Java – Does a natural comparator exist in the standard api

comparablecomparatorjava

I need a comparator as part of a strategy pattern that can either use the natural ordering of the objects or some custom ordering. For the natural ordering case, I wrote a simple comparator:

private static class NaturalComparator<T extends Comparable<? super T>> implements Comparator<T> {
    @Override
    public int compare(T o1, T o2) {
        return o1.compareTo(o2);
    }
}

Seems simple enough, but I was wondering if anyone knew of one in the standard API. I looked at TreeMap, and it does it without such a class, so when that code was written, the apparent answer would be no, but perhaps it was added later.

Best Answer

Added to Comparator in Java 8:

static <T extends Comparable<? super T>> Comparator<T> naturalOrder()

Use it like this, for example:

Comparator<Double> natural = Comparator.<Double>naturalOrder();
return natural.compare(1.0, 1.1));

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Java – How does the Java ‘for each’ loop work

for (Iterator<String> i = someIterable.iterator(); i.hasNext();) {
    String item = i.next();
    System.out.println(item);
}

Note that if you need to use i.remove(); in your loop, or access the actual iterator in some way, you cannot use the for ( : ) idiom, since the actual iterator is merely inferred.

As was noted by Denis Bueno, this code works for any object that implements the Iterable interface.

Also, if the right-hand side of the for (:) idiom is an array rather than an Iterable object, the internal code uses an int index counter and checks against array.length instead. See the Java Language Specification.

Java – When to use Comparable and Comparator

Use Comparable if you want to define a default (natural) ordering behaviour of the object in question, a common practice is to use a technical or natural (database?) identifier of the object for this.

Use Comparator if you want to define an external controllable ordering behaviour, this can override the default ordering behaviour.

Best Answer

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