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Sorting list of objects in natural order

Sort a list of objects in natural order without comparator.

Code Explanation

The java.lang.Comparable interface imposes an ordering on a collection of objects. This ordering is called natural ordering of objects. Collection of objects that implement java.lang.Comparable interface can be sorted automatically using Collections#sort() or Arrays.sort() methods without using java.util.Comparator.

Let us have Cat object and every cat has a name, age and sex. The Cat class implements java.lang.Comparable interface and implements the only method compareTo(). Then in CatNaturalOrdering class we would create a list of cats and then sort this list.

  9 public class Cat implements Comparable<Cat> {
 10
 11     /**
 12      * An enum of sex.
 13      */
 14     public enum Sex {
 15
 16         /**
 17          * Male.
 18          */
 19         MALE,
 20         /**
 21          * Female.
 22          */
 23         FEMALE;
 24     }
 25     /**
 26      * Name.
 27      */
 28     private String name;
 29     /**
 30      * Age.
 31      */
 32     private int age;
 33     /**
 34      * Sex.
 35      */
 36     private Sex sex;
 37
 38     /**
 39      * Constructs a new Cat instance.
 40      * @param name Name of the cat.
 41      * @param age Age of the cat.
 42      * @param sex Sex of the cat.
 43      */
 44     public Cat(String name, int age, Sex sex) {
 45         this.name = name;
 46         this.age = age;
 47         this.sex = sex;
 48     }
107 }
108

Now let us see the definition of compareTo() method which compares two given cats based on their name.

103     @Override
104     public int compareTo(Cat newCat) {
105         return this.name.compareTo(newCat.getName());
106     }
107

The main() method of CatNaturalOrdering class creates multiple cat objects and adds them to a list. And then Collections#sort() method is called to sort the list in natural order of Cat class.

20     public static void main(String[] args) {
21         Cat cat1 = new Cat("Magic", 5, Sex.MALE);
22         Cat cat2 = new Cat("Ajax", 8, Sex.MALE);
23         Cat cat3 = new Cat("Alpha", 6, Sex.FEMALE);
24         Cat cat4 = new Cat("Gigolo", 1, Sex.MALE);
25         Cat cat5 = new Cat("Jessica", 2, Sex.FEMALE);
26         List<Cat> cats = new ArrayList<Cat>();
27         cats.add(cat1);
28         cats.add(cat2);
29         cats.add(cat3);
30         cats.add(cat4);
31         cats.add(cat5);
32         CatNaturalOrdering sortTest = new CatNaturalOrdering();
33         sortTest.printCats("############# List Of Cats #############", cats);
34         Collections.sort(cats);
35         sortTest.printCats(
36                 "############# Natural Ordering of Cats #############", cats);
37     }
38

Input and Output

Following Cats were given as input:

    "Magic", 5, MALE
    "Ajax", 8, MALE
    "Alpha", 6, FEMALE
    "Gigolo", 1, MALE
    "Jessica", 2, FEMALE

The output of the program is:

    ############# List Of Cats #############
    name: Magic  age: 5  sex: MALE
    name: Ajax  age: 8  sex: MALE
    name: Alpha  age: 6  sex: FEMALE
    name: Gigolo  age: 1  sex: MALE
    name: Jessica  age: 2  sex: FEMALE

    ############# Natural Ordering of Cats #############
    name: Ajax  age: 8  sex: MALE
    name: Alpha  age: 6  sex: FEMALE
    name: Gigolo  age: 1  sex: MALE
    name: Jessica  age: 2  sex: FEMALE
    name: Magic  age: 5  sex: MALE

Download

You may download the complete source code from here: