- One quality measure of an abstraction is the complexity ofits interface (ie API size). - Another measure is the amount of state the abstraction enca

- One quality measure of an abstraction is the complexity ofits interface (ie API size). - Another measure is the amount of state the abstraction encapsulates. The simplest abstraction of many interfaces is two methods: setParameterForNextCall(paramName, paramValue) call(functionName) The API is small, not complex and the amount of state is small, given that parameters are only set right before a call, all parameters are cleared after every call and no other state is retained. Yet it's clear to anyone that this isn't a good abstraction and that it doesn't result in an implementation that requires few unit tests. It requires an immense amount of documentation and allows a great many variations that yield bad/unexpected results. Hence, good abstractions require few unit tests. Abstractions don't have unit tests: implementations have unit tests. It's the size and complexity of the implementation made possible by the abstraction that matters.