public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        int result = 0;
        for (int i = 0; i < arr.length; i += 2) {
            result += arr[i];
        }
        System.out.println(result);
    }
}

public abstract class Animal {
    protected String name;

    public Animal(String name) {
        this.name = name;
    }

    public abstract void makeSound(); // Абстрактный метод

    public void eat() {
        System.out.println(name + " is eating");
    }
}

public class Dog extends Animal {
    public Dog(String name) {
        super(name);
    }

    @Override
    public void makeSound() {
        System.out.println("Woof! Woof!");
    }
}

public interface Animal {
    void makeSound(); // Метод без реализации

    default void sleep() {
        System.out.println("Sleeping...");
    }
}

public class Dog implements Animal {
    @Override
    public void makeSound() {
        System.out.println("Woof! Woof!");
    }
}

public class Main {
    public static void main(String[] args) {
        // Абстрактный класс
        Dog dog = new Dog("Buddy");
        dog.makeSound(); // Выведет "Woof! Woof!"
        dog.eat(); // Выведет "Buddy is eating"

        // Интерфейс
        Animal animal = new Dog();
        animal.makeSound(); // Выведет "Woof! Woof!"
        animal.sleep(); // Выведет "Sleeping..."
    }
}

public class Main {
    public static void main(String[] args) {
        int x = 10;
        int y = 5;
        x += y; // x = x + y
        y -= 3; // y = y - 3
        x *= 2; // x = x * 2
        y /= 2; // y = y / 2
        System.out.println("x = " + x + ", y = " + y);
    }
}

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

public class Main {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        System.out.println("Сумма двух чисел: " + calc.add(2, 3)); // Выводит 5
        System.out.println("Сумма трех чисел: " + calc.add(2, 3, 4)); // Выводит 9
    }
}

public class Calculator {
  void calculate(int number1, int number2) { }
  void calculate(int number1, int number2, int number3) { }
}

public class Calculator {
  void calculate(int number1, int number2) { }
  void calculate(double number1, double number2) { }
}

public class Calculator {
  void calculate(double number1, int number2) { }
  void calculate(int number1, double number2) { }
}

package com.example.myapp;

src
└── com
    └── example
        └── myapp
            ├── Main.java
            └── Utils.java

import com.example.myapp.Utils;

import com.example.myapp.*;

public class Main {
    public static void main(String[] args) {
        com.example.myapp.Utils.printHello();
    }
}

package com.example.myapp;

public class Utils {
    public static void printHello() {
        System.out.println("Hello from Utils!");
    }
}

package com.example.myapp;

import com.example.myapp.Utils;

public class Main {
    public static void main(String[] args) {
        Utils.printHello();
    }
}

public class Main {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";
        String str3 = str1 + ", " + str2 + "!";
        str1 = str1.replace("H", "J");
        str2 = str2.substring(1, 4);
        str3 = str1 + ", " + str2 + "!";
        System.out.println(str3);
    }
}

src
└── com
    └── example
        ├── main
        │   └── Main.java
        ├── utils
        │   └── Utils.java
        └── database
            ├── DatabaseHelper.java
            └── DatabaseConfig.java

src
└── com
    └── example
        ├── controller
        │   └── UserController.java
        ├── service
        │   └── UserService.java
        ├── repository
        │   └── UserRepository.java
        └── model
            └── User.java

src
└── com
    └── example
        ├── controller
        │   └── UserController.java
        ├── service
        │   └── UserService.java
        ├── repository
        │   └── UserRepository.java
        └── model
            └── User.java

package com.example.controller;

import com.example.service.UserService;

/**
 * Контроллер для обработки запросов пользователей
 */
public class UserController {
    private UserService userService;

    public UserController() {
        userService = new UserService();
    }

    public void handleRequest() {
        userService.processUser();
    }
}

package com.example.service;

import com.example.repository.UserRepository;

/
 * Сервис для обработки логики пользователей
 */
public class UserService {
    private UserRepository userRepository;

    public UserService() {
        userRepository = new UserRepository();
    }

    public void processUser() {
        userRepository.saveUser();
    }
}

package com.example.repository;

/
 * Репозиторий для работы с базой данных пользователей
 */
public class UserRepository {
    public void saveUser() {
        System.out.println("User saved to the database");
    }
}

package com.example.model;

/**
 * Модель данных пользователя
 */
public class User {
    private String name;

    public User(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

- Throwable
  - Error
  - Exception
    - RuntimeException

public class CustomException extends Exception {
    public CustomException(String message) {
        super(message);
    }
}

public class Main {
    public static void main(String[] args) {
        try {
            validateAge(15);
        } catch (CustomException e) {
            System.out.println(e.getMessage());
        }
    }

    public static void validateAge(int age) throws CustomException {
        if (age < 18) {
            throw new CustomException("Age must be at least 18.");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        try {
            int result = divide(10, 0);
        } catch (ArithmeticException e) {
            System.out.println("Exception: " + e.getMessage());
            e.printStackTrace();
        }
    }

    public static int divide(int a, int b) {
        return a / b; // Бросает ArithmeticException при делении на ноль
    }
}