How to sort an object Array using a custom Comparator interface in Java 8?

Question

Custom implementation of Comparator

@FunctionalInterface
public interface Comparator<T>  {

int compare(T t1, T t2);

static <T> Comparator<T> comparing(
        Function<T, Comparable> f){
    return (p1, p2) -> f.apply(p1).compareTo(f.apply(p2));
}

default Comparator<T> thenComparing(
        Comparator<T> cmp){
            return (p1, p2) ->
                    this.compare(p1, p2) == 0 ?
                            cmp.compare(p1, p2) : this.compare(p1, p2);
}

default Comparator<T> thenComparing(
        Function<T, Comparable> f){
            Comparator<T> cmp = comparing(f);
    return thenComparing(cmp);
  }
}

I have created a Person class with three fields, namely the firstName, lastName and age, and their respective getters and setters. Using the custom Comparator class I want to sort an array of persons in main as under:

    Comparator<Person> cmp = Comparator
            .comparing(Person::getLastName) // Extract Property and compare
            .thenComparing(Person::getFirstName)
            .thenComparing(Person::getAge);

     Person arr[] = new Person[]{
        new Person("Sean", "Gilmore", 22),
                new Person("Aaron", "Reidy", 21),
                new Person("Jane", "Kennedy", 53),
                new Person("Mike", "English", 49)
    };


    Arrays.sort(arr, cmp);

However Arrays.sort(arr, cmp); throws a compile error no instance of type variable T exists so that Comparator<Person> conforms to Comparator<? super T>

I'm thrown off by this error and I would like to know how to sort the Person array using cmp Comparator.

shmosel · Accepted Answer · 2017-02-13 18:23:17Z

3

You can easily adapt your Comparator to the JDK's like this:

Arrays.sort(arr, cmp::compare);

I believe Guava recommended a similar pattern to adapt their functional interfaces before they were retrofitted for Java 8.

answered Feb 13, 2017 at 18:23

shmosel

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7 Comments

Chetan Kinger Over a year ago

@shmosei cmp:compare would still give you a java.util.Comparator. There are other options too but you eventually can't pass anything but a java.util.Comparator to the method in question so the answer still remains : you can't do it.

shmosel Over a year ago

@CKing I didn't interpret the question as trying to avoid the JDK's Comparator entirely. I think OP is just asking how his custom interface can be used with the built-in sort functionality.

Chetan Kinger Over a year ago

@shmosei I fell for the same argument in my head but then I read this line from the question I'm thrown off my this error and I would like to know how to sort the Person array using cmp Comparator which convinced me that the OP clearly needed an explanation for why it can't be done rather than how to do it. (The OP was thrown away that he can't pass a Comparator to a method that takes a Comparator). This can be further inferred from the comment on my answer from the OP saying So that means I'll have to implement sort to use my custom Comparator? The OP is exploring Java in general!

shmosel Over a year ago

@CKing I hear you, but I suspect you're putting more thought into OP's words than he did :)

shmosel Over a year ago

@CKing That's one statement I fully agree with.

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Chetan Kinger · Accepted Answer · 2017-02-13 16:21:47Z

3

The fully qualified name of a class in Java includes it's package name.

The Arrays.sort method expects a java.util.Comparator implementation. Your custom Comparator is not the same is java.util.Comparator.

The sort method in Arrays class is a static method so you can't extend Arrays and override this method. If you want to use Arrays.sort, use an implementation of java.util.Comparator. There is no way around it.

edited Feb 13, 2017 at 16:21

answered Feb 13, 2017 at 16:18

Chetan Kinger

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6 Comments

harpresing Over a year ago

So that means I'll have to implement sort to use my custom Comparator?

Flown Over a year ago

@HarpreetSingh Or you write a facade for your custom Comparator class. Anyway, why don't you use the genuine Comparator?

Chetan Kinger Over a year ago

@Flown That Facade is still a java.util.Comparator.

Flown Over a year ago

@CKing This is what I've meant.

ncmathsadist Over a year ago

Why are you reinventing wheels?

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