Single Responsibility Principle

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“A module (e.g. a class) should have one, and only one, reason to change.”

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Original author: Robert C. Martin

Model Format MagicDraw

The single-responsibility principle (SRP) is a computer programming principle that states that “A module (e.g. a class) should have one, and only one, reason to change.” This sentence has lately been interpreted as “A module (e.g. a class) should be responsible to one, and only one, actor.”

Another way to express this is “Gather together the things that change for the same reasons. Separate those things that change for different reasons.”

Advantages

good things

Disadvantages

bad things

Code Examples

/*
 + 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();

}