Comparable and Comparator

Java supports sorting on collection in two ways - through Comparable and Comparator(both are functional interfaces with one single abstract method).

Comparable:

To sort collection based on Comparable:

• Elements of the collection must implement Comparable interface and implement it's method "int compareTo(Object o1)".
• Then sorting can be done using Collections.sort(List)

Cons:

• Can't support multiple types of sorting, as one compareTo method can have only one implementation.
• May not have access to the element class, in which case, subclassing the element and implementing compareTo would be the only option.

Code:

public class Dog implements Comparable<Dog>{

      String name;

      String breed;

      public Dog(String name,String breed){

            this.name = name;

            this.breed = breed;

      }

        @Override

public int compareTo(Dog o) {

return name.compareTo(o.name);

}

}

...

List<Dog> dogList = new ArrayList<Dog>();

Collections.sort(dogList);

...

Comparator:

To sort collection based on Comparator:

• A class/anonymous class implements Comparator interface and implements it's method "int compare(Object o1, Object o2). 
• Then sorting can be done by passing the object of the above implmented Comparator - using Collection.sort(List, Comparator).

Pros:

• Can support multiple sorting implementations.
• Don't need to have access to elements of the collection.

Code:

public class DogBreedComparator implements Comparator<Dog> {

@Override

public int compare(Dog o1, Dog o2) {

return o1.breed.compareTo(o2.breed);

}

}

..

Collections.sort(dogList, new DogBreedComparator());

..

Lambda expression:

Collections.sort(dogList, (o1,o2)->o1.breed.compareTo(o2.breed));

..

Both compareTo and compare methods return integer - 

1  : if passed in object is greater than current object

-1: if passed in object is lesser than current object

0: if both are equal

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