ReadMe.md

Note Week 1 :

Welcome to the Cybertek School.

• Introduction to programming

• Command line

• Running your first java program from command line

• Running your program from Eclipse

• Variables and Data type

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What is Computer Program :

set of instructions to tell computer what to do. 

How does Java turn source code into machine code

1. java compiler turn your source code into --> bytecode

byte code is intermediate code between src code and machine code

2. then the bytecode get coverted to machine code so your program can eventually run in jvm(Java Virtual Machine).

bytecode is not platform specific. It can run on mac , windows, linux ..

That is how Java achieve write once and run anywhere

Editor

You can write source code in any plain text editor like notepad on windows and like textedit on mac

But as programmer we always want to have syntax highlighting and some other help to write code better

Few common text edtors are

• Sublime
• Atom
• VSCode (very popular)

Java Installation

In order to run a Java Program we need to have JRE(Java Runtime Environment), It has JVM (Java Virtual Machine) to run java code.

In order to write and develop Java Program we need to have JDK (Java Development Kit) installed on the machine.

JDK Contains JRE and JRE has JVM .

So first step is to download and install JDK on your computer. any version with 8 or above will just work in this case.

Make sure to follow canvas instruction to set up your JDK properly.

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Here we wrote our source code in sublime and saved as Hello.java on Desktop :

public class Hello {

	public static void main(String[] args){

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

	}

}

We openned the command line (or terminal) and navigate to desktop using below command.

cd Desktop

We compiled the Java source code using javac commmand

javac Hello.java

It generated byte code with .class extention as Hello.class then we used the below command to execute the program

#java YourClassNameGoesHere
java Hello

It generated the output

Hello World

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As a programmer , we always want to have a tool to compile and run our program all in one place and create and manage Java projects.

That's where IDE(Integrated Devellopment Environment)) comes in.

Few Popular options are :

• Eclispe
• IntelliJ IDEA
• NetBean and others.

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Creating First Eclipse Project

Assuming you already downloaded and openned Eclipse.

Here is how to create first Java Project.

Once we have the project, next step is to create package and class under the package.

package in Eclipse is just a folder in file system to organize your classes.

Once we have the package now we can create classes under the package and start writing the source code and run it.

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Understanding Eclipse Project Structure

MyFirstProject
    ├── .DS_Store
    ├── .classpath
    ├── .project
    ├── .settings
    │   └── org.eclipse.jdt.core.prefs
    ├── bin
    │   └── day01
    │       └── Hello.class
    └── src
        └── day01
            └── Hello.java

This is how Project structure looks like for mac users, windows users will not have .DS_Store . everything else is exactly the same.

There are a hidden file

• .classpath to specify classpath information
• .settings to store eclipse specific settings.
• .project to store project specific information
• bin folder is the folder for compiled bytecodes and it has same structore as src folder.

We do not need to do anything with above files and folders.

• src folder is where the source code .java files stored.

As a programmer , we just focus on writing source code under the source folder and let IDE take care of complation and running the compiled codes.

That's why eclipse only shows src folder rather than displaying entire files and folders like .classpath ,.settings and bin folder.

If you want to see what's inside the eclipse project directly from IDE you may follow below instruction.

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Creating sub-package (package under the package)

Sometimes we want to organize the packages even more granual manner.

For example , create a package called assignments and create another package under assignments called week1. Create a class called AnotherHello under week1 package.

Process of creation is the same the only extra thing we need to make it happen is to provide package name as

assignments.week1

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Variables

Variable is a container can store piece of information in programming.

It has 3 parts :

• Data type
• Variable name
• Variable value

For examle , to store whole number 75 in a variable named score:

int score = 75 ; 

Now we can use this variable to represent the value 75 anywhere in the program.

System.out.println(score);  // 75 on the console.

Data types : Java is strongly typed language and each variable must have data type.

**Primitive Data types : **

• The most basic data types in java to store pure values like numbers , characters , boolean
• There are 8 primitive data types in java and all starts with lowercase letters.

According to the range of numbers it can hold we have

• whole number data types byte , shorts , int , long
• fractional numbers data types float , double For characters and boolean
• char : 'A', 'B'
• boolean : true , false

Type	Range
byte	-128 to 127
short	-32,768 to 32,767
int	-2,147,483,648 to 2,147,483, 647
long	-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
float	approximately ±3.40282347E+38F (6-7 significant decimal digits)
double	approximately ±1.79769313486231570E+308 (15 significant decimal digits)
char	0 to 65,536 (unsigned)
boolean	true, false

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There is another data type very commonly used for store sequence of characters is String . String is not a primitive and yet since it's so common we will be using it early on and explain when we come to topic class and object.

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Naming in Java

It's good to build good habbit in early stage of your learning to follow good naming convention.

Rules for names

In any case ,

1. spaces are not allowed as a name in java.
2. can not start a name with number
3. Name can be letters [a-z] , [A-Z] , [0-9]

   number can be anywhere after first character

4. Only 2 special characters are allowed
   1. underscore _
   2. dollar sign $

Naming Convention

These are not rules but commonly accepted criteria to follow.

Variables names should be always descriptive and meaningful.

• Class Names
  • Always start with first letter uppercase : Hello
  • If you have more than one word, use uppercase for second Word first character : HelloPractice

    • This is also known as TitleCase

• Package Names
  • Always use lowercase : day01
  • if you have more than one word add _ in between words
• Variable Names (and methods later)
  • Always use lowecase!!!
  • More than one word you can use camelCase

    • For Example : myAwesome1stVariable

Declaring a variable :

Giving a name and data type without assigning a value, you are claiming a space in memory to store data (later) .

byte num1 ; 
short score ; 
int distance ; 
long distanceBetween2Planet ; 
float price ; 
double pi ;
char firstLetter ; 
boolean lightOn ; 

String name ; 

Initilizing the Variable

Giving a value for the first time. it can be also called assigning a value to variable.

byte num1 ; 
num1 = 123; 

short score ;
score = 12398; 

int distance ;
distance = 123123123 ;  

long distanceBetween2Planet ; 
distanceBetween2Planet = 123487537456L ;

float price ; 
price = 1.99f ; 

double pi ;
pi = 3.14897987897d ; 

char firstLetter ; 
firstLetter = 'K' ; 

boolean lightOn ;
lightOn = false ;  


String name ; 
name = "Europe Batch 4" ; 

Declaring and Initilizing the Variable

You may declare and initialize Variables at the same time.

byte num1 = 123; 
short = 12398; 
int distance = 123123123 ;  
long distanceBetween2Planet = 123487537456L ;
float price = 1.99f ; 
double pi = 3.14897987897d ; 
char firstLetter = 'K' ; 
boolean lightOn = false ;  

String name "Europe Batch 4" ;