Writing First Program in JAVA : Guide

Key Points:

1. Java Features and Portability:

   • Java is platform-independent; write once, run anywhere.

   • Java programs are portable and have consistent behavior across platforms, unlike C/C++ where data type sizes can vary.

2. Setting Up Java Development Environment:

   • JDK (Java Development Kit):

     • Essential for Java development, includes tools like compiler, debugger, etc.

     • Contains JVM (Java Virtual Machine) and JRE (Java Runtime Environment).

   • JRE:

     • Includes JVM and standard Java class libraries.

   • JVM:

     • Converts bytecode to machine-specific code using either an interpreter (line-by-line) or JIT (Just-In-Time) compilation (preferred for speed).

3. Installing JDK:

   • Download JDK from Oracle's official site, compatible with your system's configuration (Windows, Linux, macOS).

   • Once installed, you can compile and run Java programs.

4. IDEs and Editors:

   • You can use simple editors like Notepad or more sophisticated IDEs like IntelliJ, BlueJ, or Eclipse.

   • For learning or competitive programming, online compilers like ide.geeksforgeeks.org are recommended.

5. Classpath:

   • Similar to the PATH variable in OS, the CLASSPATH variable helps Java locate classes and libraries.

   • IDEs usually handle classpath settings, but it may need manual configuration when using simple editors.

Writing Your First Java Program

Example Program:

Breakdown:

1. Comments:

   • Single-line comment: // This is a comment

   • Multi-line comment: /* This is a multi-line comment */

2. Class Declaration:

   • public class Test { ... }

   • Every piece of code must be part of a class in Java.

3. Main Method:

   • public static void main(String[] args) { ... }

   • Entry point for any Java application.

   • public: Accessible from anywhere.

   • static: Can be called without creating an object.

   • void: Does not return any value.

   • String[] args: Command-line arguments.

4. Printing to Console:

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

   • Uses System class, out object, and println method to print text to the console.

Running Your First Java Program

Steps:

1. Write Code:

   • Save the code in a file named Test.java.

2. Compile Code:

   • Command: javac Test.java

   • Generates Test.class bytecode file.

3. Run Code:

   • Command: java Test

   • Executes the bytecode using JVM, outputting "Hello, World".

Interview Questions

1. What makes Java platform-independent?

   • Java's bytecode can be executed on any system with a JVM, allowing it to run on various platforms without modification.

2. Explain the role of JDK, JRE, and JVM.

   • JDK: Contains tools for Java development (compiler, debugger).

   • JRE: Provides runtime environment, includes JVM and class libraries.

   • JVM: Executes Java bytecode, converts it to machine-specific code.

3. What is the difference between JIT compilation and interpretation in JVM?

   • Interpretation: Converts bytecode to machine code line-by-line, slower.

   • JIT Compilation: Converts bytecode to machine code at runtime, faster.

4. Why is the main method in Java static?

   • So the JVM can call it without creating an instance of the class, simplifying program startup.

5. How do you set the classpath in Java?

   • Can be set manually using the CLASSPATH environment variable or specified in IDEs which configure it automatically.

6. What is the significance of the System.out.println statement?

   • It prints the specified message to the console, useful for debugging and displaying output.