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+## Test doubles
+
+Meszaros uses the term Test Double as the generic term for any kind of
+pretend object used in place of a real object for testing purposes. Meszaros
+then defined four particular kinds of double:
+
+* **Dummy** objects are passed around but never actually used. Usually they
+  are just used to fill parameter lists.
+* **Fake** objects actually have working implementations, but usually take
+  some shortcut which makes them not suitable for production (an in memory
+  database is a good example).
+* **Stubs** provide canned answers to calls made during the test, usually not
+  responding at all to anything outside what's programmed in for the test.
+  Stubs may also record information about calls, such as an email gateway
+  stub that remembers the messages it 'sent', or maybe only how many messages
+  it 'sent'.
+* **Mocks** are what we are talking about here: objects pre-programmed with
+  expectations which form a specification of the calls they are expected to
+  receive.
+
+
+## Mocks vs. Stubs
+
+* A stub uses state verification while a mock uses behavior verification.
+* In order to use state verification on the stub, we need to make some extra
+  methods on the stub to help with verification (e.g. add
+  a `MailServiceStub.numberSent()` method)
+* Mock objects always use behavior verification, a stub can go either way.
+
+---
+
+**Reference:** https://www.martinfowler.com/articles/mocksArentStubs.html