JAVA INTRODUCTION
Java Tutorial
Java Introduction
Java Install
Java Output / Print
Java Comments
Java Variables
Java Data Types
Java Type Casting
Java Operators
Java Strings
Java Math
Java Booleans
Java If ... Else
Java Switch
Java While Loop
Java For Loop
Java Break
Java Continue
Java Arrays
Java Introduction
What is Java?
Java is a popular programming language, created in 1995. It is owned by Oracle, and more than 3 billion devices run Java.
Why Use Java?
• Java works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc.)
• It is one of the most popular programming languages in the world
• It has a large demand in the current job market
• It is easy to learn and simple to use
• It is open-source and free
• It is secure, fast, and powerful
• It has huge community support (tens of millions of developers)
• Java is an object-oriented language that gives a clear structure to programs and allows code to be reused, lowering development costs
• As Java is close to C++ and C#, it makes it easy for programmers to switch to Java or vice versa
It is used for:
• Mobile applications (especially Android apps)
• Desktop applications
• Web applications
• Web servers and application servers
• Games
• Database connection
• And much, much more!
Java Getting Started
Java Install
Some PCs might have Java already installed.
To check if you have Java installed on a Windows PC, search in the start bar for Java or type the following in Command Prompt (cmd.exe):
C:\Users\Your Name>java -version
If Java is installed, you will see something like this (depending on the version):
java version "11.0.1" 2018-10-16 LTS Java(TM) SE Runtime Environment 18.9 (build 11.0.1+13-LTS) Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.1+13-LTS, mixed mode)
If you do not have Java installed on your computer, you can download it for free at oracle.com.
Note: We will write Java code in a text editor in this tutorial. However, writing Java in an Integrated Development Environment, such as IntelliJ IDEA, Netbeans, or Eclipse, is possible, which is particularly useful when managing larger collections of Java files.
Setup for Windows
To install Java on Windows: -
1. Go to "System Properties" (Can be found on Control Panel > System and Security > System > Advanced System Settings)
2. Click on the "Environment variables" button under the "Advanced" tab
3. Then, select the "Path" variable in System variables and click on the "Edit" button
4. Click on the "New" button and add the path where Java is installed, followed by \bin. By default, Java is installed in C:\Program Files\Java\jdk-11.0.1 (If nothing else was specified when you installed it). In that case, You will have to add a new path with: C:\Program Files\Java\jdk-11.0.1\bin Then, click "OK", and save the settings
5. At last, open Command Prompt (cmd.exe) and type java -version to see if Java is running on your machine Write the following in the command line (cmd.exe): C:\Users\Your Name>java -version If Java was successfully installed, you will see something like this (depending on version): java version "11.0.1" 2018-10-16 LTS Java(TM) SE Runtime Environment 18.9 (build 11.0.1+13-LTS) Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.1+13-LTS, mixed mode)
Java Quickstart
In Java, every application begins with a class name, and that class must match the filename. Let's create our first Java file, called Main.java, which can be done in any text editor (like Notepad). The file should contain a "Hello World" message, which is written with the following code:
Main.java
public class Main {
public static void main(String[] args) {
System. out.println("Hello World");
}
}
Don't worry if you don't understand the code above - we will discuss it in detail in later chapters. For now, focus on how to run the code above.
Save the code in Notepad as "Main.java". Open Command Prompt (cmd.exe), navigate to the directory where you saved your file, and type "javac Main.java":
C:\Users\Your Name>javac Main.java
This will compile your code. If there are no errors in the code, the command prompt will take you to the next line. Now, type "java Main" to run the file:
C:\Users\Your Name>java Main
The output should read:
Hello World
Java Syntax
In the previous chapter, we created a Java file called Main.java, and we used the following code to print "Hello World" to the screen:
Main.java Example explained
Every line of code that runs in Java must be inside a class. In our example, we named the class Main. A class should always start with an uppercase first letter.
Note: Java is case-sensitive: "MyClass" and "class" has a different meaning.
The name of the Java file must match the class name. When saving the file, save it using the class name and add ".java" to the end of the filename. To run the example above on your computer, make sure that Java is properly installed: Go to the Get Started Chapter for how to install Java. The output should be:
Hello World
The main Method
The main() method is required and you will see it in every Java program:
public static void main(String[] args)
Any code inside the main() method will be executed. Don't worry about the keywords before and after main. You will get to know them bit by bit while reading this tutorial.
For now, just remember that every Java program has a class name that must match the filename and that every program must contain the main() method.
System.out.println()
Inside the main() method, we can use the println() method to print a line of text to the screen:
public static void main(String[] args) {
System. out.println("Hello World");
}
Note: The curly braces {} mark the beginning and the end of a block of code.
The System is a built-in Java class that contains useful members, such as out, which is short for "output". The println() method, short for "print line", is used to print a value to the screen (or a file).
Don't worry too much about System, out, and println(). Just know that you need them together to print stuff on the screen. You should also note that each code statement must end with a semicolon (;).
Java Output / Print
Print Text
You learned from the previous chapter that you can use the println() method to output values or print text in Java:
Example
System.out.println("Hello World!");
You can add as many println() methods as you want. Note that it will add a new line for each method:
Example
System.out.println("Hello World!");
System.out.println("I am learning Java.");
System.out.println("It is awesome!");
Double Quotes
When you are working with text, it must be wrapped inside double quotations marks "". If you forget the double quotes, an error occurs:
Example
System.out.println("This sentence will work!");
System.out.println(This sentence will produce an error);
The Print() Method T
here is also a print() method, which is similar to println().
The only difference is that it does not insert a new line at the end of the output:
Example
System.out.print("Hello World! ");
System.out.print("I will print on the same line.");
Note that we add an extra space (after "Hello World!" in the example above), for better readability. In this tutorial, we will only use println() as it makes it easier to read the output of code.
Java Output Numbers
Print Numbers
You can also use the println() method to print numbers.
However, unlike text, we don't put numbers inside double quotes:
You can also perform mathematical calculations inside the println() method:
Example
System.out.println(3 + 3);
Example
System.out.println(2 * 5);
Example
System.out.println(3);
System.out.println(358);
System.out.println(50000);
Java Comments
Comments can be used to explain Java code, and to make it more readable. It can also be used to prevent execution when testing alternative code.
Single-line Comments
Single-line comments start with two forward slashes (//).
Any text between // and the end of the line is ignored by Java (will not be executed).
This example uses a single-line comment before a line of code:
Example
// This is a comment
System.out.println("Hello World");
This example uses a single-line comment at the end of a line of code:
Example
System.out.println("Hello World"); // This is a comment
Java Multi-line Comments
Multi-line comments start with /* and ends with */.
Any text between /* and */ will be ignored by Java.
This example uses a multi-line comment (a comment block) to explain the code:
Example
/* The code below will print the words Hello World to the screen, and it is amazing */
System.out.println("Hello World");
Single or multi-line comments?
It is up to you which you want to use. Normally, we use // for short comments, and /* */ for longer
Java Variables
Variables are containers for storing data values.
In Java, there are different types of variables, for example:
• String - stores text, such as "Hello". String values are surrounded by double quotes
• int - stores integers (whole numbers), without decimals, such as 123 or -123
• float - stores floating point numbers, with decimals, such as 19.99 or -19.99
• char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes
• boolean - stores values with two states: true or false
Declaring (Creating) Variables
To create a variable, you must specify the type and assign it a value:
Syntax
type variableName = value;
Where type is one of Java's types (such as int or String), and variableName is the name of the variable (such as x or name). The equal sign is used to assign values to the variable.
To create a variable that should store text, look at the following example:
Example
Create a variable called name of type String and assign it the value "John":
String name = "John";
System.out.println(name);
You can also declare a variable without assigning the value, and assign the value later:
Example
int myNum;
myNum = 15;
System.out.println(myNum);
Note that if you assign a new value to an existing variable, it will overwrite the previous value:
Example
Change the value of myNum from 15 to 20:
int myNum = 15;
myNum = 20;
// myNum is now 20 System.out.println(myNum);
To create a variable that should store a number, look at the following
Example
Create a variable called myNum of type int and assign it the value 15
int myNum = 15;
System.out.println(myNum);
Final Variables
If you don't want others (or yourself) to overwrite existing values, use the final keyword (this will declare the variable as "final" or "constant", which means unchangeable and read-only):
Example
final int myNum = 15;
myNum = 20;
// will generate an error: cannot assign a value to a final variable
Other Types
A demonstration of how to declare variables of other types:
Example
int myNum = 5;
float myFloatNum = 5.99f;
char myLetter = 'D';
boolean myBool = true;
String myText = "Hello";
Java Print Variables
Display Variables
The println() method is often used to display variables.
To combine both text and a variable, use the + character:
Example
String name = "John";
System.out.println("Hello " + name);
For numeric values, the + character works as a mathematical operator (notice that we use int (integer) variables here):
Example
int x = 5;
int y = 6;
System.out.println(x + y);
// Print the value of x + y
From the example above, you can expect:
• x stores the value 5
• y stores the value 6
• Then we use the println() method to display the value of x + y, which is 11
You can also use the + character to add a variable to another variable:
Example
String firstName = "John ";
String lastName = "Doe";
String fullName = firstName + lastName;
System.out.println(fullName);
Java Declare Multiple Variables
Example
2. short 2 bytes Stores whole numbers from -32,768 to 32,767
3. int 4 bytes Stores whole numbers from -2,147,483,648 to 2,147,483,647
4. long 8 bytes Stores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
5. float 4 bytes Stores fractional numbers. Sufficient for storing 6 to 7 decimal digits
6. double 8 bytes Stores fractional numbers. Sufficient for storing 15 decimal digits
7. boolean 1 bit Stores true or false values
8. char 2 bytes Stores a single character/letter or ASCII values
Java Non-Primitive Data Types
Non-Primitive Data Types
Non-primitive data types are called reference types because they refer to objects. The main difference between primitive and non-primitive data types are:
• Primitive types are predefined (already defined) in Java. Non-primitive types are created by the programmer and is not defined by Java (except for String).
• Non-primitive types can be used to call methods to perform certain operations, while primitive types cannot.
• A primitive type has always a value, while non-primitive types can be null.
• A primitive type starts with a lowercase letter, while non-primitive types starts with an uppercase letter.
• The size of a primitive type depends on the data type, while non-primitive types have all the same size.
Examples of non-primitive types are Strings, Arrays, Classes, Interface, etc.
Java Type Casting
Type casting is when you assign a value of one primitive data type to another type.
In Java, there are two types of casting:
• Widening Casting (automatically) - converting a smaller type to a larger type size byte -> short -> char -> int -> long -> float -> double
• Narrowing Casting (manually) - converting a larger type to a smaller size type double -> float -> long -> int -> char -> short -> byte
Widening Casting
Widening casting is done automatically when passing a smaller size type to a larger size type:
Example
public class Main {
public static void main(String[] args) {
int myInt = 9;
double myDouble = myInt; // Automatic casting: int to double
System.out.println(myInt); // Outputs 9
System.out.println(myDouble); // Outputs 9.0
}
}
Narrowing Casting
Narrowing casting must be done manually by placing the type in parentheses in front of the value:
Example
public class Main {
public static void main(String[] args) {
double myDouble = 9.78d;
int myInt = (int) myDouble; // Manual casting: double to int
System.out.println(myDouble); // Outputs 9.78
System.out.println(myInt); // Outputs 9
}
}
Java Operators
int sum3 = sum2 + sum2; // 800 (400 + 400)
The else Statement
Use the else statement to specify a block of code to be executed if the condition is false.
Syntax
if (condition) {
// block of code to be executed if the condition is true
}
else {
// block of code to be executed if the condition is false
}
Example
int time = 20;
if (time < 18) {
System.out.println("Good day.");
} else {
System.out.println("Good evening.");
} // Outputs "Good evening."
The else if Statement
Use the else if statement to specify a new condition if the first condition is false.
Syntax
if (condition1) {
// block of code to be executed if condition1 is true
}
else if (condition2) {
// block of code to be executed if the condition1 is false and condition2 is true
}
else {
// block of code to be executed if the condition1 is false and condition2 is false
}
Example
int time = 22;
if (time < 10) {
System.out.println("Good morning.");
}
else if (time < 18) {
System.out.println("Good day.");
}
else {
System.out.println("Good evening.");
} // Outputs "Good evening."
Java Short Hand If...Else (Ternary Operator)
Short Hand If...Else
There is also a short-hand if else, which is known as the ternary operator because it consists of three operands.
It can be used to replace multiple lines of code with a single line, and is most often used to replace simple if else statements:
Syntax
variable = (condition) ? expressionTrue : expressionFalse;
Instead of writing:
Example
int time = 20;
if (time < 18) {
System.out.println("Good day.");
}
else {
System.out.println("Good evening.");
}
You can simply write:
Example
int time = 20;
String result = (time < 18) ? "Good day." : "Good evening."; System.out.println(result);
Java Switch
Java Switch Statements
Instead of writing many if..else statements, you can use the switch statement. The switch statement selects one of many code blocks to be executed:
Syntax
switch(expression) {
case x: // code block break;
case y: // code block break;
default: // code block
}
This is how it works:
• The switch expression is evaluated once.
• The value of the expression is compared with the values of each case.
• If there is a match, the associated block of code is executed.
• The break and default keywords are optional, and will be described later in this chapter
The example below uses the weekday number to calculate the weekday name:
Example
int day = 4;
switch (day) {
case 1:
System.out.println("Monday");
break;
case 2:
System.out.println("Tuesday");
break;
case 3:
System.out.println("Wednesday");
break;
case 4:
System.out.println("Thursday");
break;
case 5:
System.out.println("Friday");
break;
case 6:
System.out.println("Saturday");
break;
case 7:
System.out.println("Sunday");
break;
} // Outputs "Thursday" (day 4)
The break Keyword
When Java reaches a break keyword, it breaks out of the switch block.
This will stop the execution of more code and case testing inside the block.
When a match is found, and the job is done, it's time for a break. There is no need for more testing.
A break can save a lot of execution time because it "ignores" the execution of all the rest of the code in the switch block.
The default Keyword
The default keyword specifies some code to run if there is no case match:
Example
int day = 4;
switch (day) {
case 6:
System.out.println("Today is Saturday");
break;
case 7:
System.out.println("Today is Sunday");
break;
default:
System.out.println("Looking forward to the Weekend");
} // Outputs "Looking forward to the Weekend"
Note that if the default statement is used as the last statement in a switch block, it does not need a break
Java While Loop
Loops
Loops can execute a block of code as long as a specified condition is reached. Loops are handy because they save time, reduce errors, and they make code more readable.
Java While Loop
The while loop loops through a block of code as long as a specified condition is true:
Syntax
while (condition) {
// code block to be executed
}
In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5:
Example
int i = 0;
while (i < 5) {
System.out.println(i);
i++;
}
Note: Do not forget to increase the variable used in the condition, otherwise the loop will never end!
The Do/While Loop
The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.
Syntax
do {
// code block to be executed
} while (condition);
The example below uses a do/while loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:
Example
int i = 0;
do {
System.out.println(i);
i++;
} while (i < 5);
Do not forget to increase the variable used in the condition, otherwise the loop will never end!
Java For Loop
When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop:
Syntax
for (statement 1; statement 2; statement 3) {
// code block to be executed
}
Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.
The example below will print the numbers 0 to 4:
Example
for (int i = 0; i < 5; i++) {
System.out.println(i);
}
Example explained
Statement 1 sets a variable before the loop starts (int i = 0).
Statement 2 defines the condition for the loop to run (i must be less than 5). If the condition is true, the loop will start over again, if it is false, the loop will end.
Statement 3 increases a value (i++) each time the code block in the loop has been executed.
Another Example
This example will only print even values between 0 and 10:
Example
for (int i = 0; i <= 10; i = i + 2) {
System.out.println(i);
}
Nested Loops
It is also possible to place a loop inside another loop.
This is called a nested loop. The "inner loop" will be executed one time for each iteration of the "outer loop":
Example
// Outer loop
for (int i = 1; i <= 2; i++) {
System.out.println("Outer: " + i); // Executes 2 times
// Inner loop
for (int j = 1; j <= 3; j++) {
System.out.println(" Inner: " + j); // Executes 6 times (2 * 3)
}
}
Java For Each Loop
For-Each Loop
There is also a "for-each" loop, which is used exclusively to loop through elements in an array:
Syntax
for (type variableName : arrayName) {
// code block to be executed
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
System.out.println(i);
}
Java Break and Continue
Java Break
You have already seen the break statement used in an earlier chapter of this tutorial. It was used to "jump out" of a switch statement.
The break statement can also be used to jump out of a loop.
This example stops the loop when i is equal to 4:
Example
for (int i = 0; i < 10; i++) {
if (i == 4) {
break;
}
System.out.println(i);
}
Java Continue
The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
This example skips the value of 4:
Example
for (int i = 0; i < 10; i++) {
if (i == 4) {
continue;
}
System.out.println(i);
}
Break and Continue in While Loop
You can also use break and continue in while loops:
Break Example
int i = 0;
while (i < 10) {
System.out.println(i);
i++;
if (i == 4) {
break;
}
}
Continue Example
int i = 0;
while (i < 10) {
if (i == 4)
{
i++;
continue;
}
System.out.println(i);
i++;
}
Java Arrays
Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.
To declare an array, define the variable type with square brackets:
String[] cars;
We have now declared a variable that holds an array of strings. To insert values to it, you can place the values in a comma-separated list, inside curly braces:
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
To create an array of integers, you could write:
int[] myNum = {10, 20, 30, 40};
Access the Elements of an Array
You can access an array element by referring to the index number.
This statement accesses the value of the first element in cars:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars[0]); // Outputs Volvo
Note: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Change an Array Element
To change the value of a specific element, refer to the index number:
Example
cars[0] = "Opel";
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars[0] = "Opel";
System.out.println(cars[0]); // Now outputs Opel instead of Volvo
Array Length
To find out how many elements an array has, use the length property:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"}; System.out.println(cars.length); // Outputs 4
Loop Through an Array
You can loop through the array elements with the for loop, and use the length property to specify how many times the loop should run.
The following example outputs all elements in the cars array:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (int i = 0; i < cars.length; i++) {
System.out.println(cars[i]);
}
Loop Through an Array with For-Each
There is also a "for-each" loop, which is used exclusively to loop through elements in arrays:
Syntax
for (type variable : arrayname) {
...
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
System.out.println(i);
}
The example above can be read like this: for each String element (called i - as in index) in cars, print out the value of i.
If you compare the for loop and for-each loop, you will see that the for-each method is easier to write, it does not require a counter (using the length property), and it is more readable.
Java Multi-Dimensional Arrays
Multidimensional Arrays
A multidimensional array is an array of arrays.
Multidimensional arrays are useful when you want to store data as a tabular form, like a table with rows and columns.
To create a two-dimensional array, add each array within its own set of curly braces:
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
myNumbers is now an array with two arrays as its elements.
Access Elements
To access the elements of the myNumbers array, specify two indexes: one for the array, and one for the element inside that array. This example accesses the third element (2) in the second array (1) of myNumbers:
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} }; System.out.println(myNumbers[1][2]); // Outputs 7
Remember that: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Change Element Values
You can also change the value of an element:
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
myNumbers[1][2] = 9;
System.out.println(myNumbers[1][2]); // Outputs 9 instead of 7
Loop Through a Multi-Dimensional Array
We can also use a for loop inside another for loop to get the elements of a two-dimensional array (we still have to point to the two indexes):
Example
public class Main {
public static void main(String[] args) {
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7}
};
for (int i = 0; i < myNumbers.length; ++i)
{
for(int j = 0; j < myNumbers[i].length; ++j)
{
System.out.println(myNumbers[i][j]);
}
}
}
}