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Single Responsibility Principle (SRP)

What is this?​

This principle says:

A module should have one, and just one reason to be changed

Why use this ?​

  • Helps the maintenance
  • Help to make a reusable code
  • Less git conflicts

Examples (Java)​

Bad example​

public class Facture {
    private Book book;
    private int quantity;
    private double percDiscount;
    private double percTax;
    private double total;

    public Facture(Book book, int quantity, double percDiscount, double percTax) {
        this.book = book;
        this.quantity = quantity;
        this.percDiscount = percDiscount;
        this.percTax = percTax;
        this.total = this.calculateTotal();
    }

    public double calculateTotal() {
        double price = ((book.price - book.price * percDiscount) * this.quantity);
        double taxPrice = price * (1 + percTax);
        return taxPrice;
    }

    // Print facture should not be the Facture class responsability!
    public void printFacture() {
        // do something to print the facture
    }

    // Save facture should not be the Facture class responsability!
    public void saveToFile(String nomeArquivo) {
        // do something to save the file
    }
}

Good example​

// Print Facture now have his own class and responsability!
public class PrintFacture {
  private Facture facture;

  public PrintFacture(Facture facture) {
    this.facture = facture;
  }

  public void printFacture() {
        // do something to print the facture
  }
}

// Save Facture now have his own class and responsability!
public class SaveFacture {
  Facture facture;

  public SaveFacture(Facture facture) {
    this.facture = facture;
  }

  public void saveToFile(String filename) {
        // do something to save the file
  }
}