Question

I have a list of java object that have 4 members.

int id;
String name;
String age;
int order;

I am using his code to sort object fields in ascending order according to if condition.

Collections.sort(list, new Comparator<com.MyObject>() {

          public int compare(com.MyObject object1, com.MyObject object2) {

            if(myString.equalsIgnoreCase("name")){
            return object1.getName().compareTo(object2.getName());

          }else if(myString.equalsIgnoreCase("age")){
            return object1.getAge().compareTo(object2.getAge());

          }else{
            return object1.getOrder() - object2.getOrder();
          }
      }
  });

I want to add another condition if(myString.equalsIgnoreCase("nameReverse")) If this will be the case the list should be sorted in descending order.

Answer 1

The Collections class has a reverseOrder method which returns a reverse Comparator for a given comparator. IMO, it would be more cleaner to not modify the Comparator if all you want to do is get a reverse Comparator.

A bit of sample code:

public class Person {

    public static void testComparator() {
        Person p1 = new Person("c");
        Person p2 = new Person("a");
        Person p3 = new Person("b");
        List<Person> persons = Arrays.asList(p1, p2, p3);
        Collections.sort(persons, PersonComparator.INSTANCE);
        System.out.println("After normal sorting: " + persons);
        Collections.sort(persons, Collections.reverseOrder(PersonComparator.INSTANCE));
        System.out.println("After reverse sorting: " + persons);
    }

    public Person(String name) { this.name = name; }

    public String name;

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("Person [name=").append(name).append("]");
        return builder.toString();
    }

}

class PersonComparator implements Comparator<Person> {

    public static final PersonComparator INSTANCE = new PersonComparator();

    @Override
    public int compare(Person o1, Person o2) {
        return o1.name.compareTo(o2.name);
    }

}