sort a List based on a parameter available in another class

Question

I have some design problems with Java Comparator Interface.

I have a class which contains a Set of a simple custom data structure:

Answer 1

You could use something that smells like higher order functions. That is, make a static function that takes a map of sorts from Long to int (which is the priority) or data and returns a new Comparator.

The class Foo has a static method getComparator which takes an Orange. An Orange is a class that has a method getPriority which takes an ID an return the corresponding priority. The getComparator method constructs a new Comparator object. The new Comparator object's compare method takes two IDs. It looks up the corresponding priorities of the two IDs and compares them.

public interface Orange {
    // Looks up id and returns the corresponding Priority.
    public int getPriority(Long id);
}

public class Foo {
    public static Comparator getComparator(final Orange orange) {
        return new Comparator() {
            public int compare(Long id1, Long id2) {
                // Get priority through orange, or
                // Make orange juice from our orange.
                // You may want to compare them in a different way.
                return orange.getPriority(id1) - orange.getPriority(id2);
        };
    }
}

My java is a bit rusty so the code may be flawed. The general idea should work, though.

Usage:

// This is defined somewhere. It could be a local variable or an instance
// field or whatever. There's no exception (except is has to be in scope).
Collection c = ...;
...
Orange orange = new Orange() {
    public int getPriority(Long id) {
        // Insert code that searches c.mySet for an instance of data
        // with the desired ID and return its Priority
    }
};
Collections.sort(c.myList, Foo.getComparator(orange));

I have not given an example on how an Orange could look.