Java Basics

Java is a high-level, object-oriented programming language developed by Sun Microsystems (now Oracle). It's known for its "write once, run anywhere" philosophy.

Key Features

Platform Independent: Java code runs on any platform with JVM
Object-Oriented: Everything is an object (except primitives)
Memory Management: Automatic garbage collection
Strongly Typed: Type checking at compile time
Multithreaded: Built-in support for concurrent programming

Basic Syntax

Hello World Program

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Variables and Data Types

Primitive Data Types

// Integer types
byte myByte = 100;          // 8-bit (-128 to 127)
short myShort = 5000;       // 16-bit (-32,768 to 32,767)
int myInt = 100000;         // 32-bit (-2^31 to 2^31-1)
long myLong = 15000000000L; // 64-bit (-2^63 to 2^63-1)

// Floating point types
float myFloat = 5.75f;      // 32-bit IEEE 754
double myDouble = 19.99d;   // 64-bit IEEE 754

// Other types
boolean myBool = true;      // true or false
char myChar = 'A';          // 16-bit Unicode character

Reference Types

String myString = "Hello Java";
int[] myArray = {1, 2, 3, 4, 5};
ArrayList<String> myList = new ArrayList<>();

Control Structures

Conditional Statements

// if-else statement
int age = 18;
if (age >= 18) {
    System.out.println("Adult");
} else if (age >= 13) {
    System.out.println("Teenager");
} else {
    System.out.println("Child");
}

// switch statement
int day = 3;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    case 3:
        System.out.println("Wednesday");
        break;
    default:
        System.out.println("Other day");
}

Loops

// for loop
for (int i = 0; i < 5; i++) {
    System.out.println("Count: " + i);
}

// enhanced for loop (for-each)
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    System.out.println(num);
}

// while loop
int count = 0;
while (count < 3) {
    System.out.println("Count: " + count);
    count++;
}

// do-while loop
int x = 0;
do {
    System.out.println("x = " + x);
    x++;
} while (x < 3);

Object-Oriented Programming

Classes and Objects

public class Car {
    // Instance variables
    private String brand;
    private String model;
    private int year;
    
    // Constructor
    public Car(String brand, String model, int year) {
        this.brand = brand;
        this.model = model;
        this.year = year;
    }
    
    // Getter methods
    public String getBrand() {
        return brand;
    }
    
    public String getModel() {
        return model;
    }
    
    public int getYear() {
        return year;
    }
    
    // Setter methods
    public void setBrand(String brand) {
        this.brand = brand;
    }
    
    // Instance method
    public void startEngine() {
        System.out.println(brand + " " + model + " engine started!");
    }
    
    // Static method
    public static void honk() {
        System.out.println("Beep beep!");
    }
}

// Using the class
public class Main {
    public static void main(String[] args) {
        Car myCar = new Car("Toyota", "Camry", 2023);
        myCar.startEngine();
        Car.honk(); // Static method call
    }
}

Inheritance

// Parent class
public class Animal {
    protected String name;
    
    public Animal(String name) {
        this.name = name;
    }
    
    public void eat() {
        System.out.println(name + " is eating");
    }
}

// Child class
public class Dog extends Animal {
    public Dog(String name) {
        super(name); // Call parent constructor
    }
    
    public void bark() {
        System.out.println(name + " is barking");
    }
    
    @Override
    public void eat() {
        System.out.println(name + " is eating dog food");
    }
}

Interfaces

public interface Drawable {
    void draw(); // Abstract method
    
    default void display() { // Default method
        System.out.println("Displaying...");
    }
}

public class Circle implements Drawable {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

Arrays and Collections

Arrays

// Array declaration and initialization
int[] numbers = new int[5];
int[] values = {1, 2, 3, 4, 5};

// Accessing elements
numbers[0] = 10;
int first = values[0];

// Array length
int length = numbers.length;

// 2D Array
int[][] matrix = {{1, 2, 3}, {4, 5, 6}};

Collections Framework

import java.util.*;

// ArrayList
List<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");

// HashMap
Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 25);
ages.put("Bob", 30);

// HashSet
Set<String> uniqueNames = new HashSet<>();
uniqueNames.add("John");
uniqueNames.add("Jane");

Exception Handling

public class ExceptionExample {
    public static void main(String[] args) {
        try {
            int result = divide(10, 0);
            System.out.println("Result: " + result);
        } catch (ArithmeticException e) {
            System.out.println("Error: " + e.getMessage());
        } catch (Exception e) {
            System.out.println("General error: " + e.getMessage());
        } finally {
            System.out.println("Finally block always executes");
        }
    }
    
    public static int divide(int a, int b) throws ArithmeticException {
        if (b == 0) {
            throw new ArithmeticException("Division by zero!");
        }
        return a / b;
    }
}

Input/Output

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.print("Enter your name: ");
        String name = scanner.nextLine();
        
        System.out.print("Enter your age: ");
        int age = scanner.nextInt();
        
        System.out.println("Hello " + name + ", you are " + age + " years old!");
        
        scanner.close();
    }
}

Important Concepts

Method Overloading

public class Calculator {
    public int add(int a, int b) {
        return a + b;
    }
    
    public double add(double a, double b) {
        return a + b;
    }
    
    public int add(int a, int b, int c) {
        return a + b + c;
    }
}

Access Modifiers

public: Accessible from anywhere
protected: Accessible within package and subclasses
default (no modifier): Accessible within package only
private: Accessible within the same class only

String Operations

String str = "Hello World";

// Common string methods
int length = str.length();
String upper = str.toUpperCase();
String lower = str.toLowerCase();
boolean contains = str.contains("World");
String substring = str.substring(0, 5);
String[] words = str.split(" ");

// String concatenation
String greeting = "Hello" + " " + "World";
String formatted = String.format("Hello %s, you are %d years old", "John", 25);

Best Practices

Naming Conventions:
- Classes: PascalCase (e.g., MyClass)
- Methods/Variables: camelCase (e.g., myMethod)
- Constants: UPPER_SNAKE_CASE (e.g., MAX_SIZE)
Code Organization:
- One public class per file
- Package names in lowercase
- Meaningful variable and method names
Memory Management:
- Close resources (Scanner, FileReader, etc.)
- Avoid memory leaks
- Use appropriate data structures

Java Basics

Java Basics

Key Features

Basic Syntax

Hello World Program

Variables and Data Types

Primitive Data Types

Reference Types

Control Structures

Conditional Statements

Loops

Object-Oriented Programming

Classes and Objects

Inheritance

Interfaces

Arrays and Collections

Arrays

Collections Framework

Exception Handling

Input/Output

Important Concepts

Method Overloading

Access Modifiers

String Operations

Best Practices

Oracle Java Documentation

Java Tutorials

Java API Documentation

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