Sorting a collection of objects [duplicate]

Answer 1

Implement 3 different types of Comparator.

you can add the comparator to the sort command. The comparator you define, will sort the elements by name, age, or what ever.

Collections.sort(list, new Comparator() {

        public int compare(Object arg0, Object arg1) {
            if (!(arg0 instanceof Person)) {
                return -1;
            }
            if (!(arg1 instanceof Person)) {
                return -1;
            }

            Person pers0 = (Person)arg0;
            Person pers1 = (Person)arg1;


            // COMPARE NOW WHAT YOU WANT
            // Thanks to Steve Kuo for your comment!
            return pers0.getAge() - pers1.getAge();
        }
    });

Answer 2

Implement the Comparator interface (once for each different sort order) and use the Collections.sort() method that takes a Comparator as additional parameter.

Answer 3

Using lambdaj ( http://code.google.com/p/lambdaj/ ) you can achieve what you're asking in the following way:

sort(personList, on(Person.class).getName());

sort(personList, on(Person.class).getAge());

sort(personList, on(Person.class).getCountry());

Answer 4

You could also use the BeanComparator from apache commons beanutils, like this:

Collections.sort(personList, new BeanComparator("name"));

Answer 5

I asked a very similar question (about searching rather than sorting), perhaps there is some useful information (I ended up using an enum that implements Comparator so I pass the enum value as a comparator selector).

Answer 6

Thanks to the responders. For the benefit of others, I'd like to include a complete example.

The solution is the create the following additional classes:

public class NameComparator implements Comparator<Person>
{
    public int compare(Person o1, Person o2)
    {
       return o1.getName().compareTo(o2.getName());
   }
}

public class AgeComparator implements Comparator<Person>
{
    public int compare(Person o1, Person o2)
    {
        return o1.getAge().compareTo(o2.getAge());
    }
}

public class CountryComparator implements Comparator<Person>
{
    public int compare(Person o1, Person o2)
    {
        return o1.getCountry().compareTo(o2.getCountry());
    }
}

The list can then be sorted like this:

Collections.sort(personList, new NameComparator());
Collections.sort(personList, new AgeComparator());
Collections.sort(personList, new CountryComparator());