary for the code to work correctly but we have not learned all of them yet: these are more advanced topics. For now you will understand how they work in general to know what is going on in the code.
Note: the code as is now, does not compile.

All the tasks are done here:
<div class="baza_container"><a class="baza_link baza_btn" href="https://replit.com/@CodersLab/JAVA7TaskEN" target="_blank" rel="noopener">Open <img class="baza_img" src="https://www-api.coderslab.pl/uploads/media/default/0001/05/a4c7fa1be03f24b7e7660962494535341a8c6f03.png" alt="arrow" /></a></div>

Let's go through the code:
<ul>
<li>At the beginning you can see two import instructions: one of them you probably remember from the previous lesson when we discussed receiving data from the user. The second one lets us use the functionalities the computer needs to randomize a value.</li>
<li>Under the 'main' method declaration you can see the Scanner class object declaration, and a similar one for the Random class object.</li>
</ul>

The solution of the task is <a href="https://replit.com/@CodersLab/JAVA7TasksolutionEN" target="_blank" rel="noopener">here</a>

First Game

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