Java: The Complete Beginner-to-Advanced Guide (With Practical Code Examples)

Java is one of the most influential and widely used programming languages in the world. Designed with portability, performance, security, and scalability in mind, Java powers everything from enterprise backends and Android applications to financial systems and distributed platforms.

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1. What Is Java?

Java is a high-level, statically typed, object-oriented, compiled + interpreted language developed by Sun Microsystems (now Oracle).

Core goals of Java

• Write Once, Run Anywhere (WORA)
• Platform independence
• Strong memory management
• Built-in concurrency
• Security by design

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2. Java Architecture (High-Level)

Java follows a three-layer execution architecture:

Java Source Code (.java)
        ↓
Java Compiler (javac)
        ↓
Bytecode (.class)
        ↓
Java Virtual Machine (JVM)
        ↓
Native Machine Code

Key Components

• JDK (Java Development Kit) – compiler + tools
• JRE (Java Runtime Environment) – JVM + libraries
• JVM (Java Virtual Machine) – executes bytecode

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3. Java Internal Architecture (JVM Deep Dive)

3.1 JVM Components

Class Loader
Runtime Data Areas
Execution Engine
JNI
Garbage Collector

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3.2 Class Loader Subsystem

Loads .class files into memory.

Types:

• Bootstrap ClassLoader
• Extension ClassLoader
• Application ClassLoader

Process:

1. Loading
2. Linking
3. Initialization

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3.3 Runtime Data Areas (Memory Model)

Method Area

• Class metadata
• Static variables
• Constant pool

Heap

• Objects
• Arrays

Stack

• Method frames
• Local variables

PC Register

• Tracks instruction execution

Native Method Stack

• Used by JNI

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4. Java Execution Engine

• Interpreter – executes bytecode line by line
• JIT Compiler – converts hot code to native code
• Garbage Collector – automatic memory cleanup

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5. Java Garbage Collection (GC)

Java uses automatic memory management.

Common GC Algorithms:

• Serial GC
• Parallel GC
• G1 GC
• ZGC
• Shenandoah

Example:

MyObject obj = new MyObject();
obj = null; // eligible for GC

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6. Java Language Syntax — Complete Coverage

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7. Basic Java Program Structure

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

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8. Data Types in Java

Primitive Types

Type	Size
byte	1
short	2
int	4
long	8
float	4
double	8
char	2
boolean	JVM dependent

int x = 10;
double pi = 3.14;

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Reference Types

• Class
• Array
• Interface
• Enum

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9. Variables

int a = 5;         // local
static int b = 10; // static

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10. Operators

Arithmetic:

a + b;
a - b;
a * b;
a / b;

Logical:

&& || !

Bitwise:

& | ^ << >> >>>

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11. Control Flow Statements

If-Else

if (x > 0) {
    System.out.println("Positive");
}

Switch

switch(day) {
    case 1 -> System.out.println("Mon");
}

Loops

for(int i=0;i<5;i++){}
while(true){}
do{}while(false);

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12. Methods (Functions)

static int add(int a, int b) {
    return a + b;
}

Method Overloading:

add(int a, int b)
add(double a, double b)

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13. Object-Oriented Programming in Java

Classes and Objects

class Car {
    String brand;
    void drive() {}
}

Car c = new Car();

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Encapsulation

private int age;

public int getAge() {
    return age;
}

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Inheritance

class Animal {}
class Dog extends Animal {}

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Polymorphism

Animal a = new Dog();

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Abstraction

abstract class Shape {
    abstract void draw();
}

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Interfaces

interface Fly {
    void fly();
}

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14. Constructors

class User {
    User(String name) {}
}

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15. Static Keyword

static int count;
static void print(){}

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16. Final Keyword

final int MAX = 100;
final class Secure {}

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17. Packages and Imports

package com.app.core;
import java.util.*;

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18. Exception Handling

try {
    int x = 10 / 0;
} catch (ArithmeticException e) {
    e.printStackTrace();
} finally {
    System.out.println("Done");
}

Custom Exception:

class MyException extends Exception {}

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19. Java Collections Framework

List

List<Integer> list = new ArrayList<>();

Set

Set<Integer> set = new HashSet<>();

Map

Map<String,Integer> map = new HashMap<>();

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20. Generics

class Box<T> {
    T value;
}

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21. Lambda Expressions

(a, b) -> a + b;

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22. Streams API

list.stream()
    .filter(x -> x > 5)
    .forEach(System.out::println);

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23. Multithreading & Concurrency

Thread

class MyThread extends Thread {
    public void run() {}
}

Runnable

new Thread(() -> {}).start();

Synchronization

synchronized void method() {}

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24. Memory Model & volatile

volatile boolean running;

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25. Java I/O

FileReader fr = new FileReader("a.txt");

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26. NIO (New I/O)

Files.readAllLines(Path.of("a.txt"));

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27. Serialization

class User implements Serializable {}

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28. Annotations

@Override
@Deprecated

Custom:

@interface MyAnno {}

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29. Reflection API

Class<?> c = Class.forName("MyClass");

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30. JVM Tools

• javac
• java
• javadoc
• jstack
• jmap
• jconsole

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31. Java Modules (JPMS)

module my.module {
    requires java.sql;
}

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32. Java Versions & Modern Features

• Java 8 → Lambdas, Streams
• Java 11 → LTS
• Java 17 → Records, Sealed classes
• Java 21 → Virtual Threads (Project Loom)

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33. Virtual Threads (Modern Java)

Thread.startVirtualThread(() -> {
    System.out.println("Virtual");
});

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34. Where Java Is Used

• Backend (Spring, Quarkus)
• Android
• Banking systems
• Distributed systems
• Big data (Hadoop)

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35. Final Thoughts

Java remains one of the most powerful, stable, and enterprise-ready languages ever created. Its robust architecture, rich standard library, and modern language evolution make it relevant even decades after its birth.

If you fully understand Java internals + syntax, you can confidently build scalable, high-performance, production-grade systems.