THENRETURN and doReturn are used for setting up stubs and are provided by the Mockito framework. They are generally used along with when clause as --> when-thenReturn and doReturn-when. when-thenReturn is used in most cases due to better readability. 

Following are the differences between thenReturn and doReturn:

• Type safety:
  doReturn takes OBJECT parameter, unlike thenReturn. Hence there is no type CHECK in doReturn at compile time. In the case of thenReturn, whenever the type mismatches during RUNTIME, the WrongTypeOfReturnValue exception is raised. This is why when-thenReturn is a better option whenever we know the type.
• No side-effect:
  SINCE doReturn does not have type safety, there are no side effects when it is being used. Whenever thenReturn is used, if the type mismatch occurs, an exception is thrown which might result in the abortion of test cases.