amanagomez

September 30 2022

Single Responsibility Principle

/*
 + The following class has two reasons to change: one is that
 + the product definition/behaviour changes; the other is that
 + the tax calculation (values or formula) changes
 */

class Product {
  public title : string;
  public price : double;
  public taxRate : double;

  constructor(title, price, taxRate) {
    this.title = title;
    this.price = price;
    this.taxRate = taxRate;
  }
  calculateTax() {
    return this.price * this.taxRate;
  }
}

/* 
 * A solution is to split the class in two, each one with just one
 * responsibility (reason to change). In this solution the tax rate 
 * remains in the Product class, because it is different for 
 * different products
 */

class Product {
  public title : string;
  private price : double;
  private taxRate : double;

  constructor(title, price, taxRate) {
    this.title = title;
    this.price = price;
    this.taxRate = taxRate;
  }
  getPrice() {
    return this.price;
  }
  getTaxRate() {
    return this.taxRate;
  }
}

class TaxCalculator {
  static calculateTax(product) {
    return product.getPrice() * product.getTaxRate();
  }
}

/*
 * Still the product has the tax rate "hardcoded" in it.
 * A better solution is to identify the real "owner" of the
 * tax rate in the domain model, which is the ProductCategory,
 * because in real world the tax rates depend on the category
 * of the products.
 */

class Product {
  public title : string;
  private price : double;
  private category : ProductCategory;

  constructor(title, price, category) {
    this.title = title;
    this.price = price;
    this.category = category;
  }
  getPrice() {
    return this.price;
  }
  getCategory() {
    return this.category;
  }
}

class ProductCategory {
  public title : string;
  public taxRate : double;

  constructor(title, taxRate) {
    this.title = title;
    this.taxRate = taxRate;
  }
  getTaxRate() {
    return this.taxRate;
  }
}

class TaxCalculator {
  static calculateTax(product) {
    return product.getPrice() * product.getCategory().getTaxRate();
  }
}

+ The following class has two reasons to change: one is that

+ the product definition/behaviour changes; the other is that

+ the tax calculation (values or formula) changes

class Product {

public title : string;

public price : double;

public taxRate : double;

constructor(title, price, taxRate) {

this.title = title;

this.price = price;

this.taxRate = taxRate;

}

calculateTax() {

return this.price * this.taxRate;

}

* A solution is to split the class in two, each one with just one

* responsibility (reason to change). In this solution the tax rate

* remains in the Product class, because it is different for

* different products

class Product {

public title : string;

private price : double;

private taxRate : double;

constructor(title, price, taxRate) {

this.title = title;

this.price = price;

this.taxRate = taxRate;

}

getPrice() {

return this.price;

}

getTaxRate() {

return this.taxRate;

}

class TaxCalculator {

static calculateTax(product) {

return product.getPrice() * product.getTaxRate();

}

* Still the product has the tax rate "hardcoded" in it.

* A better solution is to identify the real "owner" of the

* tax rate in the domain model, which is the ProductCategory,

* because in real world the tax rates depend on the category

* of the products.

class Product {

public title : string;

private price : double;

private category : ProductCategory;

constructor(title, price, category) {

this.title = title;

this.price = price;

this.category = category;

}

getPrice() {

return this.price;

}

getCategory() {

return this.category;

}

class ProductCategory {

public title : string;

public taxRate : double;

constructor(title, taxRate) {

this.title = title;

this.taxRate = taxRate;

}

getTaxRate() {

return this.taxRate;

}

class TaxCalculator {

static calculateTax(product) {

return product.getPrice() * product.getCategory().getTaxRate();

}

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Single Responsibility Principle