What is an example of the Single Responsibility Principle?

Question

Can someone give me an example of the Single Responsibility Principle? I am trying to understand what it means, in practice, for a class to have a single responsibility as I fear I probably break this rule daily.

Answer 1

The most effective way to break applications is to create GOD classes. Those are classes that keep track of a lot of information and have several responsibilities. One code change will most likely affect other parts of the class and therefore indirectly all other classes that use it. That in turn leads to an even bigger maintenance mess since no one dares to do any changes other than adding new functionality to it.

The following example is a TypeScript class that defines a Person, this class should not include email validation because that is not related with a person behaviour:

class Person {
    public name : string;
    public surname : string;
    public email : string;
    constructor(name : string, surname : string, email : string){
        this.surname = surname;
        this.name = name;
        if(this.validateEmail(email)) {
          this.email = email;
        }
        else {
            throw new Error("Invalid email!");
        }
    }
    validateEmail(email : string) {
        var re = /^([\w-]+(?:\.[\w-]+)*)@((?:[\w-]+\.)*\w[\w-]{0,66})\.([a-z]{2,6}(?:\.[a-z]{2})?)$/i;
        return re.test(email);
    }
    greet() {
        alert("Hi!");
    }
}

We can improve the class above by removing the responsibility of email validation from the Person class and creating a new Email class that will have that responsibility:

class Email {
    public email : string;
    constructor(email : string){
        if(this.validateEmail(email)) {
          this.email = email;
        }
        else {
            throw new Error("Invalid email!");
        }        
    }
    validateEmail(email : string) {
        var re = /^([\w-]+(?:\.[\w-]+)*)@((?:[\w-]+\.)*\w[\w-]{0,66})\.([a-z]{2,6}(?:\.[a-z]{2})?)$/i;
        return re.test(email);
    }
}

class Person {
    public name : string;
    public surname : string;
    public email : Email;
    constructor(name : string, surname : string, email : Email){
        this.email = email;
        this.name = name;
        this.surname = surname;
    }
    greet() {
        alert("Hi!");
    }
}

Making sure that a class has a single responsibility makes it per default also easier to see what it does and how you can extend/improve it.