Sorting through Comparable(Very Urgent)

[Angela D'souza]

Hi I have two classes name Name.java and NameSort.java
Code of Name.java:

import java.util.*;
public class Name implements Comparable {
    private String  firstName, lastName,number;
    public Name(String firstName, String lastName,String number) {
        if (firstName==null || lastName==null)
            throw new NullPointerException();
        this.firstName = firstName;
        this.lastName = lastName;
        this.number = number;
    }
    public String firstName(){return firstName;}
    public String lastName(){return lastName;}
    public String number(){return number;}
    public boolean equals(Object o) {
        if (!(o instanceof Name))
            return false;
        Name n = (Name)o;
        return n.firstName.equals(firstName) &&
               n.lastName.equals(lastName);
    }
    public int hashCode() {
        return 31*firstName.hashCode() + lastName.hashCode();
    }
    public String toString() {return firstName + " " + lastName + " " + number;}
    public int compareTo(Object o) {
        Name n = (Name)o;
        int lastCmp = lastName.compareTo(n.lastName);
        return (lastCmp!=0 ? lastCmp :
                firstName.compareTo(n.firstName));
    }
}

Code of NameSort.java:

import java.util.*;
class NameSort {
    public static void main(String args[]) {
Name [] n= new Name[4];
n[0] = new Name("10", "Alvin", "Lennon");
n[1] = new Name("20","CKarl", "Marx");
n[2] = new Name("90","BOscar", "Marx");
n[3] = new Name("70","DOscar", "Grouch");

List set = new ArrayList();
set.add(n[0]);
set.add(n[1]);
set.add(n[2]);
set.add(n[3]);
Collections.sort(set);
ListIterator it = set.listIterator();
while(it.hasNext()){
  Name name = (Name) it.next();
  System.out.println(name);
  }
    }
}

In above code I can do sort either by firstName or LastName. I also want to do by number(String format only). I can't. Why???
Thanks,
Angela
(Edited by Cindy to format code. You know Angela - you have been around here enough to know that you should format the code yourself using the [ code] and [ /code] tags, without the spaces of course.)
[ August 13, 2002: Message edited by: Cindy Glass ]

[Ron Newman]

Can you tell us more about what happens when you try?
One problem I see in this code is that equals() and hashCode() entirely ignore the "number" field. You'll want to change that if you ever want to put Names into into a HashSet or HashMap.

[Cindy Glass]

In your NameSort class you have your input parameters to Name strangly arranged.
n[0] has a first name of 10, last name of Alvin and number of Lennon. I doubt that that is what you intended.
Next - in your compareTo method - I guess that you are going to PRAY that the only objects coming to you are truely Name objects - because you do not code for any other situation.
Next - - in your compareTo method - you are comparing to lastname (which is Alvin ) and if those are equal then you compare to first name (which is 10) - but nowhere do you mention number at all.
[ August 13, 2002: Message edited by: Cindy Glass ]

[Angela D'souza]

I am learning this sorting of object. I just grab this code from Internet. I am trying to change it so I can sort anything I want. But I don't have clue how to start.So if someone has good example or suggestion please help me out.
Thanks again,
Angela

[Neil Laurance]

Why not create instances of Comparator for each type of sort you would like. Example code would be something like:

class Name {
  public static final Comparator
    FIRST = new Comparator() {
      public int compare(Object o1, Object o2) {
        try {
          return ((Name)o1).firstName
            .compareTo(((Name)o2).firstName)));
        } catch(ClassCastException e) {
          // do something here
        }
      }
      // do something for equals too
    },
    LAST = new Comparator() {
      public int compare(Object o1, Object o2) {
        try {
          return ((Name)o1).lastName
            .compareTo(((Name)o2).lastName)));
        } catch(ClassCastException e) {
          // do something here
        }
      }
      // do something for equals too
    },
    NUMBER = new  Comparator() {
      public int compare(Object o1, Object o2) {
        try {
          return ((Name)o1).number
            .compareTo(((Name)o2).number)));
        } catch(ClassCastException e) {
          // do something here
        }
      }
      // do something for equals too
    };
    // rest of attributes and methods
}

Then you can call the required sort mechanism using:

Collections.sort(set, Name.FIRST);
Collections.sort(set, Name.LAST);
Collections.sort(set, Name.NUMBER);

One point to make: do you really want 'number' to be a String? This would mean we perform a lexical sort as opposed to a numerical sort for this attribute.
[changed code from Comparable to Comparator]
Hope this makes sense. Cheers, Neil
[ August 13, 2002: Message edited by: Neil Laurance ]
[ August 13, 2002: Message edited by: Neil Laurance ]
[ August 13, 2002: Message edited by: Neil Laurance ]

[Angela D'souza]

Thanks Neil,
The following code does sort by length of string instead ascending order.Eg. I put August 13, August 9, August 2. It will sort like August 13, August 2, August 9 instead August 2, August 9, August 13.
Would you please help me out? If you need to view code:
Code for NameSort.java:
import java.util.*;
class NameSort {
public static void main(String args[]) {
Name [] n= new Name[4];
n[0] = new Name("10", "Rakeh", "Pandit");
n[1] = new Name("20","Rakesh", "Pandit");
n[2] = new Name("90","Angela", "Desouza");
n[3] = new Name("100", "Ed", "Lin");
List set = new ArrayList();
set.add(n[0]);
set.add(n[1]);
set.add(n[2]);
set.add(n[3]);
Collections.sort(set,Name.FIRST);
ListIterator it = set.listIterator();
while(it.hasNext()){
Name name = (Name)it.next();
System.out.println(name.lastName + " " + name.firstName);
}
}
}
Code for Name.java:
import java.util.*;
class Name
{
public String number, firstName, lastName;
public Name(String number,String firstName, String lastName)
{
if (firstName==null || lastName==null)
throw new NullPointerException();
this.firstName = firstName;
this.lastName = lastName;
this.number = number;
}
public static final Comparator FIRST = new Comparator()
{
public int compare(Object o1, Object o2)
{
try
{
Name name1 = (Name)o1;
Name name2 = (Name)o2;
return name1.firstName.compareTo(name2.firstName);
}
catch(ClassCastException e)
{
// do something here
}
return 0;
}
// do something for equals too
};
public static final Comparator LAST = new Comparator()
{
public int compare(Object o1, Object o2)
{
try
{
Name name1 = (Name)o1;
Name name2 = (Name)o2;
return name1.lastName.compareTo(name2.lastName);
}
catch(ClassCastException e)
{
// do something here
}
return 0;
}
// do something for equals too
};
public static final Comparator NUMBER = new Comparator()
{
public int compare(Object o1, Object o2)
{
try
{
Name name1 = (Name)o1;
Name name2 = (Name)o2;
return name1.number.compareTo(name2.number);
}
catch(ClassCastException e)
{
// do something here
}
return 0;
}
// do something for equals too
};
}
Please help me out.
Thanks,
Angela

[Ron Newman]

You're storing and sorting the number field as a string, so "August 13" does indeed sort before "August 2".
If it's a number, store it as an int, and compare it that way.

[Jamie Robertson]

the sort will work if you use MMDD format instead of the longer values. So August 13 becomes "0813" and August 8 becomes "0808"...etc. This will also work if you use August 08 instead of August 8.

Jamie
[ August 14, 2002: Message edited by: Jamie Robertson ]

[Angela D'souza]

Originally posted by Jamie Robertson:
the sort will work if you use MMDD format instead of the longer values. So August 13 becomes "0813" and August 8 becomes "0808"...etc. This will also work if you use August 08 instead of August 8.
[ August 14, 2002: Message edited by: Jamie Robertson ]

How can I get MMDD format???
Thanks,
Angela

[Thomas Paul]

The second article in my series on Collections has an example of setting up Comparators. Check the JavaRanch newsletter. I think it's in the July issue.

[Neil Laurance]

If you have one of your strings that always contains dates, I would convert these into a private java.util.Date attribute within the Name class. Then do your comparison using these Date values.

[Jamie Robertson]

If I were you, I would parse the complete date string ( including the year ) using a SimpleDateFormat ( using the pattern your date string is in ) and compare the Date values rather than the String values of the date as mentioned earlier. As an alternative, you could also compare the millisecond representation of the Date or as suggested by me earlier, format the date into the desired "MMdd" String pattern and compare those.
just for my information, why isn't there a year value associated with every date?
Jamie