Interfaces - Beginning Java 8 Fundamentals - page 655

Suppose each department must submit a job every day, even if they do not have something to run. It suggests
that sometimes you need an empty job or a job with nothing real to do. The developer of the Job interface may
provide a constant, which represents a trivial implementation of the Job interface as listed in Listing 17-15. It has a
nested class called EmptyJob , which implements the enclosing Job interface.
Listing 17-15. The Job Interface with a Nested Class and a Constant Field
// Job.java
package com.jdojo.interfaces;
public interface Job {
// A nested class
class EmptyJob implements Job {
private EmptyJob() {
// Do not allow outside to create its object
}
public void runJob() {
System.out.println("Nothing serious to run...");
}
}
// A constant field
Job EMPTY_JOB = new EmptyJob();
// An abstract method
void runJob();
}
If a department does not have a meaningful job to submit, it can use Job.EMPTY_JOB constant as a job. The fully
qualified name of the EmptyJob class is com.jdojo.interfaces.Job.EmptyJob . Note that the enclosing interface Job
provides an additional namespace for the EmptyJob class. Inside the Job interface, the EmptyJob class can be referred
to by its simple name of EmptyJob . However, outside the Job interface, it must be referred to as Job.EmptyJob . You
may notice that a trivial job object is represented by the Job.EMPTY_JOB constant. You have made the constructor for
the EmptyJob nested class private, so no one outside the interface can create its object. Listing 17-16 shows how to use
the class. Typically, in such cases, you would make the constructor of the Job.EmptyJob class private so its object
cannot be created outside the Job interface because the EMPTY_JOB constant already provides an object of this class.
Listing 17-16. A Test Program to Test the Job Interface and Its Nested EmptyJob Class
// JobTest.java
package com.jdojo.interfaces;
public class JobTest {
public static void main(String[] args) {
submitJob(Job.EMPTY_JOB);
}
public static void submitJob(Job job) {
job.runJob();
}
}
Nothing serious to run...