@Test
  void checkNotNullInputZeroNotNull0OutputZero() {
    // Arrange
    final Object reference = 0;
    final String errorMessageTemplate = "   ";
    final Object[] errorMessageArgs = {};
    // Act
    final Object retval = Util.checkNotNull(reference, errorMessageTemplate, errorMessageArgs);
    // Assert result
    assertThat(retval).isEqualTo(0);
  }
Ex. your {@code Foo} object would be null, but so would things like Number.
core/src/test/java/feign/UtilTest.java