Mocks and stubs are both used in testing to replace real objects. The main differences between mocks and stubs are:

Mocks:

• Are used to verify interactions with an object. For example, you can assert that a method was called a certain number of times, with certain arguments, in a certain order, etc.

• Have expectations set on them to verify the interactions. For example:

mock_object.expects(:message).with('Hello').returns('Hi')

• Raises an error if it receives an unexpected message.

Stubs:

• Are used simply to replace an object with a hardcoded response.

• Do not care about the messages they receive or verify interactions. They just return a stubbed response.

• Do not raise errors on unexpected messages.

So in summary:

Mocks:

• Set expectations

• Verify interactions

• Raise errors if those expectations are not met

Stubs:

• Simply provide stubbed responses

• Do not verify interactions

• Do not raise errors on unexpected messages

Some examples to illustrate the difference:

Mock:

mock_object = mock('name')
mock_object.expects(:message).with('Hello').returns('Hi')
mock_object.message('Hello')  # Returns 'Hi'
mock_object.message('Goodbye') # Raises error - unexpected message

Stub:

stub_object = stub('name') 
stub_object.stubs(:message).returns('Hi')
stub_object.message('Hello') # Returns 'Hi'
stub_object.message('Goodbye') # Also returns 'Hi' - stub doesn't care

cheers!