Stubbing methods that return values

In this recipe, we will stub a method that returns a value so that it returns our desired result.

Getting ready

For this recipe, our system under test will be AverageTaxFactorCalculator along with the TaxFactorFetcher class. Together, they form a unit whose purpose is to calculate the average factor. The TaxFactorFetcher class is called twice: once to get a tax factor from DB and once to get a tax factor for a given person. Then, it calculates an average out of those values. Have a look at the following code:

public class AverageTaxFactorCalculator {

    private final TaxFactorFetcher taxFactorFetcher;

    public AverageTaxFactorCalculator(TaxFactorFetcher taxFactorFetcher) {
        this.taxFactorFetcher = taxFactorFetcher;
    }

    public double calculateAvgTaxFactorFor(Person person) {
        double taxFactor = taxFactorFetcher.getTaxFactorFromDb(person);
        double anotherTaxFactor = taxFactorFetcher.getTaxFactorFor(person);
        return (taxFactor...