Newbie tries to use comparators... and fails.

[Ian Taylor]

I am trying to understand how comparators work and to that end have created a simple program that will compare the integer variables in instances of a simple object. However, the comparator method fails to recognise the instance variables in the objects passed to it, giving a 'cannot find symbol' error at line 34 in Netbeans. I tried exactly the same program without the comparator implementation and it didn't give any errors, so I assume there is an aspect of the implementation/override aspect that I am goofing up. Go ahead and tell me what rookie error I am making this time Code below.

import java.util.Comparator;

public class ListSort {
    
    public static void main (String[] args) {
        
        ObjectCreate object1 = new ObjectCreate ("Object 1", 3);
        ObjectCreate object2 = new ObjectCreate ("Object 2", 7);
        ObjectCreate object3 = new ObjectCreate ("Object 3", 2);

    }
         
}

class ObjectCreate {
    
    String objectName;
    int objectScore;
    
    ObjectCreate (String name, int score) {

        objectName = name;
        objectScore = score;             
    }
    
}


class ObjectComparator<ObjectCreate> implements Comparator<ObjectCreate> {
    
    @Override
    public int compare (ObjectCreate obj1, ObjectCreate obj2) {

       int compareVar = obj1.objectScore - obj2.objectScore;   //Error occurs here!
        
       return compareVar;
                                
    }
}

[Tony Docherty]

Your comparator class declaration should be:

class ObjectComparator implements Comparator<ObjectCreate> {

[Matthew Brown]

Tony's right. You might be interested in what the effect of your mistake is. When you write:

class ObjectComparator<ObjectCreate> implements Comparator<ObjectCreate> {  
 ...
} 

It's actually using ObjectCreate as a generic type variable, not as a concrete type. So your class is the equivalent of this:

class ObjectComparator<T> implements Comparator<T> {  
    @Override  
    public int compare (T obj1, T obj2) {  
        int compareVar = obj1.objectScore - obj2.objectScore;   //Error occurs here!  
        return compareVar;  
    }  
} 

Now you can why that error occurs. As far as the compiler is concerned, T can be anything. So it has no idea whether it will have a objectScore field or not.

[Ian Taylor]

I see!

I need to understand generics much better. I got my original syntax from the example on this page

http://tutorials.jenkov.com/java-collections/sorting.html

Corrected and it works fine now, thanks both of you!