What is the difference between Comparable and Comparator?
When to use Comparable and when to use Comparator?
2 answers
Classes implement Comparable, so that you can then sort by implementing the compareTo(Object) method.
If the class implements this interface, you can then use Collection.sort() or Arrays.sort(). The objects will be sorted based on the implementation of the compareTo(Object) method.
For example:
public class Country implements Comparable<Country>{
@Override
public int compareTo(Country country) {
return (this.countryId < country.countryId ) ? -1: (this.countryId > country.countryId ) ? 1:0 ;
}
}
When calling Collection.sort() on a collection of objects of this class, they will be compared based on compareTo(Country country).
And Comparator is used to implement sorting by custom field, tipo:
List<Country> listOfCountries = new ArrayList<Country>();
[...]
Collections.sort(listOfCountries,new Comparator<Country>() {
@Override
public int compare(Country o1, Country o2) {
return o1.getCountryName().compareTo(o2.getCountryName());
}
});
The objects will be sorted based on the comparison of the country names.
To summarize, then:
Comparable - implemented inside the class. Essentially, it defines the normal / natural order of comparing objects.
Comparator - implemented outside the class. You can implement different sorting options based on comparing different fields.
Comparable-implemented by the class itself when the natural sort order is needed. An example is the String class.
Comparator-implemented by other classes. Allows you to separate the comparison implementation from the class and make several comparison implementations for different parameters for the same class.
| Comparable | Comparator
Package | java.lang | java.util
Functional Interface | Yes | Yes
Method | int o1.compareTo(o2) | int comparator.compare(o1, o2)
-1 if | o1 < o2 | Same
1 if | o1 > o2 | Same
0 if | o1 == o2 | Same
Implement | Class itself | Separate classes
Defines | Natural ordering | Sorting by different parameters
Examples | String, Date, Wrappers | For third-party classes