overriding compareTo

[kevinn lee]

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

   int pages,year;
   String name;
   Book(int p,int y,String n){
   	pages=p;
   	year=y;
   	name=n;
   }
   public int compareTo(Book b){
   	return year.compareTo(b.year);
   	
   }
    
    
} 

How to get rid of this error and what causes it?
thanks

[Ankit Garg]

How to get rid of this error and what causes it?

What error?? Can you post the error that you are getting...

[kevinn lee]

   int cannot be dereferenced
    return year.compareTo(b.year);
               ^  

[Prithvi Sehgal]

Dear Kevin,

   

int cannot be dereferenced  
    return year.compareTo(b.year);  
 

One hint for you. Do you think that compareTo() method is defined for integers?
What you are trying to do is year.compareTo(), and nothing like compareTo exists
for integers.

   
  public int compareTo(Book b){  
   
	  if(this.year == b.year){
		  return 0 ;
	  }
	  
	  else if(this.year > b.year){
		  return 1 ;
	  }
	  
	  else{
		  return -1 ;
	  }
}  

Do you think, it might help you?
Hope this helps you,

[Dejan Miler]

# int cannot be dereferenced
# return year.compareTo(b.year);

year is primitive data type so it do not have compareTo method

Dejan

[kevinn lee]

hello Prithivi

using a wrapper class is the solution then.is it?
thanks

[kevinn lee]

Dejan Miler wrote:

# int cannot be dereferenced
# return year.compareTo(b.year);

year is primitive data type so it do not have compareTo method

Dejan

thanks

[Moguluri Ravi Kiran]

Dear Kevin Lee,

I dont know the way you want your Book class to work....
but i am telling the way how to use the compareTo()

& your error is simple , all our friends already explained.

you need here two things

1) how to use equals()
2) intention of compareTo()

for equals() go through Thinking in Java ed.4...

now for the CompareTo() the implementation must be as follows.

let b1 & b2 are your book instances

1) if b1.equals(b2) is true then b1.compareTo(b2)==0 must be true...
2) if b1.equals(b2) is false then b1.compareTo(b2) must return a non-zero...
3) any_book_obj.compareTo(null) must throw NullPointerException.
4) for incompatiable class it must throw ClassCastException.


Click Here for More

your modified program is

   import java.util.*;

class Book implements Comparable<Book> {

   int pages,year;
   String name;

   Book(int p,int y,String n){
    
   	pages=p;
   	year=y;
   	name=n;

   }


   public int compareTo(Book b){
        
        if(b!=null)
        {
            try{
   	    if( (this.pages==b.pages) && (this.year==b.year) && ((this.name).equals(b.name)) )
            return 0;
            else 
            return 1;
                }
            catch(Exception e)
            {
               throw new ClassCastException();
            }
        }
        else 
        throw new NullPointerException();
        
   	
   }

   public static void main(String a[])
   {
     Book b1 = new Book(200,2010,"xx");
     Book b2 = new Book(300,2010,"yy");
     Book b3 = new Book(200,2010,"xx");
  
     System.out.println("b1.compareTo(b2) = "+ b1.compareTo(b2));// checkout these two stmt's with equals(). & remember you also need to override the equals().
     System.out.println("b1.compareTo(b3) = "+ b1.compareTo(b3));// go through Thinking in Java for must.
     System.out.println("b1.compareTo(null) = "+b1.compareTo(null));//Exception.
   }
    
    
}  

[kevinn lee]

thank you ravi
I wanted to sort the Book objects according to the year.Thats why I used compareTo in Book claas.After making year an Integer instead of a primitive the code worked.
Is there any other means of getting it done that is better?(to be used with sort())
thanks again