Sorting using Comparator- Descending order User defined classes

Question

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

User · Answer

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.

User · Answer

String   s     a    x    y    Arrays sort s  new Comparator lt String gt            Override     public int compare String o1  String o2            return o2 compareTo o1             System out println Arrays toString s     - gt   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  lt  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 lt Person gt  new PersonComparator

User · Answer

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

User · Answer

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 lt Person gt        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 lt Person gt                public int compare Person p1  Person p2                        int age1   p1 getAge                int age2   p2 getAge                 if  age1    age2                  return 0              else if  age1  gt  age2                  return 1              else                 return -1                       public static void main String   args                List lt Person gt  people   new ArrayList lt Person gt             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

User · Answer

Using Google Collections   class Person    private int age    public static Function lt Person  Integer gt  GET AGE     new Function lt Person  Integer gt       public Integer apply Person p    return p age             public static void main String   args     ArrayList lt Person gt  people      Populate the list      Collections sort people  Ordering natural   onResultOf Person GET AGE  reverse

User · Answer

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  lt  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  lt  arr length  i               System out println arr i  age                Here is one way of doing it

User · Answer

You can do the descending sort of a user-defined class this way overriding the compare   method   Collections sort unsortedList new Comparator lt Person gt           Override     public int compare Person a  Person b            return b getName   compareTo a getName                 Or by using Collection reverse   to sort descending as user Prince mentioned in his comment   And you can do the ascending sort like this   Collections sort unsortedList new Comparator lt Person gt           Override     public int compare Person a  Person b            return a getName   compareTo b getName                 Replace the above code with a Lambda expression Java 8 onwards  we get concise   Collections sort personList   Person a  Person b  - gt  b getName   compareTo a getName        As of Java 8  List has sort   method which takes Comparator as parameter more concise     personList sort  a b - gt b getName   compareTo a getName        Here a and b are inferred as Person type by lambda expression

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

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