Question

https://stackoverflow.com/questions/63438057/using-mokito-when-for-method-with-string-and-classt-arguments

Answer

The problem is caused by mixing mocking with and without argument matchers. If you use an argument matcher to one of the mocked method arguments, you have to use a matcher for all of them. You can read more here.

I've created a simple project on GitHub with a solution - you can check it if you like, but here's the snippet:

@Test
void withoutEq() {
    DynamicProperties dynamicProperties = mock(DynamicProperties.class);
    when(dynamicProperties.getEnvironmentProperty(KEY_A, Boolean.class))
            .thenReturn(true);
    when(dynamicProperties.getEnvironmentProperty(KEY_B, Long.class))
            .thenReturn(1L);

    assertAll(
            () -> assertTrue(dynamicProperties.getEnvironmentProperty(KEY_A, Boolean.class)),
            () -> assertEquals(1, dynamicProperties.getEnvironmentProperty(KEY_B, Long.class))
    );
}

@Test
void withEq() {
    DynamicProperties dynamicProperties = mock(DynamicProperties.class);
    when(dynamicProperties.getEnvironmentProperty(eq(KEY_A), eq(Boolean.class)))
            .thenReturn(true);
    when(dynamicProperties.getEnvironmentProperty(eq(KEY_B), eq(Long.class)))
            .thenReturn(1L);

    assertAll(
            () -> assertTrue(dynamicProperties.getEnvironmentProperty(KEY_A, Boolean.class)),
            () -> assertEquals(1, dynamicProperties.getEnvironmentProperty(KEY_B, Long.class))
    );
}

As you can see - one of the methods uses eq matcher for both of the method's arguments and the other one uses none. Both tests pass.

In your case

when(dynamicProperties.getEnvironmentProperty(eq(KEY_A), anyObject())).thenReturn(true);

this mock did not cause problems, because eq and anyObject are both argument matchers, so there was no error, but

when(dynamicProperties.getEnvironmentProperty(eq(KEY_A), Boolean.class)).thenReturn(true);

has a matcher (eq) and a simple object (Boolean.class without eq).