如何使用Java 8中的自定义Comparator接口对对象进行排序？

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> 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.

Answer 1

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.

Answer 2

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.