Designing small classes with a single responsibility

Understanding Designing small classes with a single responsibility

The useful test for "this class is too big" is not line count, it is how many unrelated reasons force you to open the file. A class that holds an order's item, quantity and price and also builds the receipt text has two: pricing rules change when the business changes, and the receipt layout changes when someone wants a different look. In Java the tell is mechanical rather than aesthetic: list the fields, then mark for each method which fields it actually reads. If the marks fall into groups that never overlap, you already have two classes sharing one pair of braces.

Behaviour belongs next to the data it reads. subtotal() belongs on Order because it reads quantity and unitPrice, both private, so nothing has to be exposed for the calculation to happen. A method that reads nothing from this and only shuffles its parameters is a passenger: it belongs either on the class whose data it uses, or on a new class named after the thing it does own, such as a tax rate or an output format. The same reasoning says pagesPerDay does not belong on Book, because pace is a fact about a reader, and a Book would need one field per reader to store it.

The payoff shows up in concrete Java terms. Fields stay private because fewer classes need to see them, so getters appear only where a collaborator genuinely needs a value. You can write new Order("Keyboard", 2, 49.50) in a test without a printer, a tax table or a console, and when the receipt layout changes, Order is not edited, so nothing that relies on its invariants needs rechecking. You also gain names: new TaxPolicy(0.20) says what 0.20 means at the call site, which a bare double parameter never does.

import java.util.Locale;

class Order {
    private final String item;
    private final int quantity;
    private final double unitPrice;

    Order(String item, int quantity, double unitPrice) {
        this.item = item;
        this.quantity = quantity;
        this.unitPrice = unitPrice;
    }

    String item() {
        return item;
    }

    int quantity() {
        return quantity;
    }

    double subtotal() {
        return quantity * unitPrice;
    }
}

class TaxPolicy {
    private final double rate;

    TaxPolicy(double rate) {
        this.rate = rate;
    }

    double taxOn(double amount) {
        return amount * rate;
    }
}

class ReceiptPrinter {
    String render(Order order, TaxPolicy policy) {
        double subtotal = order.subtotal();
        double tax = policy.taxOn(subtotal);
        return String.format(Locale.US,
                "%s x%d%nsubtotal %.2f%ntax %.2f%ntotal %.2f",
                order.item(), order.quantity(), subtotal, tax, subtotal + tax);
    }
}

public class Main {
    public static void main(String[] args) {
        Order order = new Order("Keyboard", 2, 49.50);
        TaxPolicy vat = new TaxPolicy(0.20);
        System.out.println(new ReceiptPrinter().render(order, vat));
    }
}

Worked examples

class Task {
    private final String name;
    private final boolean done;

    Task(String name, boolean done) {
        this.name = name;
        this.done = done;
    }

    String name() {
        return name;
    }

    boolean done() {
        return done;
    }
}

class ChecklistView {
    String render(Task task) {
        return (task.done() ? "[x] " : "[ ] ") + task.name();
    }
}

class CsvView {
    String render(Task task) {
        return task.name() + "," + task.done();
    }
}

public class Main {
    public static void main(String[] args) {
        Task task = new Task("Write tests", true);
        System.out.println(new ChecklistView().render(task));
        System.out.println(new CsvView().render(task));
    }
}

class Book {
    private final String title;
    private final int pages;

    Book(String title, int pages) {
        this.title = title;
        this.pages = pages;
    }

    String title() {
        return title;
    }

    int pages() {
        return pages;
    }
}

class ReadingPlan {
    private final int pagesPerDay;

    ReadingPlan(int pagesPerDay) {
        this.pagesPerDay = pagesPerDay;
    }

    int daysFor(Book book) {
        return (book.pages() + pagesPerDay - 1) / pagesPerDay;
    }
}

public class Main {
    public static void main(String[] args) {
        Book book = new Book("Dune", 604);
        System.out.println(book.title() + " at 50 pages/day: " + new ReadingPlan(50).daysFor(book) + " days");
        System.out.println(book.title() + " at 80 pages/day: " + new ReadingPlan(80).daysFor(book) + " days");
    }
}

Important notes

Common mistakes

Try it yourself