Mock Objects

What are Mock Objects

§Any dummy object that stands in for a real object that is not available, or is difficult to use in a test case

§(More rigid): A mock object must have the ability to set up expectations and provide a self-validation mechanism

Why use Mock Objects?

§Help keep design decoupled

§Check code’s usage of another object

§Test code from the inside out

§Make tests run faster

§Make it easier to write code for problematic resources

§Defer having to write a class

§Test components in isolation from the rest of the system

§Promote interface-based design

§Encourage composition over inheritance

§Refine interfaces

§Test unusual, unlikely, and exceptional situations

When to use Mock Objects

§Non-deterministic or random behavior

§Difficult setup

§Behavior hard to cause

§Slow real objects

§UIs

§Test needs to make queries not supported by real object

§Non-existent real objects

How to use Mock Objects

§Create them by hand from scratch

§Create them by hand, using the MockObjects library

§Use MockMaker to generate the mock object prior to execution

§Use EasyMock to generate the mock object at runtime

By hand from scratch

§Create a class that implements the same interface as the real one

§Hard code return values

§Lots of work to create and validate expectations

By hand with MockObjects library

§Create the mocks

§Set state/expectations

§Execute code

§Have mocks verify expectations

§Lots of duplication across Mocks

MockMaker

§Given an interface or class, create a mock object for it

§Mock can then be tweaked

EasyMock

§Create mock object at runtime

§Two modes: record and replay

EasyMock adjustments

§Make the following adjustments to EasyMock example code in the book:

§EasyMock.niceControlFor should be MockControl.createControl

§control.activate should be control.replay

EasyMock Matcher

§Easymock by default uses Object.equals() type comparison

§For arrays, this doesn’t look at content

§Solution: the first time you call a method that takes an array as a parameter, set the matcher before setting return value