Practice

Example 1. A rectangle with two constructors

The constructor taking one side delegates to the two-parameter constructor. Initial dimensions are finite and positive, and the fields are final. Area and perimeter are calculated on request and are not duplicated.

final class Rectangle {
    private final double width;
    private final double height;

    Rectangle(double width, double height) {
        if (!Double.isFinite(width) || !Double.isFinite(height)
                || width <= 0 || height <= 0
                || width > 1e6 || height > 1e6) {
            throw new IllegalArgumentException("Invalid dimensions");
        }
        this.width = width;
        this.height = height;
    }

    Rectangle(double side) { this(side, side); }
    public double area() { return width * height; }
    public double perimeter() { return 2 * (width + height); }
    public String toString() { return width + " x " + height; }
}

public class Main {
    public static void main(String[] args) {
        Rectangle rectangle = new Rectangle(3, 4);
        Rectangle square = new Rectangle(2);
        System.out.println(rectangle);
        System.out.println(rectangle.area());
        System.out.println(rectangle.perimeter());
        System.out.println(square.area());
        try {
            new Rectangle(Double.NaN);
        } catch (IllegalArgumentException error) {
            System.out.println(error.getMessage());
        }
    }
}

3.0 x 4.0
12.0
14.0
4.0
Invalid dimensions

Example 2. A discounted product

The price is specified in integer kopiykas before tax. The sample rate of 20% is an explicitly defined example constant, not tax advice. First, the discount is applied with rounding down, followed by tax using the same rule. Rounding order is part of the contract.

final class Product {
    public static final int TAX_PERCENT = 20;
    private final String name;
    private long price;
    private int discount;

    Product(String name, long price) {
        if (name == null || name.isBlank()) {
            throw new IllegalArgumentException("Invalid name");
        }
        this.name = name;
        setPrice(price);
    }

    public void setPrice(long value) {
        if (value < 0 || value > 1_000_000_000L) {
            throw new IllegalArgumentException("Invalid price");
        }
        price = value;
    }

    public void setDiscount(int value) {
        if (value < 0 || value > 100) {
            throw new IllegalArgumentException("Invalid discount");
        }
        discount = value;
    }

    public long total() {
        long net = price * (100 - discount) / 100;
        return net + net * TAX_PERCENT / 100;
    }

    public String toString() { return name + ": " + total(); }
}

public class Main {
    public static void main(String[] args) {
        Product product = new Product("Notebook", 10000);
        product.setDiscount(10);
        System.out.println(product);
        try {
            product.setDiscount(101);
        } catch (IllegalArgumentException error) {
            System.out.println(error.getMessage());
        }
        System.out.println(product.total());
    }
}

Notebook: 10800
Invalid discount
10800

Example 3. Visitors and a shared total

Each entrance has its own counter, while a static field accumulates the total number of admitted visitors. This is a single-threaded model; thread synchronization is not covered here. The bound prevents overflow. The file Counter.java belongs to the ua.edu.study.counter package.

package ua.edu.study.counter;

public final class Counter {
    private static int total;
    private int local;

    public void enter(int count) {
        if (count < 0 || count > 1_000_000 - total) {
            throw new IllegalArgumentException("Invalid count");
        }
        local += count;
        total += count;
    }

    public int getLocal() { return local; }
    public static int getTotal() { return total; }
}

The file Main.java belongs to the ua.edu.study.app package.

package ua.edu.study.app;

import ua.edu.study.counter.Counter;

public class Main {
    public static void main(String[] args) {
        Counter north = new Counter();
        Counter south = new Counter();
        north.enter(3);
        south.enter(2);
        System.out.println(north.getLocal());
        System.out.println(south.getLocal());
        System.out.println(Counter.getTotal());
    }
}

3
2
5

Build the package with javac -d out, run the fully qualified class name, create a JAR with Main-Class, and run it again. Compare stdout: packaging should not change the domain result.