问题描述:

This may seem like a dumb question, but I'd like to inquire about the best way to sort in reverse order a sortable structure (any kind, could be List) of a class MyClass that doesn't implement Comparable

First off, since MyClass doesn't implement Comparable and I want to sort it, I create a Comparator, something like this:

public class MyClassComparator implements Comparator<MyClass> {

@Override

public int compare(MyClass o1, MyClass o2) {

return (o1.getMyField()).compareTo(o2.getMyField());

}

}

This, of course, sorts MyClass objects according to the natural ordering of MyClass.MyField. But I want the reverse order.

I could of course just hardcode the reverse order in the compare(MyClass o1, MyClass o2) method. Something like

if (o1.getMyField()).compareTo(o2.getMyField()) > 0 return -1;

if (o1.getMyField()).compareTo(o2.getMyField()) < 0 return 1;

return 0;

Anoter alternative is leaving the code of MyClassComparator as I initially suggested, and using public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) from java.util.Collections to pass an instance of MyClassComparator and get a reverse comparator.

And a third alternative, that I think is wrong, could be sorting my LinkedList<MyClass> with the natural order Comparator and fetch the elements from last to first. LinkedList is a doubly linked list, so it should handle this perfectly. The problem is that I would always have to get a specific Iterator instead of just using a for each loop (minor) and, of course, that I initially wanted to sort ANY sortable structure, and I can't guarantee that it will be a doubly linked list or anything that handles well iterating through it in reverse order.

Considering that I'm quite certain (can't ever be 100% certain) that I won't be using the natural order of MyClass objects, just the reverse order, what would be the best way to obtain my reverse comparator?

Thank you.

网友答案:

Using a comparator would be the correct approach here in order to avoid the overhead of reversing an array. If the classes you are comparing implement Comparable you can just reverse the order of the comparison. obj2.compareTo(obj1). A way to visualize this is to think of the two objects as integers and compareTo as subtracting them.

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