I have collection of persons:
class Person {
public String name;
public int count;
public boolean favorite;
// getters / setters ...
}
I want to sort collection of persons by:
How can I write this conditions with Comparator from 1.8 JDK and retrieve expected results?
You can use Comparator.comparing(...).thenComparing(...) to sort by more than one attribute. For sorting favourites first, you can sort by that attribute, but then have to reverse the result, as false sorts before true. Assuming that you want to sort just by whether the count is > 0, and not by the actual value of the count:
List<Person> persons = Arrays.asList(
new Person("p1", 2, true),
new Person("p2", 0, false),
new Person("p3", 0, true),
new Person("p4", 1, false),
new Person("p5", 3, true));
persons.stream().sorted(Comparator
.comparing(Person::isFavorite).reversed()
.thenComparing(p -> p.getCount() == 0))
.forEach(System.out::println);
Result:
Person(name=p1, count=2, favorite=true)
Person(name=p5, count=3, favorite=true)
Person(name=p3, count=0, favorite=true)
Person(name=p4, count=1, favorite=false)
Person(name=p2, count=0, favorite=false)
Note that the final count == 0 condition is redundant (assuming that count can not be < 0)
Or sort by count directly and reverse once at the end; this will sort p5 before p1 in the example:
persons.stream().sorted(Comparator
.comparing(Person::isFavorite)
.thenComparing(Person::getCount).reversed())
.forEach(System.out::println);
Assuming you want to sort your persons and not filter them, you could do the following:
class Person implements Comparable<Person>
{
...
@Override
int compareTo(Person person)
{
// Maye not specified
if (person == null)
return (1);
// Compare 'favorite'
// Favorites first, so flag=true means smaller
if (this.favorite && (!person.favorite))
return (-1);
else if ((!this.favorite) && person.favorite)
return (1);
// Compare 'count'
// Having count > 0 first, so count=0 means greater
// Assuming count is never negative
if ((this.count == 0) && (person.count > 0))
return (1);
if ((this.count > 0) && (person.count == 0))
return (-1);
// At this point, both persons have the same 'favorite' flag
// And both have either count=0 or count>0
// Order by name in that case
return (this.name.compareTo(person.name));
} // compareTo
} // class Person
After that, sort your persons:
List<Person> my_persons;
my_persons = ....
Collections.sort(my_persons);
Comparator.comparing(...).thenComparing(...)?countin descending order.