Mockito doAnswer Vs thenReturn

Question

I am using Mockito for service later unit testing  I am confused when to use doAnswer vs thenReturn    Can anyone help me in detail  So far  I have tried it with thenReturn

User · Answer

You should use thenReturn or doReturn when you know the return value at the time you mock a method call  This defined value is returned when you invoke the mocked method      thenReturn T value  Sets a return value to be returned when the method is called       Test public void test return   throws Exception       Dummy dummy   mock Dummy class       int returnValue   5          choose your preferred way     when dummy stringLength  dummy    thenReturn returnValue       doReturn returnValue  when dummy  stringLength  dummy        Answer is used when you need to do additional actions when a mocked method is invoked  e g  when you need to compute the return value based on the parameters of this method call      Use doAnswer   when you want to stub a void method with generic Answer       Answer specifies an action that is executed and a return value that is returned when you interact with the mock     Test public void test answer   throws Exception       Dummy dummy   mock Dummy class       Answer lt Integer gt  answer   new Answer lt Integer gt              public Integer answer InvocationOnMock invocation  throws Throwable               String string   invocation getArgumentAt 0  String class               return string length     2                           choose your preferred way     when dummy stringLength  dummy    thenAnswer answer       doAnswer answer  when dummy  stringLength  dummy

User · Answer

doAnswer and thenReturn do the same thing if    You are using Mock  not Spy The method you re stubbing is returning a value  not a void method    Let s mock this BookService  public interface BookService       String getAuthor        void queryBookTitle BookServiceCallback callback       You can stub getAuthor   using doAnswer and thenReturn   BookService service   mock BookService class   when service getAuthor    thenReturn  Joshua       or   doAnswer new Answer          Override     public Object answer InvocationOnMock invocation  throws Throwable           return  Joshua            when service  getAuthor      Note that when using doAnswer  you can t pass a method on when      Will throw UnfinishedStubbingException doAnswer invocation - gt   Joshua   when service getAuthor       So  when would you use doAnswer instead of thenReturn  I can think of two use cases    When you want to  stub  void method    Using doAnswer you can do some additionals actions upon method invocation  For example  trigger a callback on queryBookTitle   BookServiceCallback callback   new BookServiceCallback          Override     public void onSuccess String bookTitle            assertEquals  Effective Java   bookTitle            doAnswer new Answer          Override     public Object answer InvocationOnMock invocation  throws Throwable           BookServiceCallback callback    BookServiceCallback  invocation getArguments   0           callback onSuccess  Effective Java               return null because queryBookTitle is void         return null           when service  queryBookTitle callback   service queryBookTitle callback      When you are using Spy instead of Mock   When using when-thenReturn on Spy Mockito will call real method and then stub your answer  This can cause a problem if you don t want to call real method  like in this sample   List list   new LinkedList    List spy   spy list      Will throw java lang IndexOutOfBoundsException  Index  0  Size  0 when spy get 0   thenReturn  java    assertEquals  java   spy get 0      Using doAnswer we can stub it safely   List list   new LinkedList    List spy   spy list   doAnswer invocation - gt   java   when spy  get 0   assertEquals  java   spy get 0      Actually  if you don t want to do additional actions upon method invocation  you can just use doReturn   List list   new LinkedList    List spy   spy list   doReturn  java   when spy  get 0   assertEquals  java   spy get 0

[java] Mockito : doAnswer Vs thenReturn

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