List<String> mockedList = Mockito.mock(List.class);
Mockito.when(mockedList.size()).thenReturn(10);
mockedList.add("item");
Mockito.verify(mockedList).add("item");
<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-core</artifactId>
<version>2.0.0</version>
<scope>test</scope>
</dependency>
