How to write mock test for method with logger?

I have the following interface:

public interface SomeProcessor<T> {
void process(Stream<T> someStream);
default <R> R someMap(Function<T, R> function,
T message,
Logger log) {
try {
return function.apply(message);
} catch (Exception exception) {
log.error("Mapping error for message => {}", message, exception);
return null;
}
}
}

And want to cover log part of the method by mock test but unfortunately it always fails.
Now I have the following test:

@ExtendWith(MockitoExtension.class)
class SomeProcessorTest<T> implements SomeProcessor<T> {
@Mock
private Logger mockLogger;
@Override
public void process(Stream<T> someStream) {
}
@Override
public <R> R someMap(Function<T, R> function, T message, Logger log) {
try {
return function.apply(message);
} catch (Exception exception) {
log.error("Mapping error for message => {}", message, exception);
return null;
}
}
@Test
void testWithException() {
SomeProcessor<String> processor = new SomeProcessor <String>() {
@Override
public void process(Stream<String> someStream) {
}
@Override
public <R> R someMap(Function<String, R> function, String message, Logger log) {
throw new RuntimeException("test exception");
}
};
Function<String, Integer> function = Integer::parseInt;
String message = "123";
assertThrows(RuntimeException.class, () -> processor.someMap(function, message, mockLogger));
ArgumentCaptor<String> someCaptor = ArgumentCaptor.forClass(String.class);
ArgumentCaptor<Throwable> exceptionCaptor = ArgumentCaptor.forClass(Throwable.class);
verify(mockLogger).error(eq("Mapping error for message => {}"), eq(message), any(RuntimeException.class));
assertEquals("Mapping error for message => {}", someCaptor.getValue());
assertTrue(exceptionCaptor.getValue() instanceof RuntimeException);
}}

But I always see errors related to Wanted but not invoked: mockLogger.error...
and I cannot cover the following lines in my interface

catch (Exception exception) {
log.error("Mapping error for message => {}", message, exception);
return null;

What should I change im my mock test to cover these lines and not catching an error?
Give me a piece of advice please.

There seems to be a lot of mixed up things in your code and the rationale behind it is not clear at first glance. The problem with mockLogger.error not being called has its source in the fact that you're overriding the default implementation of the someMap method from the interface within the anonymous class inside your test method:

@Override
public <R> R someMap(Function<String, R> function, String message, Logger log) {
    throw new RuntimeException("test exception");
}

Because of that the code you're trying to test is not called at all. Instead, you should call super.someMap(...) from your anonymous class to invoke the actual interface method default implementation.

When you do that though, the assertion of RuntimeException being thrown will fail as the original method consumes the exception and returns null - no exception is thrown.

Removing the assertion of throwing an exception still would not make the test pass, because the function you're testing (Integer::parseInt) should work fine for the message you're providing ("123"), so the catch block will not be executed as no exception is thrown. To solve that you should pass a message value that would cause parsing failure - not a number (e.g. "abc").

Finally - I don't understand why your class implements SomeProcessor<T>, but that seems to be redundant and may be removed.