Comparable and compareTo

[swaraj gupta]

This is from KS & BB book..

class DVDInfo implements Comparable<DVDInfo> {   // Line 1 
  // existing code 
  public int compareTo(DVDInfo d) {                                    // Line 2
    return title.compareTo(d.getTitle());     
} }

Please let me know the exact relationship in-between Comparable<DVDInfo> and compareTo(DVDInfo d). How should i read them?
As whenever i try to differ the Comparable's concrete type and compareTo() methods argument, it gives a compile time error.

This is what given in the book: "In line 1 we declare that class DVDInfo implements Comparable in such a way
that DVDInfo objects can be compared to other DVDInfo objects". which is turning out to be an insufficient information for me..

[Prasad Kharkar]

@swaraj
what you are trying to implement using different types is unknown to us
so please post it here

and
about line 1
that surely says that, we can compare objects of type DVDInfo

what you have written is correct , DVDInfo objects getting compared means that
we can determine whether two DVDInfo objects are equal or less than or greater than with each other on the basis of its instance variable
for example

class DVDInfo implements comparable<DVDInfo>{
    int price;
    public int getPrice(){
        return price;
    }
    public int compareTo(DVDInfo d){
        return this.compareTo(d.getPrice());
    }
}

here,
compareTo() method is called whenever we sort the array of the DVDInfo objects and that sorting order is determined using the price

hope this is clear to you now

and post your code please
hth

[swaraj gupta]

This is what I was trying to do...as i didn't get the point...
case 1:

import java.util.*;
class Book implements Comparable<Object>
{
	String name;
	String editor;	

        public int compareTo(Book b)
  	      {
		return name.compareTo(b.name);	
	       }
   .......
} 

case 2:

import java.util.*;
class Book implements Comparable<Book>
{
	String name;
	String editor;	

        public int compareTo(Object b)
  	      {
		return name.compareTo(b.name);	
	       }
   .......
} 

Please let me know the problem with these cases.

[Matthew Brown]

Well, the interface definition is:

public interface Comparable<T> {
    int compareTo(T o); 
}

So, if you implement Comparable<DVDInfo>, the interface definition tells you that there should be a method compareTo(DVDInfo). Anything else and you'll get a compiler error (unless it's an abstract class) because you simply haven't implemented the interface. As far as the compiler is concerned, it's exactly the same situation as if you'd omitted the compareTo method entirely.