How do I sort a list by different parameters at different timed

Question

I have a class named Person with multiple properties, for example:

public class Person {     private int id;     private String name, address;     // Many more properties. } 

A lot of Person-objects are stored in an ArrayList<Person>. I want to sort this list by multiple sort parameters, and different from time to time. For instance I might one time want to sort by name ascending and then address descending, and another time just by id descending.

And I don't want to create my own sort methods (i.e., I want to use Collections.sort(personList, someComparator). What is the most elegant solution that achieves this?

Answer 1

I think your enum approach is basically sound, but the switch statements really need a more object oriented approach. Consider:

enum PersonComparator implements Comparator<Person> {     ID_SORT {         public int compare(Person o1, Person o2) {             return Integer.valueOf(o1.getId()).compareTo(o2.getId());         }},     NAME_SORT {         public int compare(Person o1, Person o2) {             return o1.getFullName().compareTo(o2.getFullName());         }};      public static Comparator<Person> decending(final Comparator<Person> other) {         return new Comparator<Person>() {             public int compare(Person o1, Person o2) {                 return -1 * other.compare(o1, o2);             }         };     }      public static Comparator<Person> getComparator(final PersonComparator... multipleOptions) {         return new Comparator<Person>() {             public int compare(Person o1, Person o2) {                 for (PersonComparator option : multipleOptions) {                     int result = option.compare(o1, o2);                     if (result != 0) {                         return result;                     }                 }                 return 0;             }         };     } } 

An example of usage (with a static import).

public static void main(String[] args) {     List<Person> list = null;     Collections.sort(list, decending(getComparator(NAME_SORT, ID_SORT))); } 

Answer 2

You can create comparators for each of properties you might want to sort and then try "comparator chaining" :-) like this:

public class ChainedComparator<T> implements Comparator<T> {     private List<Comparator<T>> simpleComparators;      public ChainedComparator(Comparator<T>... simpleComparators) {         this.simpleComparators = Arrays.asList(simpleComparators);     }     public int compare(T o1, T o2) {         for (Comparator<T> comparator : simpleComparators) {             int result = comparator.compare(o1, o2);             if (result != 0) {                 return result;             }         }         return 0;     } }