Sorting problem cannot instantiate abstract object overriding compareTo() Method.

Question

I'm trying to establish a new sort criteria, in this case by name. I'm facing an error when I call the sort method...

this is a separated class (SortByName) in package "package":

-----------------------CLASS SortByName---------------------------

package package;

import java.util.*;

public abstract class SortByName implements Comparator{

public int compareTo(Object o1, Object o2){
    String n1 = ((Ficha)o1).getName();
    String n2 = ((Ficha)o2).getName();
    return n1.compareTo(n2);

}

and then inside an ActionPerformed event I have this:

----------------IN THE ACTION EVENT BUTTON----------------------------

Collections.sort( list , new SortByName() );

"package.SortByName is abstract,> cannot be instantiated"

I tried changing the "abstract" type in the class definition (SortByName) , but it complies about not overriding the compareTo() method.

thanks for reading.

Answer 1

You need to remove "abstract" and implement the equals method as well.

Code should look like:

package package;

import java.util.*;

public class SortByName implements Comparator{

public int compare(Object o1, Object o2){
    String n1 = ((Ficha)o1).getName();
    String n2 = ((Ficha)o2).getName();
    return n1.compareTo(n2);
}

public boolean equals(Object o1) {
 // code which compares the current comparator (!) with the object given
}

Note that you might need to use specialized comparators if compiling against a version of java which supports generics. Not sure if this is correct but I think you can do something like:

public class SortByName implements Comparator <Ficha> {
    public int compare(Ficha o1, Ficha o2){
       // comparing code here
    }
}

But again, not entirely sure on that part, you need to test it a bit.

If you check out the Comparator API that will explain how you must implement equals.

Also note that the Comparator interface defines compare, not compareTo.

Answer 2

The method you have to implement is called compare, not compareTo.

Abstract classes cannot be instantiated.

Also, Comparator is generic, so you'd better do the following:

public class SortByName implements Comparator<Ficha>{

    public int compareTo(Ficha f1, Ficha f2){
        String n1 = f1.getName();
        String n2 = f2.getName();
        return n1.compareTo(n2);
    }
}

Answer 3

Comparator needs to implement compare(), not compareTo() method.

Answer 4

class SortByName implements Comparator<Ficha>{
@Override
public int compare(Ficha o1, Ficha o2) {
    String n1 = o1.getName();
    String n2 = o2.getName();
    return n1.compareTo(n2);
}

}

Answer 5

Laura you were right I had to add the equals method.( u missed the return value) also remove "abstract" at class definition. and change compareTo() with compare().

thanks a lot Laura, PROBLEM SOLVED !!!!