Core Java - Difference between Comparator and Comparable in Java

Comparable interface: Class whose objects to be sorted must implement this interface.In this,we have to implement compareTo(Object) method.

For example:

	public class Country implements Comparable{
	@Override

	public int compareTo(Object arg0) {
	Country country=(Country) arg0;

	return (this.countryId < country.countryId ) ? -1: (this.countryId > country.countryId ) ? 1:0 ;
	}}

If any class implements comparable inteface then collection of that object can be sorted automatically using Collection.sort() or Arrays.sort().Object will be sort on the basis of compareTo method in that class.

Objects which implement Comparable in java can be used as keys in a SortedMap like TreeMap or SortedSet like TreeSet without implementing any other interface.

Comparator interface: Class whose objects to be sorted do not need to implement this interface.Some third class can implement this interface to sort.

Ex:- CountrySortByIdComparator class can implement Comparator interface to sort collection of country object by id. For example:

	public class CountrySortByIdComparator implements Comparator<Country>{

	@Override
	public int compare(Country country1, Country country2) {

	return (country1.getCountryId() < country2.getCountryId() ) ? -1: (country1.getCountryId() > country2.getCountryId() ) ? 1:0 ;

	}

}

Using Comparator interface,we can write different sorting based on different attributes of objects to be sorted.You can use anonymous comparator to compare at particular line of code. For example:

	Country indiaCountry=new Country(1, 'India');
	Country chinaCountry=new Country(4, 'China');

	Country nepalCountry=new Country(3, 'Nepal');
	Country bhutanCountry=new Country(2, 'Bhutan');

	List<Country> listOfCountries = new ArrayList<Country>();

	listOfCountries.add(indiaCountry);
	listOfCountries.add(chinaCountry);

	listOfCountries.add(nepalCountry);
	listOfCountries.add(bhutanCountry);

	//Sort by countryName

	Collections.sort(listOfCountries,new Comparator<Country>() {

	@Override

	public int compare(Country o1, Country o2) {

	return o1.getCountryName().compareTo(o2.getCountryName());
	}

});