How to mock a certain method while doing junit?

Question

Below is my method which gives me the date of either recent Monday or recent Thursday in the format of YYYYMMDD.

Today is Saturday, so it should return date for Thursday in the format of YYYYMMDD so it will be - 2014027

public static String getDate() {

    Calendar cal = Calendar.getInstance();

    try {
        int dow = cal.get(Calendar.DAY_OF_WEEK);
        switch (dow) {
        case Calendar.THURSDAY:
        case Calendar.FRIDAY:
        case Calendar.SATURDAY:
        case Calendar.SUNDAY:
        while (cal.get(Calendar.DAY_OF_WEEK) != Calendar.THURSDAY) {
            cal.add(Calendar.DATE, -1);
        }
        break;
        case Calendar.MONDAY:
        case Calendar.TUESDAY:
        case Calendar.WEDNESDAY:
        while (cal.get(Calendar.DAY_OF_WEEK) != Calendar.MONDAY) {
            cal.add(Calendar.DATE, -1);
        }
        break;
        }
    } catch (Exception ex) {
        // log error
    }

    return toDateFormat.format(cal.getTime());
}

So my question is, how do I mock the Calendar getInstance method so that I can easily junit getDate method? I have never mocked it before so I am facing some problem?

Can anyone provide a simple example if possible on my example?

Answer 1

If this was my code I would extract the Calendar variable into another method, overloading the method so that there is one version that takes the Calendar as a parameter and one version with no arguments, like this:

public static String getDate() {
    return getDate(Calendar.getInstance());
}

public static String getDate(Calendar cal} {
// your example code
}

This way you can write tests exercising your logic in getDate without having to use any mocks. If you really want to cover the no-args version of the method then you can use PowerMock to mock Calendar's getInstance method (here's a question with examples of that).