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Prefer Testing Public APIs Over Implementation-Detail Classes

2015-01-14

Public APIs can be called by many users. Implementation details are only called by public APIs. If the public APIs are well tested, as they should be, then the implementation details will get tested by association.

Heavy testing against implementation details can cause a couple problems:

  • Unlike public APIs, implementation details are vulnerable to refactoring. Tests for implementation details can fail even though the behavior from the public API is fine.
  • Testing implementation details can give false confidence. Even if an implementation detail is well tested, that doesn't mean the pubic API behaves properly.