Chapter 10: Modules

Question 1

What is a module?

Answer 1

A module is a group of one or more packages plus a special file called module-info.java

Question 2

Given a class in a package ‘zoo.animal.feeding’, create a module-info.java file with no body for that module.

Answer 2

file contents:

module zoo.animal.feeding { }

Question 3

Where must the file module-info.java be placed?

Answer 3

In the root directory of the module.

Question 4

Given a module ‘zoo.animal.feeding’ with a file Task.java, in the root directory ‘test’ and a directory ‘mods’ containing custom module files.

Compile the code.

Answer 4

javac -p mods test/zoo/animal/feeding/Task.java test/module-info.java

or

javac –module-path mods test/zoo/animal/feeding/Task.java test/module-info.java

Question 5

Given a module ‘zoo.animal.feeding’ with a file Task.java in the root directory ‘test’ and a directory ‘mods’ containing custom module files.

Compile the code and place the class files in the directory ‘out’.

Answer 5

javac -p mods -d out test/zoo/animal/feeding/Task.java test/module-info.java

or

javac –module-path mods -d out test/zoo/animal/feeding/Task.java test/module-info.java

Question 6

Suppose there is a module named book.module which was compiled in the directory ‘test’. Inside that module is a package named com.sybex, which has a class named OCP with a main() method.

Using the above information, run the module.

Answer 6

java –module-path test –module book.module/com.sybex.OCP

–module-path can be replaced by -p and –module can be replaced by -m

Question 7

Package everything under the ‘test’ directory to the mods directory under the name zoo.animal.feeding

Answer 7

jar -cvf mods/zoo.animal.feeding.jar -C test/ .

Question 8

Given the following module-info.java file, export the zoo.animal.feeding module so that the module will be available for other modules.

module zoo.animal.feeding { }

Answer 8

module zoo.animal.feeding { 
    exports zoo.animal.feeding;
}

Question 9

Given the module zoo.animal.care, create a module-info.java file that exports the package zoo.animal.care.medical and uses the zoo.animal.feeding module.

Answer 9

module zoo.animal.care {
exports zoo.animal.care.medical;
requires zoo.animal.feeding;
}

Question 10

Given the module zoo.animal.care, create a module-info.java file that exports the package zoo.animal.care.medical to only the module zoo.staff.

Answer 10

module zoo.animal.care {
    exports zoo.animal.care.medical to zoo.staff;
}

Question 11

What does the private access control mean with multiple modules?

Answer 11

The private access modifier applied to an element does not allow any outside modules to access the element.

Question 12

What does the default (package-private) access control mean with multiple modules?

Answer 12

The default access modifier applied to an element does not allow any outside modules to access the element.

Question 13

What does the protected access control mean with multiple modules?

Answer 13

Accessible to subclasses only if package is exported

Question 14

What does the public access control mean with multiple modules?

Answer 14

Accessible only if package is exported

Question 15

What does ‘requires transitive’ do?

Answer 15

While ‘requires’ specifies that the current module depends on another module, ‘requires transitive’ specifies that the current module depends on another module plus all the modules that the other module requires.

Question 16

Given the modules zoo.animal.feeding, zoo.animal.talks, zoo.animal.care and zoo.staff, does the following code compile?

module zoo.staff {
    exports zoo.staff;
    requires zoo.animal.feeding;
    requires zoo.animal.care;
    requires zoo.animal.talks;
    requires transitive zoo.animal.care;
}

Answer 16

Java doesn’t allow you to repeat the same module in a requires clause. It is redundant and most like an error in coding.

Question 17

What does the provides ‘keyword’ do?

Answer 17

The provides keyword specifies that a class provides an implementation of a service. (Think of a service as a fancy interface).

provides zoo.staff.ZooApi with zoo.staff.ZooImpl

Question 18

What does the provides ‘uses’ do?

Answer 18

The uses keyword specifies that a module is relying on a service.

uses zoo.staff.ZooApi

Question 19

What does the provides ‘opens’ do?

Answer 19

Allows any module using the module where opens is used to use reflection (inspect and call code at runtime_

Question 20

What command can we use to find out what the module structure is of a jar file?

Answer 20

java -p {module-path here} -d {module here}

or

java -p {module-path here} –describe-module {module here}

Question 21

What command can we use to list available modules?

Answer 21

java –list-modules
/
java -p mods –list-modules

Question 22

What flag can we use to print debugging information when running the modules?

Answer 22

–show-module-resolution

Question 23

What command can we use to get information about dependencies within a module that are actually used (and not just declared)?

Answer 23

The jdeps command

Question 24

Use a command to get information about actual dependencies of the module zoo.staff in the directory mods.

Answer 24

jdeps mods/zoo.staff.jar

Question 25

Use a command to get a summary of the information about actual dependencies of the module zoo.staff in the directory mods.

Answer 25

jdeps -s mods/zoo.staff.jar

or

jdeps -summary mods/zoo.staff.jar

Question 26

Name all important JMOD operations

Answer 26

create, extract, describe, list, hash