Unit testing a class with a Java 8 Clock
java, java-8, java-time, unit-testing
Solution
I don't want to add a Clock argument to the method because the real code shouldn't be concerned with passing in a clock.
No... but you might want to consider it as a constructor parameter. Basically you're saying that your class needs a clock with which to work... so that's a dependency. Treat it as you would any other dependency, and inject it either in a constructor or via a method. (I personally favour constructor injection, but YMMV.)
As soon as you stop thinking of it as something you can easily construct yourself, and start thinking of it as "just another dependency" then you can use familiar techniques. (I'm assuming you're comfortable with dependency injection in general, admittedly.)
Problem
Java 8 introduced `java.time.Clock` which can be used as an argument to many other `java.time` objects, allowing you to inject a real or fake clock into them. For example, I know you can create a `Clock.fixed()` and then call `Instant.now(clock)` and it will return the fixed `Instant` you provided. This sounds perfect for unit testing! However, I'm having trouble figuring out how best to use this. I have a class, similar to the following: ``` public class MyClass { private Clock clock = Clock.systemUTC(); public void method1() { Instant now = Instant.now(clock); // Do something with 'now' } } ``` Now, I want to unit test this code. I need to be able to set `clock` to produce fixed times so that I can test `method()` at different times. Clearly, I could use reflection to set the `clock` member to specific values, but it would be nice if I didn't have to resort to reflection. I could create a public `setClock()` method, but that feels wrong. I don't want to add a `Clock` argument to the method because the real code shouldn't be concerned with passing in a clock. What is the best approach for handling this? This is new code so I could reorganize the class. Edit: To clarify, I need to be able to construct a single `MyClass` object but be able to have that one object see two different clock values (as if it were a regular system clock ticking along). As such, I cannot pass a fixed clock into the constructor.