Introduction to Java 

Java is a high-level, object-oriented programming language prescribed in the ICSE Class 10 Computer Applications syllabus. It helps students learn programming in a structured and logical way while preparing them for board examinations.

Java was developed by Sun Microsystems in 1995 and is now maintained by Oracle. One of its most powerful features is platform independence, meaning a Java program can run on any system using the Java Virtual Machine (JVM).

For ICSE students, Java focuses on logic building, program structure, and problem-solving, rather than just memorizing syntax. Topics like loops, arrays, strings, and methods are frequently asked in exams, making Java a high-scoring subject.

Types of Java Programs :

1. Applet Programs and Application Programs

In this article, we will clearly explain:

• What is a Java Application Program?

• What is a Java Applet Program?

• Differences between Applet and Application

• ICSE exam-focused explanation

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Java Application 

A Java Application Program is a standalone program that runs independently on a computer using the Java Virtual Machine (JVM).

These programs contain the main() method and are executed using the command prompt or IDE like BlueJ.

Key Features of Application Programs

•  Contains public static void main(String args[])
•  Runs independently
•  Can take input from user
•  Uses Scanner class
•  Mostly used in ICSE syllabus

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Example of Java Application Program

import java.util.Scanner;

class Addition {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        
        System.out.print("Enter first number: ");
        int a = sc.nextInt();
        
        System.out.print("Enter second number: ");
        int b = sc.nextInt();
        
        int sum = a + b;
        System.out.println("Sum = " + sum);
    }
}

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Java Applet:

A Java Applet is a small Java program that runs inside a web browser or applet viewer. It does not use the main() method.

Applets are mainly used to create:

• Graphics

• Animations

• Drawing shapes

• Interactive web applications

Key Features of Applet Programs

•  Does NOT contain main() method
•  Extends Applet class
•  Uses paint() method
•  Runs inside browser or applet viewer
•  Used for graphics and GUI

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Example of Java Applet Program

import java.applet.*;
import java.awt.*;

public class HelloApplet extends Applet {
    
    public void paint(Graphics g) {
        g.drawString("Hello ICSE Students!", 50, 50);
    }
}

📌 This program displays text in a browser window.

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Difference Between Applet and Application (Important for ICSE Exam)

Basis	Application Program	Applet Program
Main Method	Required	Not Required
Execution	Runs independently	Runs inside browser
Package	No need to extend Applet	Must extend Applet
Usage	General programs	Graphics & Web-based
ICSE Focus	Practical + Theory	Mostly Theory

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Program Execution in Java:

Program execution means the process through which a Java program is written, compiled, and finally executed to produce output.

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Phases of Program Execution in Java

There are mainly three important phases:

1️⃣ Writing the Java Program (Source Code)
2️⃣ Compilation
3️⃣ Execution

Let’s understand each phase in detail.

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Java Program (Source Code Phase)

In this phase:

• The programmer writes Java code.

• The file is saved with .java extension.

• The file name must match the class name.

Example:

If class name is:

class TestProgram

Then file name must be:

TestProgram.java

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Compilation Phase:

After writing the program, the next step is compilation.

Compilation is done using:

javac filename.java

What happens during compilation?

• Java compiler (javac) checks for syntax errors.

• If errors are found → Compilation fails.

• If no errors → Bytecode file is created.

The compiler creates a file with .class extension.

Example:

TestProgram.class

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Execution Phase:

After successful compilation, the program is executed using:

java filename

What happens during execution?

• JVM (Java Virtual Machine) reads the bytecode.

• JVM converts bytecode into machine code.

• Output is displayed on screen.

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Diagram of Java Program Execution

Source Code (.java)

⬇

Compiler (javac)

⬇

Bytecode (.class)

⬇

JVM

⬇

Output

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Java is Platform Independent:

Because:

• Java converts code into bytecode.

• Bytecode runs on JVM.

• JVM is available for different operating systems.

That’s why Java follows:

👉 “Write Once, Run Anywhere”

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Example of Complete Execution Process

Step 1: Write Code

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

Step 2: Compile

javac Hello.java

Step 3: Run

java Hello

Output:

Hello ICSE Students

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Common Errors in Program Execution 

• Syntax Error – Mistake in writing code
• Runtime Error – Error during execution
• Logical Error – Wrong output due to wrong logic

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Difference Between Source Code and Bytecode 

Basis	Source Code	Bytecode
Definition	Original program written by programmer	Code generated after compilation
File Extension	.java	.class
Readability	Human-readable	Not human-readable
Created By	Programmer	Java Compiler (javac)
Execution	Cannot run directly	Executed by JVM
Purpose	Writing program	Running program

JVM (Java Virtual Machine)

JVM (Java Virtual Machine) is a virtual machine that runs Java bytecode. When you compile a Java program, it generates bytecode (.class file). This bytecode cannot run directly on your computer. It runs on the JVM.

Main Functions of JVM:

• Executes Java program
• Makes Java platform independent
• Manages memory
• JVM is responsible for “Write Once, Run Anywhere”.

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JRE (Java Runtime Environment)

In Simple Words:
JRE = JVM + Libraries

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JDK (Java Development Kit)

JDK = JRE + Development Tools

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Relationship Between JDK, JRE, and JVM
⬇
Contains JRE
⬇
JRE contains JVM
JRE = JVM + Libraries

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Difference Between JDK, JRE, and JVM (ICSE Table Format)

Basis	JVM	JRE	JDK
Full Form	Java Virtual Machine	Java Runtime Environment	Java Development Kit
Purpose	Executes bytecode	Runs Java programs	Develops & runs Java programs
Contains	Execution engine	JVM + Libraries	JRE + Compiler
Used For	Running program	Running program	Writing & compiling program
Required By	All Java users	End users	Programmers

Main Features of Java 

1️⃣ Simple

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2️⃣ Object-Oriented

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3️⃣ Platform Independent

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4️⃣ Secure

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5️⃣ Robust

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6️⃣ Portable

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7️⃣ Multithreaded

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8️⃣ Distributed

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9️⃣ Dynamic

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Features of Java 

✔ Object-Oriented
✔ Platform Independent
✔ Secure
✔ Robust
✔ Portable
✔ Multithreaded
✔ Distributed
✔ Dynamic

Read More: OOP's Principal

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