Single purpose class

One class should only do one thing. This helps to understand them. Check your classes if they really only have one purpose.

A single-purpose class does exactly one thing and does it well. When a class tries to do too much, it becomes hard to understand, test, and maintain. Keeping responsibilities focused makes your code easier to read, safer to change, and easier to reuse.

For example, instead of this multipurpose class:

public class UserManager {
    public void createUser(User user) { ... }
    public void sendWelcomeEmail(User user) { ... }
    public void logUserActivity(User user, String activity) { ... }
}

Each method belongs to a different concern - user creation, emailing, logging. Refactor it into focused classes:

public class UserService {
    public void createUser(User user) { ... }
}

public class EmailService {
    public void sendWelcomeEmail(User user) { ... }
}

public class ActivityLogger {
    public void logUserActivity(User user, String activity) { ... }
}

Now:

UserService only handles user-related operations.
EmailService only handles sending emails.
ActivityLogger only handles logging activity.

Practical habits:

Check each class: “Does this do only one thing?”
Avoid mixing business logic with technical concerns like logging or persistence.
Small, focused classes are easier to test, extend, and reuse.
Group related behavior, but don’t overload classes with unrelated responsibilities.

Every commit should leave classes more focused, making the code easier to understand, maintain, and evolve. Single-purpose classes are a cornerstone of clean, maintainable architecture.