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About Bhagvan Kommadi

Bhagvan Kommadi
Bhagvan Kommadi is the Founder of Architect Corner & has around 19 years’ experience in the industry, ranging from large scale enterprise development to helping incubate software product start-ups. He has done Masters in Industrial Systems Engineering at Georgia Institute of Technology (1997) and Bachelors in Aerospace Engineering from Indian Institute of Technology, Madras (1993). He is member of IFX forum,Oracle JCP and participant in Java Community Process. He founded Quantica Computacao, the first quantum computing startup in India. Markets and Markets have positioned Quantica Computacao in ‘Emerging Companies’ section of Quantum Computing quadrants. Bhagvan has engineered and developed simulators and tools in the area of quantum technology using IBM Q, Microsoft Q# and Google QScript.

How to Create and Run Your First Java Program

1. Introduction

This is an in-depth article on how to create and run your first java program. Java compiler is used to compile java code. Java class is the output of the compilation. To execute the program you need java runtime virtual machine.

2. Run Your First Java Program

2.1 Prerequisites

Java 7 or 8 is required on the linux, windows or mac operating system.

2.2 Download

You can download Java 7 from Oracle site. On the other hand, You can use Java 8. Java 8 can be downloaded from the Oracle web site .

2.3 Setup

You can set the environment variables for JAVA_HOME and PATH. They can be set as shown below:

Environmental Variables setup
JAVA_HOME=”/desktop/jdk1.8.0_73″
export JAVA_HOME
PATH=$JAVA_HOME/bin:$PATH
export PATH

2.4 First Program

2.4.1 Checking the running of the program

You need to create a java class "FirstJavaProgram". The class needs to have static method "main" which takes string array as the arguments. The first program in java is shown below. It prints “Checking First java program”.

First Java Program
 
public class FirstJavaProgram 
{

  public static void main(String[] args)
  {
    System.out.println("checking First java ");
  }
}

The command below executes the above code snippet:

Run Command
javac FirstJavaProgram.java
java FirstJavaProgram

The output of the executed command is shown below.

Run Your First Java Program
First Java Program

2.4.2 Command Line Arguments

You can pass the command line arguments to the java program. The static method "main" has the string array which will have the command line arguments. The code is modified to handle command line arguments.

JavaProgramArguments
public class JavaProgramArguments 
{
  public static void main(String[] args)
  {
    System.out.println("checking the arguments ");
      
      
    int i=0;
    for(String arg: args)
    {
        i++;
       System.out.println("argument "+i+" value "+ arg);
    }
  }
}

The command below executes the above code snippet:

Run Command
javac JavaProgramArguments.java
java JavaProgramArguments 11 22 33

The output of the executed command is shown below.

Run Your First Java Program - with Command LineArguments
Java Program with Command LineArguments

2.4.3 Java Class with Constructor

The java program can have a constructor. The "Program" class can have a public method "output". In the "main" method, program is instantiated and the method on the object is invoked. The code is shown below.

Program Class
public class Program 
{
  
  public Program()
  {
  
  }
  
  public void output(String value)
  {
  
    System.out.println(value);
  }
  
  
  public static void main(String[] args)
  {
    Program program = new Program();
    
    program.output("checking class with constructor");
  }
}

The command below executes the above code snippet:

Run Command
javac Program.java
java Program

The output of the executed command is shown below.

Run Your First Java Program - with Constructor
Java Class with Constructor

2.4.4 Java Class with setter and getter

You can create a java class Person which has id as a private member. It can have a default constructor with setter and getter methods for "id". The code for the java class is shown below.

Person Class
public class Person 
{
  private String id;
  public Person()
  {
  
  }
  
  public String getId()
  {
     return id;
  }
  
  public void setId(String id)
  {
     this.id = id;
  }
  public static void main(String[] args)
  {
    Person person = new Person();
    
    person.setId("34567");
    
    System.out.println("Person's id" + person.getId());
  }
}

The command below executes the above code snippet:

Run Command
javac Person.java
java Person

The output of the executed command is shown below.

Run Your First Java Program - Person class
Person class

2.5 Error Handling

Errors can happen during compilation or runtime. Compile time errors happen during the compilation of the program. Run-time errors can happen when you run the program. Logical errors are related to the code and the wrong results.

2.6 Input

You can use the "java.util.Scanner" class to input values to the java program. The "java.util.Scanner" has methods to handle long, float, double and String types. The methods are "nextLong()", "nextFloat()", "nextDouble()" and "next()". The code below shows how "java.util.Scanner" class can be used.

Java Input
import java.util.Scanner;

public class JavaInput {

    public static void main(String[] args) {
    	
    	Scanner scanner = new Scanner(System.in);
    	
    	System.out.print("Enter a number: ");
    	double number = scanner.nextDouble();
    	System.out.println("The number inputted is " + number);
    }
}

The command below executes the above code snippet:

Run Command
javac JavaInput.java
java JavaInput

The output of the executed command is shown below.

Run Your First Java Program - Java Input
Java Input

2.7 Expressions

You can use expressions in the code. Expressions are variables, operators, literals and method calls. They evaluate to one value. The sample code is shown below

Expressions
public class JavaExpressions {

    public static void main(String[] args) {
    	
    	double number; 
        number = 65.0;
        
        System.out.println(number);
        
        double newNumber;
        newNumber = 45.0;
        
        if(newNumber < number)
        {
        
          System.out.println("45 is less than 65");
        }
      
        
    }
}

The command below executes the above code snippet:

Run Command
javac JavaExpressions.java
java JavaExpressions

The output of the executed command is shown below.

Run Your First Java Program - Java Expressions
Java Expressions

2.8 Blocks

A block of code consists of one or more statements. These statements are enclosed in curly braces { } in if condition, for, do-while and while loops.

2.9 Comments

In the program, you can add comments at the class level and the methods. You can use block comments or single line comments. The suggested best practices for commenting code can be accessed at the oracle website.

start command
/**
    Person class
*/
public class Person 
{
  // id property  
  private String id;

  /**
   * default constructor
   */
  public Person()
  {
  
  }
  /**
   * getter method for Id
   */ 
  public String getId()
  {
     return id;
  }
  /**
   * setter method for Id
   */
  public void setId(String id)
  {
     this.id = id;
  }
  /**
   * static method main
   */
  public static void main(String[] args)
  {
    Person person = new Person();
    
    person.setId("34567");
    
    System.out.println("Person's id is " + person.getId());
  }
}

3. Download the Source Code

Download
You can download the full source code of this example here: How to Create and Run Your First Java Program
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