Using Matchers.any() to match a mock object

Foo mockFoo1 = mock(Foo.class);
Foo mockFoo2 = mock(Foo.class);
when(((Foo) any()).someMethod()).thenReturn("Hello");

In the above sample code, line 3 fails with a NullPointerException. Why so?

My thought on this:

EITHER.. any() should be used for matching parameters rather than matching the objects on which methods are triggered.

OR .. any() works only for real concrete objects and not mock objects.

You need to do:

Foo mockFoo1 = mock(Foo.class);
Foo mockFoo2 = mock(Foo.class);
when(mockFoo1).someMethod().thenReturn("Hello");
when(mockFoo2).someMethod().thenReturn("Hello");

any() (shorter alias to anyObject()) is an Mockito argument matcher that matches any argument and only should be used as follows:

when(mockFoo1.someMethod(any())).thenReturn("Hello");

any() returns null, so your code was equivalent to

when(((Foo) null).someMethod()).thenReturn("Hello");