Expert: Neal Ziring - 3/16/2007

Question
Hi,

   I solved the problem to sort the Employee first based on Employee Id and then based on Employee Name...I have got 4 class to do this...

1) my Employee calss,which has the Get/Set methods.

2) my EmployeeNameCompare class which implements the Comparator and in my compare methods i'm sorting based on Name..

public int compare(Object obj1, Object obj2) {
String name = ((Employee) obj1).getName();
String name1 = ((Employee) obj2).getName();
return name.compareTo(name1);
}
3) my EmployeeIdCompare class which implements the Comparator and in my compare methods i'm sorting based on ID..

4) And finally my Utility calss which calls the Collection.Sort(list,new EmployeeNameCompare() );
Collection.Sort(list,new EmployeeIdCompare () );

But my problem is if i need to sort on the basis of age,sex,place i need to have 3 more Comparator class for these.
Is there any way to reduce the number of comparatorclass and sort as many atributes i want. I want to make the comparatorclass generic.  

Answer
Mary,

You can easily combine the results from several Comparators
for an object class into one Comparator.

First, create a Comparator class for each
different aspect of the Employee class that you might need.
For example:

    public class EmployeeAgeComparator
       implements Comparator
   {
           public int compare(Object o1, Object o2) {
                int age1 = ((Employee)o1).getAge();
                int age2 = ((Employee)o2).getAge();
                if (age1 < age2) return -1;
                if (age2 < age1) return  1;
                return 0;
           }
     }

Once you've got EmployeeIdComparator, EmployeeNameComparator and all the rest, then use this class:

     public class MultiComparator
            implements Comparator
     {
          Vector comparators;
          public MultiComparator() {
             comparators = new Vector();
          }
          public void addComparator(Comparator c) {
             comparators.add(c);
          }
          public void removeComparator(Comparator c) {
             comparators.remove(c);
          }
          public int compare(Object obj1, Object obj2) {
             int result, i;
             Comparator c;
             result = 0;
             for(i = 0; i < comparators.size(); i++) {
                c = (Comparator)(comparators.elementAt(i));
                result = c.compare(obj1,obj2);
                if (result != 0) break;
             }
             return result;
          }
      }

Now that you have the multi-comparator, and individual
comparators for all the fields, you can create a comparator
that has any properties you want.  For example, to sort
on EmployeeID, Name, and Age, in that order, you would do
this:

           MultiComparator mc;
           mc = new MultiComparator();
           mc.add(new EmployeeIdComparator());
           mc.add(new EmployeeNameComparator());
           mc.add(new EmployeeAgeComparator());
           Collections.sort(list, mc);


The solution I have outlined above is the most general and
the most flexible.  You can reduce the number of comparator
classes, but it will make your solution less flexible.  For
example:

     public class EmployeeNameAndAgeComparator() {
          public int compare(Object o1, Object o2) {
             int result;
             Employee e1, e2;
             e1 = (Employee) o1;
             e2 = (Employee) o2;
             result = e1.getName().compareTo(e2.getName());
             if (result != 0) return result;
             if (e1.getAge() < e2.getAge()) return -1;
             if (e2.getAge() < e1.getAge()) return  1;
             return 0;
         }
     }


There is no other simple way to make a comparator that
sorts on many attributes.  There are complex ways to do
this, but they depend on the structure and value ranges
of the attributes.

Hope this helps...

...nz