Sorting using Comparator- Descending order (User defined classes)

Question

Hi,

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 to do? Please give me a solution,

Thanks.

Answer 1

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()).

Answer 2

The Java.util.collections class has a sort method that takes a list and a custom Comparator. You can define you comparter to sort your person object however you like.

Answer 3

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.

Answer 4

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.

Answer 5

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());
}

Answer 6

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);
    }
}