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<Long> getComparator(final Orange orange) {
        return new Comparator<Long>() {
            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.

Answer 2

I assume that you have a List<Data> stored somewhere.. In Comparator, you need to invoke a getDataById method from your data class, and sort through Priority..

Check the below code.. I have used a single class for many purpose..

Ideally you would want to break it into more classes.. But this is just a Demo, how to achieve what you want..

class Container {
    // List of Data  instances created..
    // This list has to be static, as it is for a class, 
    // and not `instance specific`
    public static List<Data> dataList = new ArrayList<Data>();

    // List of Ids, that you want to sort.
    private List<Long> idList = new ArrayList<Long>();

    // Populate both the list..

    // Have a method that will iterate through static list to 
    // find Data instance for a particular id

    public static Data getDataById(long id) {
        // Find Data with id from the list

        // Return Data
    }

    public void sortList() {
        Collections.sort(idList, new MyComparator());
    }
}

public MyComparator implements Comparator<Long> {

    public int compare(Long int1, Long int2) {
        Data data1 = Container.getDataById(int1);
        Data data2 = Container.getDataById(int2);

        return data1.getPriority() - data2.getPriority();
    }
}