java: how to mock Calendar.getInstance()?

Question

In my code I have something like this:

private void doSomething() {
   Calendar today = Calendar.getInstance();
   ....
}

How can I \"mock\

Answer 1

Don't mock it - instead introduce a method you can mock that gets dates. Something like this:

interface Utility {

    Date getDate();
}

Utilities implements Utility {


    public Date getDate() {

        return Calendar.getInstance().getTime();
    }

}

Then you can inject this into your class or just use a helper class with a bunch of static methods with a load method for the interface:

public class AppUtil {

    private static Utility util = new Utilities();

    public static void load(Utility newUtil) {

         this.util = newUtil;
    }

    public static Date getDate() {

        return util.getDate();
    }

}

Then in your application code:

private void doSomething() {
   Date today = AppUtil.getDate();
   ....
}

You can then just load a mock interface in your test methods.

@Test
public void shouldDoSomethingUseful() {
     Utility mockUtility = // .. create mock here
     AppUtil.load(mockUtility);

     // .. set up your expectations

     // exercise the functionality
     classUnderTest.doSomethingViaAPI();

     // ... maybe assert something 

}

See also Should you only mock types you own? and Test smell - everything is mocked