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☕ Java Programming Foundations

This repository contains my personal study notes on Java. It covers the history, features, and the core architecture of the Java platform.

1. Introduction to Java

Java is a high-level, robust, and object-oriented programming language developed by James Gosling at Sun Microsystems in 1995.

  • Historical Note: It was originally named Oak (after an oak tree outside Gosling's office), but later renamed to Java.
  • Core Philosophy: "Write Once, Run Anywhere" (WORA).

2. Key Features of Java

Feature Description
Simple Syntax is clean and based on C / C++, but without complex features like pointers.
OOPs (Object-Oriented Programming) Everything in Java is an Object. It follows concepts like Inheritance, Polymorphism, Abstraction, and Encapsulation.
Platform Independent Unlike many other languages, Java is compiled into platform-neutral bytecode. "Write Once, Run Anywhere."
Secure Runs in a sandbox environment; no explicit pointers prevent unauthorized memory access.
Robust Strong memory management, lack of pointers, automatic Garbage Collection, and Exception Handling.
Architecture Neutral Primitive data types have a fixed size regardless of the architecture (e.g., int is always 4 bytes in both 32-bit and 64-bit systems).
Multi-threaded Supports the execution of two or more parts of a program concurrently to maximize CPU utilization.

3. The Java Environment: JDK, JRE, & JVM

To understand how Java works, you have to understand the "Russian Doll" relationship between these three components:

🔹 JVM (Java Virtual Machine)

  • What it is: An abstract machine/runtime environment.
  • Role: It loads, verifies, and executes the bytecode. It is platform-dependent (different versions for Windows, Mac, Linux), but it makes the code platform-independent.

🔹 JRE (Java Runtime Environment)

  • Formula: JRE = JVM + Libraries (rt.jar)
  • Role: It provides the minimum requirements to run a Java application. It does not contain development tools like compilers.

🔹 JDK (Java Development Kit)

  • Formula: JDK = JRE + Development Tools
  • Role: This is the full package needed to develop and run Java programs.
  • Tools included: * javac: The compiler (converts .java to .class).
    • jar: Archiver for packaging libraries.
    • jdb: The debugger.

JDK||JRE||JVM|| Structure Example.

4. Java Data Types

Java is a statically-typed language, meaning all variables must be declared before they can be used.

Java Data Types Diagram

Primitive Data Types (Predefined)

Non-Primitive Data Types (Reference Types)

🚀 How a Java Program Runs

  1. Source Code: You write code in a .java file.
  2. Compilation: The JDK uses javac to turn that code into Bytecode (.class file).
  3. Execution: The JVM (inside the JRE) interprets that bytecode so the computer can understand it.