Writing First Program in JAVA : Guide

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:

  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.

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