You need to model an expiration date. Which object will you use? TL;DR: Model real word expiration dates with... expiration dates Problems Bijection Violation
Unexpected Behavior
Small Objects Missing
Primitive Obsession Solutions Honor the bijection and model correct behavior Context In many systems, the expiry date of a credit card is often represented by simply using a Date object. This can lead to potential issues and misunderstandings, especially when dealing with operations such as comparisons and calculations based on the expiry date. It's generally considered a better practice to represent the expiration date with an adequate object. Sample Code Wrong import java.util.Date;

public class CreditCard {
    private String cardNumber;
    private Date expiryDate;

    public CreditCard(String cardNumber, Date expiryDate) {
        // Not a complete date
        this.cardNumber = cardNumber;
        this.expiryDate = expiryDate;
    }

    public boolean isExpired() {
        Date currentDate = new Date();
        return expiryDate.before(currentDate);
        // How will it work? 
        // using the last day of the month?
    }
} public class CreditCard {
    private String cardNumber;
    private int expiryMonth;
    private int expiryYear;

    public CreditCard(String cardNumber, int expiryMonth, int expiryYear) {
        this.cardNumber = cardNumber;
        this.expiryMonth = expiryMonth;
        this.expiryYear = expiryYear;
        // No validations on number ranges?
    }

    public boolean isExpired(int currentMonth, int currentYear) {
        return (expiryYear < currentYear) ||
            (expiryYear == currentYear && expiryMonth < currentMonth);
    }
    // Inappropriate intimacy code smell
 
} Right class CreditCard {
  private String number;
  private MonthOfYear expiryDate;
  // expiryDate is the role
  // MonthOfYear is the type
}

class MonthOfYear {
  private Month month;  
  private Year year;
  // These are other small objects

  public MonthOfYear(Month month, Year year) {
    // You don't need to add validations since 
    // month is a valid month
    // year is a valid year
    this.month = month;
    this.year = year;
  } 
  
 public boolean isBeforeEndOfMonth(Date date) {
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(date);
    return (calendar.get(Calendar.YEAR) < year.value()) ||
           (calendar.get(Calendar.YEAR) == year.value() &&
            calendar.get(Calendar.MONTH) < month.value())      
    // Notice there are no days involved   
  }
  
  // This protocol is just for MonthOfYears  
  public Day[] getDaysInMonth() { }
  
  public boolean isLeapYear() { } // ...
  
  public void iterateDays() { } // ...
  
} Detection [x]Manual This is a design smell Tags Primitive Obsession Level [x]Intermediate This is not an obvious design decision AI Generation Actual AI assistants are not very good at creating these kinds of small objects AI Detection Gemini detected this is a possible smell Conclusion ExpiryDate explicitly separates month and year, making the code more readable and making it easier to understand the specific information needed for expiry. It can also encapsulate logic specific to expiry dates, such as calculating days remaining, validating expiry, or iterating through days in the month. While Date objects offer date manipulation functionalities, they don't inherently represent the specific concept of a credit card expiry. Finding small objects with concrete behavior is always a difficult task. Relations Code Smell 122 - Primitive Obsession Code Smell 177 - Missing Small Objects More Info The One and Only Software Design Principle Disclaimer Code Smells are my opinion. Credits Photo by CardMapr.nl on Unsplash Thank you, Hernan Wilkinson, for this tip Standards are always out of date. That’s what makes them standards. Alan Bennett https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true This article is part of the CodeSmell Series. How to Find the Stinky Parts of your Code Also published here. You need to model an expiration date. Which object will you use? You need to model an expiration date. Which object will you use? TL;DR: Model real word expiration dates with... expiration dates TL;DR: Model real word expiration dates with... expiration dates expiration dates Problems Bijection Violation Unexpected Behavior Small Objects Missing Primitive Obsession Bijection Violation Bijection Unexpected Behavior Small Objects Missing Small Objects Primitive Obsession Primitive Obsession Solutions Honor the bijection and model correct behavior Honor the bijection and model correct behavior Context In many systems, the expiry date of a credit card is often represented by simply using a Date object. This can lead to potential issues and misunderstandings, especially when dealing with operations such as comparisons and calculations based on the expiry date. It's generally considered a better practice to represent the expiration date with an adequate object. Sample Code Wrong import java.util.Date;

public class CreditCard {
    private String cardNumber;
    private Date expiryDate;

    public CreditCard(String cardNumber, Date expiryDate) {
        // Not a complete date
        this.cardNumber = cardNumber;
        this.expiryDate = expiryDate;
    }

    public boolean isExpired() {
        Date currentDate = new Date();
        return expiryDate.before(currentDate);
        // How will it work? 
        // using the last day of the month?
    }
} import java.util.Date;

public class CreditCard {
    private String cardNumber;
    private Date expiryDate;

    public CreditCard(String cardNumber, Date expiryDate) {
        // Not a complete date
        this.cardNumber = cardNumber;
        this.expiryDate = expiryDate;
    }

    public boolean isExpired() {
        Date currentDate = new Date();
        return expiryDate.before(currentDate);
        // How will it work? 
        // using the last day of the month?
    }
} public class CreditCard {
    private String cardNumber;
    private int expiryMonth;
    private int expiryYear;

    public CreditCard(String cardNumber, int expiryMonth, int expiryYear) {
        this.cardNumber = cardNumber;
        this.expiryMonth = expiryMonth;
        this.expiryYear = expiryYear;
        // No validations on number ranges?
    }

    public boolean isExpired(int currentMonth, int currentYear) {
        return (expiryYear < currentYear) ||
            (expiryYear == currentYear && expiryMonth < currentMonth);
    }
    // Inappropriate intimacy code smell
 
} public class CreditCard {
    private String cardNumber;
    private int expiryMonth;
    private int expiryYear;

    public CreditCard(String cardNumber, int expiryMonth, int expiryYear) {
        this.cardNumber = cardNumber;
        this.expiryMonth = expiryMonth;
        this.expiryYear = expiryYear;
        // No validations on number ranges?
    }

    public boolean isExpired(int currentMonth, int currentYear) {
        return (expiryYear < currentYear) ||
            (expiryYear == currentYear && expiryMonth < currentMonth);
    }
    // Inappropriate intimacy code smell
 
} Right class CreditCard {
  private String number;
  private MonthOfYear expiryDate;
  // expiryDate is the role
  // MonthOfYear is the type
}

class MonthOfYear {
  private Month month;  
  private Year year;
  // These are other small objects

  public MonthOfYear(Month month, Year year) {
    // You don't need to add validations since 
    // month is a valid month
    // year is a valid year
    this.month = month;
    this.year = year;
  } 
  
 public boolean isBeforeEndOfMonth(Date date) {
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(date);
    return (calendar.get(Calendar.YEAR) < year.value()) ||
           (calendar.get(Calendar.YEAR) == year.value() &&
            calendar.get(Calendar.MONTH) < month.value())      
    // Notice there are no days involved   
  }
  
  // This protocol is just for MonthOfYears  
  public Day[] getDaysInMonth() { }
  
  public boolean isLeapYear() { } // ...
  
  public void iterateDays() { } // ...
  
} class CreditCard {
  private String number;
  private MonthOfYear expiryDate;
  // expiryDate is the role
  // MonthOfYear is the type
}

class MonthOfYear {
  private Month month;  
  private Year year;
  // These are other small objects

  public MonthOfYear(Month month, Year year) {
    // You don't need to add validations since 
    // month is a valid month
    // year is a valid year
    this.month = month;
    this.year = year;
  } 
  
 public boolean isBeforeEndOfMonth(Date date) {
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(date);
    return (calendar.get(Calendar.YEAR) < year.value()) ||
           (calendar.get(Calendar.YEAR) == year.value() &&
            calendar.get(Calendar.MONTH) < month.value())      
    // Notice there are no days involved   
  }
  
  // This protocol is just for MonthOfYears  
  public Day[] getDaysInMonth() { }
  
  public boolean isLeapYear() { } // ...
  
  public void iterateDays() { } // ...
  
} Detection [x]Manual [x] Manual [x] This is a design smell Tags Primitive Obsession Primitive Obsession Level [x]Intermediate [x] Intermediate [x] This is not an obvious design decision AI Generation Actual AI assistants are not very good at creating these kinds of small objects AI Detection Gemini detected this is a possible smell Conclusion ExpiryDate explicitly separates month and year, making the code more readable and making it easier to understand the specific information needed for expiry. It can also encapsulate logic specific to expiry dates, such as calculating days remaining, validating expiry, or iterating through days in the month. While Date objects offer date manipulation functionalities, they don't inherently represent the specific concept of a credit card expiry. Finding small objects with concrete behavior is always a difficult task. Relations Code Smell 122 - Primitive Obsession Code Smell 122 - Primitive Obsession Code Smell 177 - Missing Small Objects Code Smell 177 - Missing Small Objects More Info The One and Only Software Design Principle The One and Only Software Design Principle Disclaimer Code Smells are my opinion . opinion Credits Photo by CardMapr.nl on Unsplash CardMapr.nl Unsplash Thank you, Hernan Wilkinson, for this tip Hernan Wilkinson, Standards are always out of date. That’s what makes them standards. Standards are always out of date. That’s what makes them standards. Alan Bennett Alan Bennett https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true This article is part of the CodeSmell Series. How to Find the Stinky Parts of your Code How to Find the Stinky Parts of your Code Also published here. Also published here . here

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