Mockito is a widely used Java library for creating mock objects in unit tests. Mocking enables us to replace the behaviour of an object or class with a simulated version, making it easier to test how our code interacts with other components. However, when working with generic lists, we may encounter unchecked warnings.
This article explores using Mockito to mock list matchers with generics effectively and avoid compiler warnings.
1. How Is Mockito Set Up?
To use Mockito in a Maven project, add the following dependency to your pom.xml:
1 2 3 4 5 6 | <dependency> <groupId>org.mockito</groupId> <artifactId>mockito-core</artifactId> <version>5.16.1</version> <scope>test</scope></dependency> |
2. Reproducing the Compiler Warning
When mocking a method that takes a generic list as an argument, using Matchers.any(List.class) can trigger an unchecked assignment warning because it does not retain type information.
Consider the following example, where we have a PaymentService class responsible for processing a list of transactions:
Code That Triggers the Warning
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 | public class PaymentService { public String summarizeTransactions(List transactions) { return String.join(", ", transactions); }}public class MockitoUncheckedWarningTest { @Test void testUncheckedWarning() { // Create a mock of PaymentService PaymentService mockService = mock(PaymentService.class); // This line triggers an unchecked assignment warning when(mockService.summarizeTransactions(any(List.class))).thenReturn("TXN1, TXN2, TXN3"); assertEquals("TXN1, TXN2, TXN3", mockService.summarizeTransactions(new ArrayList<String>())); }} |
With Maven, we can enable compiler warnings by adding the following plugin configuration to our pom.xml.
01 02 03 04 05 06 07 08 09 10 11 12 13 14 | <build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.13.0</version> <configuration> <compilerArgs> <arg>-Xlint:unchecked</arg> </compilerArgs> </configuration> </plugin> </plugins></build> |
Compiler Warning Output
When we run the test, it passes, but the compiler throws the following warning:
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | [WARNING] /Users/omozegieaziegbe/NetBeansProjects/mockito-generic-list-matchers/src/test/java/com/jcg/examples/MockitoUncheckedWarningTest.java:[27,47] unchecked method invocation: method summarizeTransactions in class com.jcg.examples.PaymentService is applied to given types required: java.util.List<java.lang.String> found: java.util.List[WARNING] /Users/omozegieaziegbe/NetBeansProjects/mockito-generic-list-matchers/src/test/java/com/jcg/examples/MockitoUncheckedWarningTest.java:[27,51] unchecked conversion required: java.util.List<java.lang.String> found: java.util.List[INFO] [INFO] --- surefire:3.2.5:test (default-test) @ mockito-generic-list-matchers ---[INFO] Using auto detected provider org.apache.maven.surefire.junitplatform.JUnitPlatformProvider[INFO] [INFO] -------------------------------------------------------[INFO] T E S T S[INFO] -------------------------------------------------------[INFO] Running com.jcg.examples.MockitoUncheckedWarningTestJava HotSpot(TM) 64-Bit Server VM warning: Sharing is only supported for boot loader classes because bootstrap classpath has been appended[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 8.405 s -- in com.jcg.examples.MockitoUncheckedWarningTest[INFO] [INFO] Results:[INFO] [INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0[INFO] [INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------ |
This warning occurs because ArgumentMatchers.any(List.class) does not retain the generic type <String>, causing the compiler to treat it as List<?>, which leads to an unchecked assignment.
This can be resolved by using more specific matchers that maintain type safety.
3. Correct Approach for Java 8+
In Java 8 and later, the type inference capabilities of the Java compiler are more advanced, allowing for a cleaner way to handle generic list matchers in Mockito. By using anyList(), we can avoid the unchecked warnings and ensure that the type is correctly inferred.
1 2 3 4 5 6 7 | @Testvoid testListMatcherJava8() { PaymentService mockService = mock(PaymentService.class); // Use anyList() to avoid compiler warnings when(mockService.summarizeTransactions(anyList())).thenReturn("TXN1, TXN2, TXN3"); assertEquals("TXN1, TXN2, TXN3", mockService.summarizeTransactions(new ArrayList()));} |
The anyList() matcher safely matches any List<?> without triggering unchecked warnings, ensuring type safety in Mockito tests. In this case, it allows the summarizeTransactions(List<String> transactions) method to be mocked to return "TXN1, TXN2, TXN3" when called. The test then verifies that summarizeTransactions() is correctly invoked with a List<String>.
This eliminates the unchecked warning.
4. Solution for Java 7 and Below
In Java 7 and earlier, generic type inference is less advanced, and using anyList() may still trigger unchecked warnings. To explicitly specify the generic type, use ArgumentMatchers.<String>anyList(), ensuring type safety and avoiding compiler warnings.
1 2 3 4 5 6 7 | @Testpublic void testListMatcherJava7() { PaymentService mockService = mock(PaymentService.class); // Correct approach for Java 7: Use ArgumentMatchers.<String>anyList() when(mockService.summarizeTransactions(ArgumentMatchers.<String>anyList())).thenReturn("TXN1, TXN2, TXN3"); assertEquals("TXN1, TXN2, TXN3", mockService.summarizeTransactions(new ArrayList<String>()));} |
Using ArgumentMatchers.<String>anyList() ensures that the compiler correctly identifies the expected type as List<String>, preventing unchecked assignment warnings while maintaining backward compatibility with Java 7.
5. Conclusion
In this article, we explored how to effectively handle Mockito generic list matchers while avoiding compiler warnings. We discussed the common issue of unchecked warnings when mocking methods that take generic lists as parameters and provided solutions for both Java 7 and Java 8+. In Java 8 and later, anyList() offers a straightforward, type-safe way to mock generic lists, while in Java 7 and below, using ArgumentMatchers.<String>anyList() ensures compatibility and eliminates unchecked assignment warnings. By applying these techniques, we can write cleaner tests in Mockito, ensuring type safety across different versions of Java.
6. Download the Source Code
This article covered Mockito generic list matchers.
You can download the full source code of this example here: mockito generic list matchers
