java-mockito

repositories { jcenter() }
dependencies { testCompile "org.mockito:mockito-core:2.+" }

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>2.13.0</version>
</dependency>

import static org.mockito.Mockito.*;
// mock creation
List mockedList = mock(List.class);
// using mock object - it does not throw any "unexpected interaction" exception
mockedList.add("one");
mockedList.clear();
// selective, explicit, highly readable verification
verify(mockedList).add("one");
verify(mockedList).clear();

// you can mock concrete classes, not only interfaces
LinkedList mockedList = mock(LinkedList.class);
// stubbing appears before the actual execution
when(mockedList.get(0)).thenReturn("first");
// the following prints "first"
System.out.println(mockedList.get(0));
// the following prints "null" because get(999) was not stubbed
System.out.println(mockedList.get(999));