Mockito: List Matchers with generics

Mockito and Generics: Combining powers

When Generics Met Matchers

Utilize ArgumentMatchers.any() method when working with Java 8 and above. It's more succinct and leverages type inference:

when(mockList.addAll(ArgumentMatchers.<String>any())).thenReturn(true);  // Did you infer that, Sherlock? :)
verify(mockList).addAll(ArgumentMatchers.<String>anyList());
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BONUS: generic types are inferred and remain **strongly typed** without verbose casting. Win-win!

Handing Overload with Grace

Overloaded methods can cause hiccups with Mockito. Put those worries to rest by using generic type matchers to slice through ambiguity:

// When there's more on the menu...
verify(mockedList).add(ArgumentMatchers.<MyClass>any());
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Back to the Future (or Past)

In Java 7 and below, any() doesn't infer types. Use Matchers.anyListOf(Class<T> clazz) to maintain the peace (and functionality):

when(mockList.addAll(Matchers.<String>anyListOf(String.class))).thenReturn(true); 
verify(mockList).addAll(Matchers.<String>anyListOf(String.class));  // Smells like legacy code
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In Raw We Trust (not really)

Avoid raw types to guard type checks. But when you must deal with them, arm yourself with the correct casting spells:

when(mockList.addAll((List) any())).thenReturn(true);  // The Wild West of Generics
verify(mockList).addAll((List)Matchers.anyList());
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ArgumentMatchers: The Swiss Knife

When wrestling with method parameters, ArgumentMatchers API is your lifesaver providing a pragmatic way to articulate the intended type and simplify your life using static imports:

import static org.mockito.ArgumentMatchers.*;  // Now you see me...

when(mockList.addAll(anyList())).thenReturn(true);
verify(mockList).addAll(anyList());  // Now you don't
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Diving deeper: Advanced Mocking Strategies

Pinpoint Matching with Nested Generics

For as complex as it gets scenarios, say nested generics, go for the Matchers API with fully blown explicit generics:

when(mockSomeObject.someMethod(Matchers.<List<List<String>>>any())).thenReturn(someResult);  // You had me at nested generics!
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Capturing the Buttery Goodness with ArgumentCaptor

Get the actual values passed to your mock using ArgumentCaptor, adding another tool in your testing kit:

ArgumentCaptor<List<String>> captor = ArgumentCaptor.forClass((Class) List.class);  // Captor, I choose you!
verify(mockList).addAll(captor.capture());
assertTrue(captor.getValue().containsAll(expectedList));  // Mind reader in action
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