Java Introduction

Java Syntax

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

Example explained

Every line of code that runs in Java must be inside a class. In our example, we named the class MyClass. A class should always start with an uppercase first letter.

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. You don't have to understand 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.

console.log(Object.getPrototypeOf({}) == Object.prototype);
// → true
console.log(Object.getPrototypeOf(Object.prototype));
// → null

Many objects don’t directly have Object.prototype as their prototype but instead have another object that provides a different set of default properties. Functions derive from Function.prototype, and arrays derive from Array.prototype.

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");
}

Java Variables

Java Variables

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:

type variable = value;

Where type is one of Java's types (such as int or String), and variable 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:

String name = "John";
System.out.println(name);

To create a variable that should store a number, look at the following example:

int myNum = 15;
System.out.println(myNum);

Other Types

A demonstration of how to declare variables of other types:

type variable = value;

Where type is one of Java's types (such as int or String), and variable 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:

int myNum = 5;
float myFloatNum = 5.99f;
char myLetter = 'D';
boolean myBool = true;
String myText = "Hello";

Java Operators

Java Operators

Operators are used to perform operations on variables and values.

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

int x = 100 + 50;

Java divides the operators into the following groups:

• Arithmetic operators
• Assignment operators
• Comparison operators
• Logical operators
• Bitwise operators

For more informations, please visit w3schools

Java If ... Else

Java Conditions and If Statements

Java supports the usual logical conditions from mathematics:

• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b
• Equal to: a == b
• Not Equal to: a != b

You can use these conditions to perform different actions for different decisions.

Java has the following conditional statements:

• Use if to specify a block of code to be executed, if a specified condition is true
• Use else to specify a block of code to be executed, if the same condition is false
• Use else if to specify a new condition to test, if the first condition is false
• Use switch to specify many alternative blocks of code to be executed

Short Hand If...Else (Ternary Operator)

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. It is often used to replace simple if else statements:

variable = (condition) ? expressionTrue :  expressionFalse;

Instead of writing:

int time = 20;
if (time < 18) {
    System.out.println("Good day.");
    } else {
    System.out.println("Good evening.");
    }