Question

I have to write a program that takes an Integer and converts it into its English word version.

For example:

Input:  21
Output: twenty one
Input:  110 
Output: one hundred and ten

I need the program to demonstrate TDD so I want to use mocking.

I have written a class that has a function that does the conversion (based on 2 arrays of English words). What I need now to design the program in such a way that I can demonstrate with Easymock.

Therefore, I need create an interface to be the subject of my mock. Can anyone give me any pointers on how I would design my program?

Would this be suitable?

1. Write a Converter class that has a reference to an interface called ConverterInterface. I could then mock the interface and set it into my Converter class.

Any help is welcome.

Answer 1

You can demonstrate TDD without mocking. In fact, mocking can confuse people that are new to TDD. I would simply start by test driving the functionality you are trying to develop, and then worry about mocking later. Let’s assume you’ve test driven the numeric to English converter (which it doesn’t appear you have done yet based on your description) and you have a class that looks something like this:

public class NumberConverterTest { ... }

public class NumberConverter {
  public String toEnglish(int number) { ... }
}

You are likely to also have some sort of main class:

public class NumberConverterMain {
  public void main(String[] args) {
    NumberConverter converter = new NumberConverter();
    System.out.println(converter.toEnglish(args[0]);
  }
}

You have now demonstrated TDD without mocking. In trying to practice TDD the mocking question will invariably come up. In order to demonstrate mocking, you can add some arbitrary business rule around the toEnglish method such as “All requests for numbers larger than 1000 must be logged to the large number department”. Knowing that the large number department is hosted on another server that we don’t want to have a dependency on we can test drive the interface and mock it out.

public class NumberConverterTest {
  // ..
  @Test public void theLargeNumberDepartmentIsNotifiedForLargeNumbers() {
     LargeNumberDepartment department = new MockLargeNumberDepartment(1000);
     NumberConverter converter = new NumberConverter(department);
     converter.toEnglish(1000);
     assertTrue(department.wasNotifiedWith(1000));
  }

  public static MockLargeNumberDepartment implements LargeNumberDepartment {

     private int valueRequested;

     public void MockLargeNumberDepartment(int threshhold) {
       this.threshold = threshold;
     }

     public int notificationThreshold() {
       return this.threshold;
     }

     public void largeNumberReceived(int value) {
       valueRequested = value;
     }

     public boolean wasNotifiedWith(int value) {
       assertEquals(value, valueRequested);
       return true;
     }
  }
}

// In NumberConverter.java
public class NumberConverter {
  public NumberConverter(LargeNumberDepartment department) {
    this.department = department;
  }

  public String toEnglish(int value) {
    if(value > department.notificationThreshold())
      department.largeNumberReceived(value);
    return convertIt(value);  
  }
}