when I run mockito test occurs WrongTypeOfReturnValue Exception

Question

According to https://groups.google.com/forum/?fromgroups#!topic/mockito/9WUvkhZUy90, you should rephrase your

when(bar.getFoo()).thenReturn(fooBar)

to

doReturn(fooBar).when(bar).getFoo()

Another reason for similar error message is trying to mock a final method. One shouldn't attempt to mock final methods (see Final method mocking).

I have also confronted the error in a multi-threaded test. Answer by gna worked in that case.

Very interested problem. In my case this problem was caused when I tried to debug my tests on this similar line:

Boolean fooBar;
when(bar.getFoo()).thenReturn(fooBar);

The important note is that the tests were running correctly without debugging.

In any way, when I replaced above code with below code snippet then I was able to debug the problem line without problems.

doReturn(fooBar).when(bar).getFoo();

For me this meant I was running this:

a = Mockito.mock(SomeClass.class);
b = new RealClass();
when(b.method1(a)).thenReturn(c); 
// within this method1, it calls param1.method2() -- note, b is not a spy or mock

So what was happening is that mockito was detecting that a.method2() was being called, and telling me I couldn't return c from a.method2() which is wrong.

Fix: use the doReturn(c).when(b).method1(a) style syntax (instead of when(b.method1(a)).thenReturn(c);), which will help you discover the hidden bug more concisely and quickly.

Or in this particular case, after doing that it started showing the more accurate "NotAMockException", and I changed it to not longer try to set a return value from a non-mock object.

I recently had this issue. The problem was that the method I was trying to mock had no access modifier. Adding public solved the problem.

I had this error because in my test I had two expectations, one on a mock and one on concrete type

MyClass cls = new MyClass();
MyClass cls2 = Mockito.mock(Myclass.class);
when(foo.bar(cls)).thenReturn(); // cls is not actually a mock
when(foo.baz(cls2)).thenReturn();

I fixed it by changing cls to be a mock as well

In my case the problem was caused by trying to mock a static method and forgetting to call mockStatic on the class. Also I forgot to include the class into the @PrepareForTest()