🧱 Variables & Data Types

Java is strongly typed — every variable must have a declared type.

✏️ Declaring Variables

Variables.java

int age = 12;
double price = 9.99;
boolean isActive = true;
char grade = 'A';
String name = "Progress Java";

System.out.println(age + ", " + name);

Each declaration includes:

• a type (int, double, boolean, etc.)
• a name (identifier)
• and optionally, an initial value

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🔢 Primitive Data Types

Type	Example	Description
byte	byte b = 10;	small integers
short	short s = 3000;	rarely used
int	int n = 42;	most common integer
long	long big = 1000000L;	add L suffix
float	float f = 3.14f;	decimal, less precise
double	double d = 2.718;	more precise
char	char c = 'J';	single character
boolean	boolean ok = true;	true or false

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🧩 Reference Types

Reference types store objects, not raw values.

String message = "Hello, Java!";
Integer number = 42;
List<String> names = List.of("Suren", "Kallem", "Maya");

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🧠 Try It

1. Create a Java file named Variables.java.
2. Declare all 8 primitive types.
3. Print them using System.out.println().
4. Change a few values and recompile.

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### 🔁 `/docs/fundamentals/control-flow.md`
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# 🔁 Control Flow

Control flow statements decide *what happens next* in your program.

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## 🧭 If / Else

```java title="IfElse.java"
int number = 7;

if (number % 2 == 0) {
  System.out.println("Even");
} else {
  System.out.println("Odd");
}

---

🔀 Switch (Modern Syntax)

Switch.java

String role = "ADMIN";

switch (role) {
  case "ADMIN" -> System.out.println("Full Access");
  case "USER"  -> System.out.println("Limited Access");
  default      -> System.out.println("Guest");
}

---

🔂 Loops

for (int i = 0; i < 3; i++) System.out.println(i);

int j = 0;
while (j < 3) {
  System.out.println(j);
  j++;
}

for (String s : List.of("A", "B", "C")) {
  System.out.println(s);
}

---

💡 Try It

• Write a loop that prints the sum of numbers 1–100.
• Convert an if/else chain to a switch expression.

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### 👤 `/docs/fundamentals/objects-and-classes.md`
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# 👤 Objects & Classes

Everything in Java revolves around **classes** and **objects**.

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## 🧱 A Simple Class Example

```java title="PersonDemo.java"
class Person {
  String name;
  int age;

  Person(String name, int age) {
    this.name = name;
    this.age = age;
  }

  String greet() {
    return "Hi, I'm " + name + " (" + age + ")";
  }
}

public class PersonDemo {
  public static void main(String[] args) {
    Person p = new Person("Suren", 12);
    System.out.println(p.greet());
  }
}

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🔒 Encapsulation

Hide data and expose it safely through methods.

class Account {
  private double balance;

  public void deposit(double amt) {
    if (amt > 0) balance += amt;
  }

  public double getBalance() {
    return balance;
  }
}

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🐾 Inheritance

class Animal { String speak() { return "???"; } }
class Dog extends Animal { String speak() { return "Woof!"; } }

Animal a = new Dog();
System.out.println(a.speak()); // "Woof!"

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🧠 Try It

• Add a toString() method to Person.
• Create a subclass Employee that extends Person with a salary field.

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### 📚 `/docs/fundamentals/collections.md`
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# 📚 Collections & Generics

Collections let you store and manage groups of objects efficiently.

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## 🧺 Common Interfaces

| Interface | Description | Implementations |
|:--|:--|:--|
| `List` | Ordered, allows duplicates | `ArrayList`, `LinkedList` |
| `Set` | Unique elements | `HashSet`, `TreeSet` |
| `Map` | Key-value pairs | `HashMap`, `TreeMap` |

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## 💻 Example

```java title="CollectionsDemo.java"
List<String> names = new ArrayList<>();
names.add("Suren");
names.add("Kallem");

Set<String> unique = new HashSet<>(names);

Map<String, Integer> ageByName = new HashMap<>();
ageByName.put("Suren", 12);

System.out.println(names.get(0));
System.out.println(unique.contains("Suren"));
System.out.println(ageByName.get("Suren"));

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🧩 Generics

List<Integer> numbers = List.of(1, 2, 3);
int first = numbers.get(0); // type-safe

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🧠 Streams (Preview)

List<String> upper =
  names.stream()
       .filter(n -> n.length() > 4)
       .map(String::toUpperCase)
       .toList();

System.out.println(upper);

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💡 Try It

• Build a Map<String, List<Integer>> of subjects → test scores.
• Compute averages using loops or streams.

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### ⚠️ `/docs/fundamentals/exceptions.md`
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# ⚠️ Exceptions & Error Handling

Exceptions help your program recover gracefully when things go wrong.

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## 🧱 Try–Catch

```java title="ExceptionsDemo.java"
try {
  int x = 10 / 0;
  System.out.println(x);
} catch (ArithmeticException e) {
  System.out.println("Error: " + e.getMessage());
}

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✅ Checked vs Unchecked

• Checked: must be declared or handled (IOException)
• Unchecked: runtime errors like NullPointerException

void readFile() throws IOException {
  Files.readString(Path.of("missing.txt"));
}

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💾 Try-With-Resources

Automatically closes files and streams.

try (var reader = Files.newBufferedReader(Path.of("data.txt"))) {
  System.out.println(reader.readLine());
} catch (IOException e) {
  System.out.println("File issue: " + e.getMessage());
}

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🧩 Custom Exceptions

class DomainException extends RuntimeException {
  DomainException(String msg) {
    super(msg);
  }
}

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💡 Try It

• Wrap a file read in try-with-resources.
• Throw a custom exception on invalid input.