Collections.sort() : need guidance for my code

[Rahul Sudip Bose]

I have the following class which i want to sort based on String name or int year (of release) for movie DVD's :
How do i make the compare method for a comparator for this class ?

class DVD 
{
	String name;
	int year; // will not use string instead.
	
	public DVD(String name, int year)
	{
	 this.name = name;
	 this.year = year;
	}
	
	public String toString()
	{
	 String info = "\n\nDVD :" + "name " + name + "year " + year + "\n\n";
	 return info;
	}
	
}

[Mohamed Sanaulla]

Have a look at the Comparable Interface. You need to override compareTo method-

[Sunny Bhandari]

You can also use Comparable interface. Depends upon the situation.
Comparable based solution will be faster.

[Rob Spoor]

Sunny, if you meant Comparator, then the statement about speed is nonsense. If there is any speed difference between using Comparable and using Comparator it is negligible.

[Rahul Sudip Bose]

Rob Spoor wrote:Sunny, if you meant Comparator, then the statement about speed is nonsense. If there is any speed difference between using Comparable and using Comparator it is negligible.

speed...how do we measure that in practice ?

[Rahul Sudip Bose]

Using the below code i can sort objects based on name, which is a string. How to modify my code if it needs to be sorted on the basis of year, an int ?
Please help.

import java.util.*; 

class DVD
{
	String name;
	int year; 
	
	public DVD(String name, int year)
	{
	 this.name = name;
	 this.year = year;
	}
	
	public String toString()
	{
	 String info = "\n\n@DVD :" + ">name: " + name + "\t>year: " + year + "\n\n";
	 return info;
	}
	
}//Class DVD ends

// a comparator to use for sorting DVD's by name (or year)

class DVDsort implements Comparator<DVD>
{
	public int compare(DVD one,DVD two)
	{
	 return(one.name.compareTo(two.name));
	}

}//class ends

public class GenericSort
{
	public static void main(String[] args)
	{
	 List<DVD> movies = new ArrayList<DVD>();
	 DVDsort dv = new DVDsort();
	 movies.add(new DVD("thing",2010));
	 movies.add(new DVD("fling",2009));
	 movies.add(new DVD("nah",2011));
	 Collections.sort(movies,dv);
	 System.out.println(movies);
	 
	}//main ends

}//class ends

[Matthew Brown]

Rahul Sudip Bose wrote:Using the below code i can sort objects based on name, which is a string. How to modify my code if it needs to be sorted on the basis of year, an int ?

class DVDsortByYear implements Comparator<DVD>  {  
    public int compare(DVD one,DVD two) {  
        return one.year - two.year;
    }  
}

[Rahul Sudip Bose]

Matthew Brown wrote:

Rahul Sudip Bose wrote:Using the below code i can sort objects based on name, which is a string. How to modify my code if it needs to be sorted on the basis of year, an int ?

class DVDsortByYear implements Comparator<DVD>  {  
    public int compare(DVD one,DVD two) {  
        return one.year - two.year;
    }  
}

Thanks. I want to be able to choose whether want i want to Sort on the basis of string and an int not either. compare() can return only once , either the comparison of strings (my) code or ints(above). It want this code to be able to choose whether it should sort on the basis of int or string.

pseudocode :

compare() // or some other method
{ 
   if(selected_year) return comparison of year;
   else if(selected_name) return comparison of name;
   else return comparison of <whatever-you-wish>
}

Please advise.
rb

[Matthew Brown]

Rahul Sudip Bose wrote:Thanks. I want to be able to choose whether want i want to Sort on the basis of string OR an int and not either. compare can return only once , either the comparison of strings (my) code or ints(above). It does not choose the basis for sorting.

You do that by choosing which Comparator to use. Create classes for them both, and instantiate the one you need.

[Rahul Sudip Bose]

Matthew Brown wrote:

Rahul Sudip Bose wrote:Thanks. I want to be able to choose whether want i want to Sort on the basis of string OR an int and not either. compare can return only once , either the comparison of strings (my) code or ints(above). It does not choose the basis for sorting.

You do that by choosing which Comparator to use. Create classes for them both, and instantiate the one you need.

ta da... DOH !!! why didnt i think the obvious ! thanks a million !

[Rahul Sudip Bose]

done. here is the code and the output :

import java.util.*;

class DVD 
{
	String name;
	int year; 
	
	public DVD(String name, int year)
	{
	 this.name = name;
	 this.year = year;
	}
	
	public String toString()
	{
	 String info = "\n\n@DVD :" + ">name: " + name + "\t>year: " + year + "\n\n";
	 return info;
	}
	
}//Class DVD ends

// a comparator to use for sorting DVD's by name.

class NameSort implements Comparator<DVD>
{
	public int compare(DVD one,DVD two)
	{
	 return(one.name.compareTo(two.name));
	}

}
	
// a comparator to use for sorting DVD's by year.

class YearSort implements Comparator<DVD>
{
	public int compare(DVD one,DVD two)
	{
	 return(one.year-two.year);
	}
	
}//class ends

public class GenericSort
{
	public static void main(String[] args)
	{
	 List<DVD> movies = new ArrayList<DVD>();
	 NameSort ns = new NameSort();
	 YearSort ys = new YearSort();
	 movies.add(new DVD("thing",2010));
	 movies.add(new DVD("fling",2009));
	 movies.add(new DVD("nah",2011));
	 System.out.println("\nSorting movies by String name...\n");
	 Collections.sort(movies,ns);
	 System.out.println(movies);
	 System.out.println("\nSorting movies by Int year...\n");
	 Collections.sort(movies,ys);
	 System.out.println(movies);
	 
	}//main ends

}//class ends

/*OUTPUT 

Sorting movies by String name...

[

@DVD :>name: fling      >year: 2009

,

@DVD :>name: nah        >year: 2011

,

@DVD :>name: thing      >year: 2010

]

Sorting movies by Int year...

[

@DVD :>name: fling      >year: 2009

,

@DVD :>name: thing      >year: 2010

,

@DVD :>name: nah        >year: 2011

]

*/

[Matthew Brown]

Well done! One suggestion (it doesn't change how it works, but it tidies things up a bit): you might consider making the Comparators (static) nested classes of DVD. That way you can refer to them as DVD.NameSort and DVD.YearSort, which ties them closely together with the DVD class (which makes sense since they're useless without it). As it is, it isn't obvious from the name (if the application gets bigger) that YearSort sorts only DVDs.

[Rahul Sudip Bose]

Matthew Brown wrote:Well done! One suggestion (it doesn't change how it works, but it tidies things up a bit): you might consider making the Comparators (static) nested classes of DVD. That way you can refer to them as DVD.NameSort and DVD.YearSort, which ties them closely together with the DVD class (which makes sense since they're useless without it). As it is, it isn't obvious from the name (if the application gets bigger) that YearSort sorts only DVDs.

Long ago i had a post "what are the uses of nested classes" and i did not get a simple answer which would not cover topics outside my knowledge. So can this be a good example of why we need nested classes ? Or is there a simpler example to show the need for inner classes ?

thanks
rb

PS : here is that old post
http://www.coderanch.com/t/528900/java/java/Practical-real-world-situations-where#2398461

[Mohamed Sanaulla]

Rahul Sudip Bose wrote:
Long ago i had a post "what are the uses of nested classes" and i did not get a simple answer which would not cover topics outside my knowledge. So can this be a good example of why we need nested classes ? Or is there a simpler example to show the need for inner classes ?
...

I did provide you an example

[Rahul Sudip Bose]

Mohamed Sanaulla wrote:

Rahul Sudip Bose wrote:
Long ago i had a post "what are the uses of nested classes" and i did not get a simple answer which would not cover topics outside my knowledge. So can this be a good example of why we need nested classes ? Or is there a simpler example to show the need for inner classes ?
...

I did provide you an example

Yes, but unfortunately i have not done GUI yet. So, i could not understand the meaning. Thanks for helping me.

regards
rb

[Campbell Ritchie]

In which case you ought to have asked for more explanation at the time.

[Tom Reilly]

This code will not stand the test of time :-) For the following code,

public int compare(DVD one,DVD two)  
{  
    return(one.year-two.year);  
}  

What happens, for example, if one.year is Integer.MIN_VALUE and two.year is 1? Your code will claim them to be equal (subtracting one from Integer.MIN_VALUE results in zero). I point this out not for this example but if you have a situation where you are comparing ints that could be negative, you could have a bug.

[Rahul Sudip Bose]

Campbell Ritchie wrote:In which case you ought to have asked for more explanation at the time.

Yes. At that time i thought that i will study a bit of swing to understand the examples put before me. The k&b-scjp book also uses a theoretical GUI chat-client example which i did not understand. So, i decided to wait till i studied swing or if someone gave a newbie example.

[Rahul Sudip Bose]

Tom Reilly wrote:This code will not stand the test of time :-) For the following code,

public int compare(DVD one,DVD two)  
{  
    return(one.year-two.year);  
}  

What happens, for example, if one.year is Integer.MIN_VALUE and two.year is 1? Your code will claim them to be equal (subtracting one from Integer.MIN_VALUE results in zero). I point this out not for this example but if you have a situation where you are comparing ints that could be negative, you could have a bug.

I did not get 0 after doing what you said.

Code :

class IntegerMinimum
{
	public static void main(String [] args)
	{
	 Integer min = Integer.MIN_VALUE; 
	 System.out.println("\nint min is " + min + "\nsubtracting 1 from min\n");
	 min = min-1; 
	 System.out.println("min is " + min);
	
	}

}

output :

int min is -2147483648
subtracting 1 from min

min is 2147483647

[Campbell Ritchie]

I think there was a mistake about getting 0.