Discussion of Activity 5.5 - Using an interface

Sample solutions

1. Comparing Employee objects by salary:

   The class header must be modified as follows:

   public class Employee implements Comparable<Employee>
   

   An example method implementation is as follows:

   public int compareTo(Employee emp)
   {
       if (salary > emp.salary)
       {
          return 1;
       }
       if (salary < emp.salary)
       {
          return -1;
       }
   
       return 0;
   }
   

In fact there is a more elegant way of doing this that returns a more useful value, and that is simply to find the difference between the two employee salaries like this:

public int compareTo(Employee emp)
{
    return this.salary - emp.salary;
}

Not only does this method return a useful value (the difference between the employee's salaries), but it is easy to reverse sorting using this approach:

public int compareTo(Employee emp)
{
    return emp.salary - this.salary;
}

2. The class header cannot be changed if we are to use Arrays.sort as this method requires the items in the array to implement Comparable for their own type.

   A possible method implementation is now as follows:

   public int compareTo(Employee emp)
   {   
      return emp.name.compareTo(name);
   }
   

   Here we make use of the fact that the String class already has a suitable compareTo method defined (this compareTo method accepts a String argument only).