Could you help me in understanding what this code means ?

Question

Hi, I was reading an article about Comparing Objects. Well, here's the link:
http://www.onjava.com/lpt/a/3286
The link I found it somewhere in this forum too...it's about sorting an array based on another sorted array...I can't understand what is happening inside this code...Well, I knew it is a method, because it actually a member of a class Person. The problem is that, everytime I delete this part of method in a class which implements Comparable...it always throws an error...is it possible if I don't use this method below ?
public int compareTo(Object anotherPerson) throws ClassCastException {
    if (!(anotherPerson instanceof Person))
      throw new ClassCastException("A Person object expected.");
    int anotherPersonAge = ((Person) anotherPerson).getAge(); 
    return this.age - anotherPersonAge;   
  }Thank you very much !
Sincerely,
Chris

Answer 1

Thank you, I am new in Java Programming, and my class will start talk about Sorting Array next week, and I know that I am so weak in Sorting things...so, I have to encourage myself, so that I could understand what they will discuss in the class....
//WHAT IS THE "!" sign before the anotherPerson instanceof Person means ?
if (!(anotherPerson instanceof Person))
      throw new ClassCastException("A Person object expected.");
* I know that Person is the class name and anotherPerson is an object of
* class Person but: ((Person)anotherPerson)...what is that mean ?
* so the getAge() method refers to anotherPerson's age ?
* or the Person's age ?
* Say that I don't want to sort it based on the age, but I want to sort it,
* for example:
* by its character's length, do I have to create a method to get the
* characters length ?
    int anotherPersonAge = ((Person) anotherPerson).getAge(); 
    return this.age - anotherPersonAge;
   }Thank you...
Chris