Time-independent JUnit Test Cases

Question

I have a Util class with 1 method which calculates age using joda-time:

public static int getAge(LocalDate birthdate) {
    LocalDate today = new LocalDate();
    Period period = new Period(birthdate, today, PeriodType.yearMonthDay());
    return period.getYears();
}

I've written an extensive JUnit test class but how do I make it time-independent? If I create a test case to calculate someone's age if they were born today in 1970 then the result would be 46 years old. But 1 year from now if i run the test case it would fail because the result would be 47.

So how do I make these test cases time-independent. I was thinking of having some sort of calendar interface that the test cases would create a date object from. I also stumbled across this post which is another possible solution to this but I'm not really sure how to go about it.

Answer 1

A common practice (described in the book Growing Object Oriented Software guided by Tests) is to delegate the responsability of creating the date instance to a collaborator.

Imagine you have a class like this:

public class Clock {
  public LocaleDate now(){
    return  new LocalDate();
  }
}

then your code could become

public static int getAge(LocalDate birthdate, Clock clock) {
    LocalDate today = clock.now();
    Period period = new Period(birthdate, today, PeriodType.yearMonthDay());
    return period.getYears();
}

I would be fine not testing Clock#now as it is trivially correct, but I can now pass in a mocked clock instance to test the getAge method.

Now this means a class using the static getAge must provide a clock, the clock would be a dependency for this class. Such dependencies can be injected in various mockable ways.

I personnally am of the injection by constructor school, but anything from injection by reflection to proctected factory method (which can therefore be overloaded in an anonymous subclass used to create a test instance) works.

This problem and the design I suggest are discussed at length in the book, in many blog posts and in stack overflow (it looks like java 8 has direct support for it if you are willing to move to java.time)

Answer 2

Use fixed date :

    LocalDate today = new LocalDate(2016, 1, 5);