Replace CompareToBuilder with Java 8's Comparator.comparing(...).thenComparing(...)

Question

Before Java 8, we implemented Comparable.compareTo(...) like this:

public int compare(Person a, Person b) {
    return new CompareToBuilder()
            .append(a.getLastName(), b.getLastName())
            .append(a.getFirstName(), b.getFirstName())
            .toComparison();
}

As of Java 8, we can do it like this:

public int compare(Person a, Person b) {
    return Comparator
            .comparing(Person::getLastName)
            .thenComparing(Person::getFirstName)
            .compare(a, b);
}

The new Java 8 way might allow us to drop the commons-lang3 dependency. Is that new Java 8 way faster? Is there a way to automatically migrate? I didn't find an IntelliJ intention for it.

---

Notice that it becomes a bit more complex when there are reverse orders and non natural comparison is involved:

public int compare(SingleBenchmarkResult a, SingleBenchmarkResult b) {
    return new CompareToBuilder()
            .append(b.hasAnyFailure(), a.hasAnyFailure()) // Reverse
            .append(a.getAverageScore(), b.getAverageScore(), resilientScoreComparator)
            .toComparison();
}

becomes

public int compare(SingleBenchmarkResult a, SingleBenchmarkResult b) {
    return Comparator
            .comparing(SingleBenchmarkResult::hasAnyFailure, Comparator.reverseOrder()) // Reverse
            .thenComparing(SingleBenchmarkResult::getAverageScore, resilientScoreComparator)
            .compare(a, b);
}

Answer 1

If you write it this way

public int compare(Person a, Person b) {
    return Comparator
            .comparing(Person::getLastName)
            .thenComparing(Person::getFirstName)
            .compare(a, b);
}

you are wasting performance by constructing a new Comparator for each comparison. And it should be obviously nonsensical when looking at the surrounding code. The compare(Person a, Person b) method surely is part of a class implementing Comparator<Person>, which you instantiate at some place to get the desired comparator. You should replace that instance by a sole Comparator.comparing(Person::getLastName).thenComparing(Person::getFirstName) instance instead, used throughout the entire operation.

E.g.

// reusable
static final Comparator<Person> By_NAME = Comparator
             .comparing(Person::getLastName).thenComparing(Person::getFirstName);

or ad hoc

listOfPersons.sort(Comparator.comparing(Person::getLastName)
                             .thenComparing(Person::getFirstName));

---

If you use it that way, it’s very likely to be faster. However, you should see, that there is no simple pattern-based replacement possible. You have to replace the use sites of the class with that simple declarative construct and make a decision whether to use a shared comparator instance for multiple use sites or create it ad-hoc. Then, you can remove the entire old implementation class or at least, remove the comparator functionality from it if it still serves other purposes.

Answer 2

I don't think there is any pre-defined inspection for that. You might try to use IntelliJ's structural-search, although I think it might by quite tricky to do that for every possible case. One possibility for a simple case with two comparisons might be the following:

search template (occurence count of $TYPE$ and $z$ is 2):

$ReturnType$ $MethodName$($TYPE$ $z$) {
        return new CompareToBuilder()
                .append($A$.$m$(), $B$.$m$())
                .append($A$.$m1$(), $B$.$m1$())
                .toComparison();
    }

replacement template:

$ReturnType$ $MethodName$($TYPE$ $z$) {
    return java.util.Comparator
            .comparing($TYPE$::$m$)
            .thenComparing($TYPE$::$m1$)
            .compare($A$, $B$);
}

I am not an expert on structural search, but I guess you would have to make another pattern for calls with more or less comparisons.