Introduction to Java

🔹 Java Overview

Keypoints:

High-level, multi-platform, object-oriented programming language.
Created by Sun Microsystems in 1995.
Common uses: web apps, mobile apps, enterprise systems, AI, IoT, big data, cloud computing.

Importance:

Write once, run anywhere (WORA).
Widely used with huge community support.

Example:

Apps like Minecraft and Android applications are built with Java.

🔹 Java Runtime Environment (JRE)

Keypoints:

Provides class libraries & resources needed for Java programs to run.
Needed on any device that executes Java apps.

Advantages of Java:

Active community & support.
Many learning resources.
Built-in functions & libraries.
Strong security system.
Platform independence (WORA).

🔹 Java Virtual Machine (JVM)

Keypoints:

Runtime engine that executes Java programs.
Converts Java bytecode → machine language.
Combines compiler + interpreter.

Process:

Source code (.java file).
Compiler → converts to bytecode.
Interpreter (java.exe) → executes line by line on OS.

Importance:

Makes Java cross-platform.

🔹 Java APIs

Keypoints:

APIs = pre-written Java classes & methods.
Add ready-made functionalities (e.g., date, math, text manipulation).
Java has ~50 keywords but thousands of API classes/methods.

Example:

Math.sqrt(25) → uses API method to calculate square root.

🔹 Java Skeleton Program

Example Code:

class Skeleton {
    public static void main(String[] args){
        System.out.println("First Java Application"); //for display
    }
}

Key Concepts:

class = blueprint of objects.
Identifiers = names for classes, objects, variables (rules: start with letter/_/$, case-sensitive, no reserved words).
Reserved words = keywords like class, public, static, void.

Main Method Breakdown:

public → makes method accessible.
static → can run without creating an object.
void → no return value.
main(String[] args) → entry point of program.

Other Notes:

String = sequence of characters.
Array = collection of same data types.
Comments:
- // single line
- /* multi-line */

🔹 Saving, Compiling & Running Java Programs

Steps:

Write program in text editor (e.g., Notepad).
Save as .java file (e.g., Demo.java).
Open Command Prompt (cmd).
Set JDK path → set path=%path%;[JDK bin directory].
Compile → javac Demo.java.
Run → java Demo.

Output Example:

First program in Java

Importance:

Basic workflow every Java programmer must know.