Java for Beginner Documentation

Java Introduction

Java is a popular programming language, created in 1995. It is owned by Oracle, and more than 3 billon devices run Java.

It is used for:

Mobile applications (specially Android apps)
Dekstop applications
Web applications
Web servers and application servers
Games
Database connection
And much, much more!

Why Use Java ?

Java works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc.)
It is one of the most popular programming language 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 a huge community support (tens of millions of developers)
Java is an object oriented language which 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

Hello World

In Java, every application begins with a class name, and that class must match the filename. Let's create our first Java file to print a "Hello World" message on the screen!

public class Main {
                     public static void main(String[] args) {
                          System.out.println("Hello World");
                         }
                }

Explained Code :

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 which must match the filename, and that every program must contain the main() method.

Inside the main() method, we use the println() method to print a line of text to the screen:

public static void main(String[] args) {
                        System.out.println("Hello World");
                      }

Java Comment

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. There are two ways to write a comment in Java :

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:

// This is a comment
                            System.out.println("Hello World");

This example uses a single-line comment at the end of a line of code:

System.out.println("Hello World"); // This is a comment

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:

/* The code below will print the words Hello World
                            to the screen, and it is amazing */
                            System.out.println("Hello World");

Java Data Types and 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

Java Declaring Variables

To create a variable, you must specify the type and assign it a value:

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.

Java Operators

Operators are used to perform operations on variables and values. Java divides the operators into the following groups:

Arithmetic Operators

Arithmetic operators are used to arithmetic values to variable.

In the example below, we use the + operator to add together two values:

int x = 100 + 50;
                            // x = 150

Assignment Operators

Assignment operators are used to assign values to variables.

In the example below, we use the assignment operator (=) to assign the value 10 to a variable called x:

int x = 10;

Comparison Operators

Comparison operators are used to compare two values (or variables).

The return value of a comparison is either true or false. These values are known as Boolean values. In the following example, we use the greater than operator (>) to find out if 5 is greater than 3:

int x = 5;
                            int y = 3;
                            System.out.println(x > y); // returns true, because 5 is higher than 3

Logical Operators

You can also test for true or false values with logical operators. Logical operators are used to determine the logic between variables or values.

Java Booleans

Boolean Values

A boolean type is declared with the boolean keyword and can only take the values true or false:

boolean isJavaFun = true;
                        boolean isFishTasty = false;
                        System.out.println(isJavaFun);     // Outputs true
                        System.out.println(isFishTasty);   // Outputs false

Boolean Expression

A Boolean expression returns a boolean value: true or false. For example, you can use a comparison operator, such as the greater than (>) operator, to find out if an expression (or a variable) is true or false:

int x = 10;
                        int y = 9;
                        System.out.println(x > y); // returns true, because 10 is higher than 9

or even easier:

System.out.println(10 > 9); // returns true, because 10 is higher than 9

Java If..Else

Java has the following conditional statements:

Use if to specify a block of code to be executed, if a specified condition is true

if (condition) {
                         // block of code to be executed if the condition is true
                      }

Use else to specify a block of code to be executed, if the same condition is false

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
                      }

Use else if to specify a new condition to test, if the first condition is false 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 }

Java Switch Case

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:

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

Java Loop

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:

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:

int i = 0;
                        while (i < 5) {
                          System.out.println(i);
                          i++;
                        }

Java 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.

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:

int i = 0;
                        do {
                          System.out.println(i);
                          i++;
                        }
                        while (i < 5);

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:

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:

for (int i = 0; i < 5; i++) {
                          System.out.println(i);
                      }

Reference

All the documentation in this page is taken from W3School.