Comprehensive understanding of Java technology: A guide from beginner to proficient

int age = 25;
double price = 12.5;
char grade = 'A';
boolean isValid = true;
String name = "John";

if (score >= 90) {
    System.out.println("优秀");
} else if (score >= 80) {
    System.out.println("良好");
} else {
    System.out.println("及格");
}

for (int i = 0; i < 10; i++) {
    System.out.println(i);
}

while (count > 0) {
    System.out.println(count);
    count--;
}

int[] numbers = new int[5];
numbers[0] = 1;
numbers[1] = 2;
numbers[2] = 3;
numbers[3] = 4;
numbers[4] = 5;

String[] names = {"Alice", "Bob", "Charlie"};

public class Person {
    private String name;
    private int age;

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

    public void sayHello() {
        System.out.println("Hello, my name is " + name);
    }
}

Person person = new Person("John", 25);
person.sayHello();

public class Animal {
    public void makeSound() {
        System.out.println("Animal makes sound");
    }
}

public class Cat extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Cat says Meow");
    }
}

Animal animal = new Cat();
animal.makeSound();

public interface Drawable {
    void draw();
}

public abstract class Shape implements Drawable {
    protected int x;
    protected int y;

    public Shape(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public void draw() {
        System.out.println("Drawing shape at (" + x + ", " + y + ")");
    }

    public abstract double area();
}

public class Circle extends Shape {
    private double radius;

    public Circle(int x, int y, double radius) {
        super(x, y);
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }
}

Shape shape = new Circle(0, 0, 5);
shape.draw();
System.out.println("Area: " + shape.area());

try {
    int result = divide(10, 0);
    System.out.println(result);
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
}

public int divide(int num1, int num2) {
    if (num2 == 0) {
        throw new ArithmeticException("Divisor cannot be zero");
    }
    return num1 / num2;
}

try (BufferedReader reader = new BufferedReader(new FileReader("file.txt"))) {
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
} catch (IOException e) {
    System.out.println("Error: " + e.getMessage());
}

try (BufferedWriter writer = new BufferedWriter(new FileWriter("file.txt"))) {
    writer.write("Hello, World!");
} catch (IOException e) {
    System.out.println("Error: " + e.getMessage());
}

public class MyThread extends Thread {
    @Override
    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.println(Thread.currentThread().getName() + ": " + i);
        }
    }
}

MyThread thread1 = new MyThread();
MyThread thread2 = new MyThread();
thread1.start();
thread2.start();