Introduction

If you're beginning your programming journey, Java is one of the most popular and beginner-friendly programming languages to learn. It is widely used in software development, coding interviews, backend systems, Android applications, and enterprise software.

Before diving into Data Structures and Algorithms (DSA), it is important to understand the fundamentals of Java and how Java programs actually run behind the scenes.

In this article, we'll explore what Java is, why it's widely used, how Java executes programs, the role of JVM, JRE, and JDK, and finally write our very first Java program.

What is Java?#

Java is a high-level, object-oriented programming language developed by Sun Microsystems in 1995. It was designed with a simple goal:

Write code once and run it anywhere.

Unlike many programming languages that depend heavily on the operating system, Java programs can run on different platforms without requiring any changes to the source code.

This feature is known as Write Once, Run Anywhere (WORA).

Today, Java powers millions of applications worldwide and remains one of the most in-demand programming languages.

Why Learn Java?#

Java continues to be one of the most preferred programming languages for beginners and professionals because it provides the perfect balance between simplicity, performance, and industry relevance.

Some reasons to learn Java are:

• Easy to learn and beginner-friendly
• Platform independent
• Strong Object-Oriented Programming support
• Large developer community
• Rich collection of libraries and frameworks
• Widely used in coding interviews and placements
• Strong demand in the software industry

Java vs Other Popular Languages#

As a beginner, you might wonder why we are learning Java instead of languages like C, C++, or Python.

Java vs C#

C	Java
Procedural Programming Language	Object-Oriented Programming Language
Platform Dependent	Platform Independent
Manual Memory Management	Automatic Memory Management
No JVM Support	Uses JVM

Java vs C++#

C++	Java
Supports Pointers	Hides Pointers from Developers
Manual Memory Management	Automatic Garbage Collection
Platform Dependent	Platform Independent
More Complex Syntax	Simpler and Cleaner Syntax

Java vs Python#

Python	Java
Simpler Syntax	More Structured Syntax
Interpreted Language	Compiled and Executed Through JVM
Popular in AI and Data Science	Popular in Enterprise and Backend Development
Generally Slower	Generally Faster

Each language has its own strengths, but Java provides a strong foundation for programming concepts, making it an excellent choice for learning DSA and software development.

Applications of Java#

Java is used in many areas of software development.

Some common applications include:

• Web Applications
• Android Applications
• Desktop Applications
• Enterprise Software
• Banking Systems
• Backend Development
• Cloud-Based Applications

Its versatility is one of the main reasons for its popularity.

How Java Programs Run#

When we write a Java program, the computer cannot directly understand the code we write.

Java follows a special execution process:

1. Write the program in a .java file.
2. Compile the code using the Java compiler.
3. The compiler generates bytecode (.class file).
4. The JVM reads the bytecode and executes it.

Java Execution Flow#

                                  Java Code (.java)
                                         ↓
                                   Compiler (javac)
                                         ↓
                                  Bytecode (.class)
                                         ↓
                                        JVM
                                         ↓
                                       Output

This execution process is one of the reasons Java is platform independent.

Understanding JVM, JRE, and JDK#

Many beginners hear the terms JVM, JRE, and JDK together and get confused. Let's understand them one by one.

What is JVM?#

JVM stands for Java Virtual Machine.

It is responsible for running Java programs.

The JVM reads the bytecode generated by the compiler and converts it into machine-level instructions that the computer can understand.

Real-Life Analogy#

Imagine writing a letter in English.

People from different countries may speak different languages, but a translator can convert your message into a language they understand.

Similarly:

• Java Code → Letter
• JVM → Translator
• Computer → Reader

Because every operating system has its own JVM implementation, the same Java program can run on Windows, Linux, and macOS.

This is the foundation of Java's Write Once, Run Anywhere principle.

What is JRE?#

JRE stands for Java Runtime Environment.

It provides everything required to run a Java application.

JRE contains:

• JVM
• Supporting Libraries
• Runtime Files

If you only want to run Java programs, JRE is enough.

What is JDK?#

JDK stands for Java Development Kit.

It is used by developers to create Java applications.

JDK contains:

• JRE
• JVM
• Java Compiler (javac)
• Development Tools

If you want to write, compile, and run Java programs, you need the JDK.

Relationship Between JVM, JRE, and JDK#

JDK
 └── JRE
      └── JVM

You can think of it like this:

• JVM → Runs Java Programs
• JRE → Provides Environment to Run Java Programs
• JDK → Provides Tools to Develop Java Programs

Your First Java Program#

Now that we understand how Java works, let's write our first Java program.

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

Output#

Hello World

Understanding the Program#

Let's break the program into smaller parts.

public class Main#

This creates a class named Main.

The file name should match the class name.

Main.java

public static void main(String[] args)#

This is the entry point of every Java program.

Whenever a Java program starts running, execution begins from the main() method.

For now, simply remember:

This is where the program starts executing.

System.out.println()#

This statement is used to display output on the screen.

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

The text inside double quotes is printed to the console.

print() vs println()#

Java provides two commonly used methods for displaying output.

Example#

System.out.print("Hello ");
System.out.println("World");
System.out.println("Welcome to Java");

Output#

Hello World
Welcome to Java

Difference Between print() and println()#

print()	println()
Prints output on the same line	Prints output and moves to the next line
Does not add a new line automatically	Adds a new line automatically

Check Your Understanding#

1. What is Java, and why is it called a platform-independent language?

2. Explain the difference between JVM, JRE, and JDK in your own words.

3. What happens when a Java program is compiled and executed? Explain the complete Java execution flow.

4. Is Java a 100% Object-Oriented Programming Language? Research this question and try to justify your answer with an example.

With these fundamentals in place, you're now ready to explore comments, identifiers, keywords, data types, and other core Java concepts.