Author: zahur

Java variables are containers which hold values during execution of java program. These are names of a memory locations where values are temporarily saved.
A variable can hold:
simple text called String
Single character represented as char
a whole number called integer and represented by int
a number with fraction such as 2.65 called floating point numbers and represent as float
values with two states: true or false represented as boolean

How to create or Declare a variable

To use a variable in java we first need to declare it. To declare a variable first of all data type of variable is mentioned e.g. int, String, char, double followed by variable name and a termination character i.e. semicolon (;).

Syntax:


DataType variableName ;


Example

Following java code will create a variable named age with data type int and put a value “10” into it.

int age;
age = 10;

We can create and assign values in a single statement as follows:

int age = 10;


We can add comments in java code. They are not compiled. Java compiler simply ignores the java comments and only compiles the java code. There are two types of java comments.

Single line java comments

Multiline java comments

Single line java comments

Any line in java code starting with // is a java comment and java compile will ignore it while compiling.
Following example shows a single line java comment:

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

We can add java comments at the end of a java code statement. Following example shows a single line java comment at the end of a java code statement:

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

Java Multi-line Comments

We can add comments in a java program which span more than one lines.

Multi-line comments start with /* and ends with */. Any text between /* and */ will be ignored by Java.
Following example shows multi-line java comments:

/* It is a multi-line java comment
It is second line of java comment
It is third line of java comment
You can add as much lines of java comments as you want */
System.out.println("Hello World");

In previous post we created a java file named FirstJavaProgram.java which prints “Hello Word” on screen when we run it using java command  after compilation using javac command.


class FirstJavaProgram {

public static void main(String args[]){

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

}

}

FirstJavaProgram.java explained

In Java every line of code must be inside a class. In above example, we named the class FirstJavaProgram. It is good programming practice to start each word in the class name with an uppercase letter.
Java is case-sensitive therefore “FirstClass” and “firstClass” has different meaning.

Java class name and file name in should be same . 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

Main Method in Java Program

public static void main(String[] args)

Java program execution starts from main method. public, static, void are keywords of java which will be explained later. String[] args are command line arguments which be explained later in detail. For now you should only remember that main method is necessary for every java program with above syntax.

The code line System.out.println(“Hello World”); prints a line of text on command line. You can note a semicolon ( ; ) at the end of the statement. It is mandatory at the end of each java statement. The curly braces {} are used to start and stop a class and method name. You can note starting and ending curly braces of class and main method separately.

All applications in java begin with a class name, and that class must match the filename. Now let’s create our fist java program. Name of class will be FirstJavaProgram and name of file will FirstJavaProgram.java. We can do it in any text editor like notepad or an IDE like Netbeans. It will contain a main method which will print “Hello Word” on screen.

class FirstJavaProgram {

public static void main(String args[]){

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

}

}


Save the file as FirstJavaProgram.java
Open terminal, point it to directory where you have saved the above java file and type following command:
javac FirstJavaProgram.java
above command will compile the java file and create an new file named FirstJavaProgram.class. Now let’s run this file by type following command on termial:
java FirstJavaProgram.class
the above command will print the string “Hello World” on screen and will be terminated.

In this section we will describe the process to setup Java on your local machine.

Option 1 setting path to use javac compiler:

You can write java program in any text editor, compile it using javac command and run by java command on command line. To install java environment go to here and follow the instructions. For windows operating system download latest jdk version install it and setup path as follows:

Assuming you have installed Java in c:\Program Files\java\jdk directory −

• • • • Right-click on ‘My Computer’ and select ‘Properties’.
      • Click the ‘Environment variables’ button under the ‘Advanced’ tab.
      • Now, alter the ‘Path’ variable so that it also contains the path to the Java executable. Example, if the path is currently set to ‘C:\WINDOWS\SYSTEM32’, then change your path to read ‘C:\WINDOWS\SYSTEM32;c:\Program Files\java\jdk\bin’.

Option 2 using an IDE

use the IDE and leave the rest to IDE to set java path. Following are some popular java IDEs you can freely download and use to write and compile the java programs.

• • • • Netbeans
      • Eclipse
      • Oracle JDeveloper
      • BlueJ