关于接口调用的问题

请问从Comparable接口实现过来的compareTo方法在，它是什么时被调用的
import java.util.*;public class EmployeeSortTest
{
 
public static void main(String args[])
{
Employee [] staff=new Employee[3];
staff[0]=new Employee("Harry Hacker",35000);
staff[1]=new Employee("Carl Cracker",75000);
staff[2]=new Employee("Tony Tester",38000);
Arrays.sort(staff);
for(int i=0;i<staff.length;i++)
{
Employee e=staff[i];
System.out.println("name="+e.getName()+" "+"salary="+e.getSalary());
}
}
}class Employee implements Comparable
{
Employee(String n,double s)
{
name=n;
salary=s;
}

public double getSalary()
{
return salary;
}
public String getName()
{
return name;
}
public int compareTo(Object otherObject)
{
Employee other=(Employee)otherObject;
if(salary<other.salary) return -1;
if(salary>other.salary) return 1;
return 0;
}
private String name;
private double salary;
}