Quiz yourself: Declaring and accessing modules (intermediate)

July 20, 2020 | Download a PDF of this article
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If you have worked on our quiz questions in the past, you know none of them is easy. They model the difficult questions from certification examinations. The “intermediate” and “advanced” designations refer to the exams rather than to the questions, although in almost all cases, “advanced” questions will be harder. We write questions for the certification exams, and we intend that the same rules apply: Take words at their face value and trust that the questions are not intended to deceive you but to straightforwardly test your knowledge of the ins and outs of the language.

This intermediate-level Java SE 11 quiz tests your ability to declare modules and enable access between modules, especially when it comes to the java.base module.

Imagine you are developing a Java Date/Time manipulation framework. The framework module is named date.time.utils and contains classes in the com.acme.utils package. The Java Date/Time API is located in the java.time package, which is in the java.base module.

Which of the following steps is mandatory to properly define your module for your clients? Choose one.

1. Add the following to the date.time.utils module descriptor:
   requires transitive java.base;.
   
2. Add the following to the date.time.utils module descriptor:
   requires java.base;.
   Also inform your clients to add the following in their modules:
   requires java.base;.
   
3. Add the following to the date.time.utils module descriptor:
   exports date.time.utils;.
   
4. Add the following to the date.time.utils module descriptor:
   exports com.acme.utils;.
   

Answer. The question provides the required information that the Java Date/Time API belongs to the java.base module. You should know for the exam that the java.base module is always implicitly required by any other module.

The question further investigates how a module makes elements of itself available for other modules to use. In particular, it’s important to know that elements declared in a module are effectively hidden inside that module unless they are deliberately and expressly made accessible to clients of the module.

Option A suggests adding a requires transitive dependency on the java.base module. This directive would declare that the date.time.utils module, and any module using it, requires java.base. However, java.base is always implicitly required for all modules.

The requires transitive directive also states that clients of this module automatically have access to the exported features of the module required by this module. But again, since all modules have implicit access to java.base, this directive has no value in this case.

In addition to the lack of effect of the suggested directive, nothing in option A actually grants clients of this module the chance to use anything inside the module. Because of this, option A is incorrect.

Option B is very similar to option A, only differing in the absence of the transitive modifier and the admonition that users of this library should expressly add a dependency on java.base. Since java.base is always implicitly required, both actions listed here are without effect. Further, neither action does anything to grant access to elements of the module to users, so option B is also incorrect.

Option C suggests adding an exports directive, but that directive appears to refer to a module name. The compiler treats what follows the exports keyword as a package. It is an error to try to export a package that does not exist, and since no package in this example has the same name as that module name, option C is incorrect.

Option D correctly describes the only required step for the described scenario and in general for any module whose classes are expected to be used outside, which is to add one or more exports directives that specify a package that should be accessible to external clients of the module. The simplest module definition for this question’s scenario will follow the guidance provided by option D and will look like this:

module date.time.utils {
    exports com.acme.utils ;
}

The correct answer is option D.