Java Language Natural (comparable) vs explicit (comparator) sorting


Example

There are two Collections.sort() methods:

  • One that takes a List<T> as a parameter where T must implement Comparable and override the compareTo() method that determines sort order.
  • One that takes a List and a Comparator as the arguments, where the Comparator determines the sort order.

First, here is a Person class that implements Comparable:

public class Person implements Comparable<Person> {         
    private String name;  
    private int age;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }         

    @Override
    public int compareTo(Person o) {
        return this.getAge() - o.getAge();
    }
    @Override
    public String toString() {
        return this.getAge()+"-"+this.getName();
    }

}

Here is how you would use the above class to sort a List in the natural ordering of its elements, defined by the compareTo() method override:

//-- usage
List<Person> pList = new ArrayList<Person>();
            Person p = new Person();
            p.setName("A");
            p.setAge(10);
            pList.add(p);
            p = new Person();
            p.setName("Z");
            p.setAge(20);
            pList.add(p);
            p = new Person();
            p.setName("D");
            p.setAge(30);
            pList.add(p);
            
            //-- natural sorting i.e comes with object implementation, by age
            Collections.sort(pList);

            System.out.println(pList);

Here is how you would use an anonymous inline Comparator to sort a List that does not implement Comparable, or in this case, to sort a List in an order other than the natural ordering:

            //-- explicit sorting, define sort on another property here goes with name
            Collections.sort(pList, new Comparator<Person>() {

                @Override
                public int compare(Person o1, Person o2) {
                    return o1.getName().compareTo(o2.getName());
                }
            });            
            System.out.println(pList);