Question

1. Do the following

a. Write a class Student that has the following attributes:
- name: String, the student's name ("Last, First" format)
- enrollment date (a Date object)

The Student class provides a constructor that saves the student's name and enrollment date.

Student(String name, Date whenEnrolled)

The Student class provides accessors for the name and enrollment date.

Make sure the class is immutable. Be careful with that Date field -- remember what to do when sharing mutable instance variables -- issued discussed in Chapter 3 (class Employee).

Write contracts for all methods: preconditions/postconditions. Write the class invariant in the class javadoc comment.

b. Implement a static method in class Student

public static Comparator<Student> getCompByName()

that returns a new comparator object for Student that compares 2 Students objects by the attribute 'name'.

Implement a static method in class Student

public static Comparator<Student> getCompByDate()

that returns a new comparator object for Student that compares 2 Students objects by their enrollment date.

You must use anonymous classes that implement the Comparator interface.

c. Write a public static main() method in class Student that:
- creates an ArrayList<Student> object called students
- adds 4 new Student objects to the students list, with some made up names and dates
- sort the students list by name and display the sorted collection to System.out. use function getCompByName()
- sort the students list by enrollment date and display the sorted collection to System.out. use function getCompByDate()

Answer 1

import java.util.Comparator;
import java.util.Date;

public final class Student {

   private final String name;
   private final Date enrollment_date;

   /*@invariant name != null && name.length>0; @*/ //class invariant
/*@invariant enrollment_date != null; @*/ //class invariant
   public Student(String name, Date enrollment_date) {
       super();
      
       assert name != null : "Precondition: name != null";
       assert enrollment_date != null : "Precondition: enrollment_date != null";
       assert name.indexOf(",") != -1 : "Precondition: name.indexOf(\",\") != -1";
      
       this.name = name;
       this.enrollment_date = enrollment_date;
      
       assert getName() == name : "Postcondition: getName() == name";
       assert getEnrollment_date() == enrollment_date : "Postcondition: getEnrollment_date() == enrollment_date";
   }

   public String getName() {
       return name;
   }

   public Date getEnrollment_date() {
       return enrollment_date;
   }

   public static Comparator<Student> getCompByName =
           new Comparator<Student>(){

       @Override
       public int compare(Student o1, Student o2) {

           assert o1.name != null : "Precondition: o1.name != null";
           assert o2.name != null : "Precondition: o2.name != null";
          
           return o1.name.compareTo(o2.name);
       }

   };

   public static Comparator<Student> getCompByDate =
           new Comparator<Student>(){

       @Override
       public int compare(Student p, Student q) {

           if (p.getEnrollment_date().before(q.getEnrollment_date())) {
               return -1;
           } else if (p.getEnrollment_date().after(q.getEnrollment_date())) {
               return 1;
           } else {
               return 0;
           }
       }

   };

}