Basics of JAVA

JAVA PROGRAMMING

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.

It is used for:

  • Mobile applications (specially Android apps)
  • Desktop 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 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


Java Syntax 

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

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.
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.
 

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

Java Multi-line Comments

Multi-line comments start with /* and ends with */.

Any text between /* and */ will be ignored by Java.

Example

/* The code below will print the words Hello World to the screen, 

and it is amazing */ 

System.out.println("Hello World"); 

 

 

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

 

Java Data Types

 

Data types are divided into two groups:

  • Primitive data types - includes byte, short, int, long, float, double, boolean and char
  • Non-primitive data types - such as Strings  ,Arrays , Classes

 

Java Operators

Operators are used to perform operations on variables and values.

Java divides the operators into the following groups:

  • Arithmetic operators
  • Assignment operators
  • Comparison operators
  • Logical operators
  • Bit-wise operators

 

 

Java If ... Else :

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 

We can use these conditions to perform different actions :

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

Example

int time = 22;
if (time < 10) {
  System.out.println("Good morning.");
} else if (time < 20) {
  System.out.println("Good day.");
} else {
  System.out.println("Good evening.");
}
 

Java While Loop

The while loop loops through a block of code as long as a specified condition is true.
 

Example

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

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.

Example

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

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