[java] Sorting using Comparator- Descending order (User defined classes)

I want to sort my objects in descending order using comparator.

class Person {
 private int age;
}

Here I want to sort a array of Person objects.

How can I do this?

Using Google Collections:

class Person {
 private int age;

 public static Function<Person, Integer> GET_AGE =
  new Function<Person, Integer> {
   public Integer apply(Person p) { return p.age; }
  };

}

public static void main(String[] args) {
 ArrayList<Person> people;
 // Populate the list...

 Collections.sort(people, Ordering.natural().onResultOf(Person.GET_AGE).reverse());
}

For whats its worth here is my standard answer. The only thing new here is that is uses the Collections.reverseOrder(). Plus it puts all suggestions into one example:

/*
**  Use the Collections API to sort a List for you.
**
**  When your class has a "natural" sort order you can implement
**  the Comparable interface.
**
**  You can use an alternate sort order when you implement
**  a Comparator for your class.
*/
import java.util.*;

public class Person implements Comparable<Person>
{
    String name;
    int age;

    public Person(String name, int age)
    {
        this.name = name;
        this.age = age;
    }

    public String getName()
    {
        return name;
    }

    public int getAge()
    {
        return age;
    }

    public String toString()
    {
        return name + " : " + age;
    }

    /*
    **  Implement the natural order for this class
    */
    public int compareTo(Person p)
    {
        return getName().compareTo(p.getName());
    }

    static class AgeComparator implements Comparator<Person>
    {
        public int compare(Person p1, Person p2)
        {
            int age1 = p1.getAge();
            int age2 = p2.getAge();

            if (age1 == age2)
                return 0;
            else if (age1 > age2)
                return 1;
            else
                return -1;
        }
    }

    public static void main(String[] args)
    {
        List<Person> people = new ArrayList<Person>();
        people.add( new Person("Homer", 38) );
        people.add( new Person("Marge", 35) );
        people.add( new Person("Bart", 15) );
        people.add( new Person("Lisa", 13) );

        // Sort by natural order

        Collections.sort(people);
        System.out.println("Sort by Natural order");
        System.out.println("\t" + people);

        // Sort by reverse natural order

        Collections.sort(people, Collections.reverseOrder());
        System.out.println("Sort by reverse natural order");
        System.out.println("\t" + people);

        //  Use a Comparator to sort by age

        Collections.sort(people, new Person.AgeComparator());
        System.out.println("Sort using Age Comparator");
        System.out.println("\t" + people);

        //  Use a Comparator to sort by descending age

        Collections.sort(people,
            Collections.reverseOrder(new Person.AgeComparator()));
        System.out.println("Sort using Reverse Age Comparator");
        System.out.println("\t" + people);
    }
}

package com.test;

import java.util.Arrays;

public class Person implements Comparable {

private int age;

private Person(int age) {
    super();
    this.age = age;
}

public int getAge() {
    return age;
}

public void setAge(int age) {
    this.age = age;
}

@Override
public int compareTo(Object o) {
    Person other = (Person)o;
    if (this == other)
        return 0;
    if (this.age < other.age) return 1;
    else if (this.age == other.age) return 0;
    else return -1;

}

public static void main(String[] args) {

    Person[] arr = new Person[4];
    arr[0] = new Person(50);
    arr[1] = new Person(20);
    arr[2] = new Person(10);
    arr[3] = new Person(90);

    Arrays.sort(arr);

    for (int i=0; i < arr.length; i++ ) {
        System.out.println(arr[i].age);
    }
}

}

Here is one way of doing it.

I would create a comparator for the person class that can be parametrized with a certain sorting behaviour. Here I can set the sorting order but it can be modified to allow sorting for other person attributes as well.

public class PersonComparator implements Comparator<Person> {

  public enum SortOrder {ASCENDING, DESCENDING}

  private SortOrder sortOrder;

  public PersonComparator(SortOrder sortOrder) {
    this.sortOrder = sortOrder;
  }

  @Override
  public int compare(Person person1, Person person2) {
    Integer age1 = person1.getAge();
    Integer age2 = person2.getAge();
    int compare = Math.signum(age1.compareTo(age2));

    if (sortOrder == ASCENDING) {
      return compare;
    } else {
      return compare * (-1);
    }
  }
}

(hope it compiles now, I have no IDE or JDK at hand, coded 'blind')

Edit

Thanks to Thomas, edited the code. I wouldn't say that the usage of Math.signum is good, performant, effective, but I'd like to keep it as a reminder, that the compareTo method can return any integer and multiplying by (-1) will fail if the implementation returns Integer.MIN_INTEGER... And I removed the setter because it's cheap enough to construct a new PersonComparator just when it's needed.

But I keep the boxing because it shows that I rely on an existing Comparable implementation. Could have done something like Comparable<Integer> age1 = new Integer(person1.getAge()); but that looked too ugly. The idea was to show a pattern which could easily be adapted to other Person attributes, like name, birthday as Date and so on.

The java.util.Collections class has a sort method that takes a list and a custom Comparator. You can define your own Comparator to sort your Person object however you like.

String[] s = {"a", "x", "y"};
Arrays.sort(s, new Comparator<String>() {

    @Override
    public int compare(String o1, String o2) {
        return o2.compareTo(o1);
    }
});
System.out.println(Arrays.toString(s));

-> [y, x, a]

Now you have to implement the Comparator for your Person class. Something like (for ascending order): compare(Person a, Person b) = a.id < b.id ? -1 : (a.id == b.id) ? 0 : 1 or Integer.valueOf(a.id).compareTo(Integer.valueOf(b.id)).

To minimize confusion you should implement an ascending Comparator and convert it to a descending one with a wrapper (like this) new ReverseComparator<Person>(new PersonComparator()).