Sort a List of objects by multiple fields

Question

I have a List of Java objects that I want to sort according to more than one field   public class graduationCeremony       String campus      String faculty      String building      Is it possible to use a Comparator or the Comparable interface to sort the list according to multiple fields  All the examples I have seen sort according to only one field  In other words  one can sort by  campus  OR  faculty  OR  building   I want to sort by  campus   then  faculty   then  building   as it exists in SQL  ORDER BY campus  faculty  building   I think this question has been asked before  but I don t understand the accepted answer  Can someone expand or illustrate this answer

User · Answer

If you know in advance which fields to use to make the comparison, then other people gave right answers.
What you may be interested in is to sort your collection in case you don't know at compile-time which criteria to apply. Imagine you have a program dealing with cities:



    protected Set<City> cities;
    (...)
    Field temperatureField = City.class.getDeclaredField("temperature");
    Field numberOfInhabitantsField = City.class.getDeclaredField("numberOfInhabitants");
    Field rainfallField = City.class.getDeclaredField("rainfall");
    program.showCitiesSortBy(temperatureField, numberOfInhabitantsField, rainfallField);
    (...)
    public void showCitiesSortBy(Field... fields) {
        List<City> sortedCities = new ArrayList<City>(cities);
        Collections.sort(sortedCities, new City.CityMultiComparator(fields));
        for (City city : sortedCities) {
            System.out.println(city.toString());
        }
    }

where you can replace hard-coded field names by field names deduced from a user request in your program.

In this example, City.CityMultiComparator<City> is a static nested class of class City implementing Comparator:



    public static class CityMultiComparator implements Comparator<City> {
        protected List<Field> fields;

        public CityMultiComparator(Field... orderedFields) {
            fields = new ArrayList<Field>();
            for (Field field : orderedFields) {
                fields.add(field);
            }
        }

        @Override
        public int compare(City cityA, City cityB) {
            Integer score = 0;
            Boolean continueComparison = true;
            Iterator itFields = fields.iterator();

            while (itFields.hasNext() && continueComparison) {
                Field field = itFields.next();
                Integer currentScore = 0;
                if (field.getName().equalsIgnoreCase("temperature")) {
                    currentScore = cityA.getTemperature().compareTo(cityB.getTemperature());
                } else if (field.getName().equalsIgnoreCase("numberOfInhabitants")) {
                    currentScore = cityA.getNumberOfInhabitants().compareTo(cityB.getNumberOfInhabitants());
                } else if (field.getName().equalsIgnoreCase("rainfall")) {
                    currentScore = cityA.getRainfall().compareTo(cityB.getRainfall());
                }
                if (currentScore != 0) {
                    continueComparison = false;
                }
                score = currentScore;
            }

            return score;
        }
    }

You may want to add an extra layer of precision, to specify, for each field, whether sorting should be ascendant or descendant. I guess a solution is to replace Field objects by objects of a class you could call SortedField, containing a Field object, plus another field meaning ascendant or descendant.

User · Answer

Yes  you absolutely can do this  For example   public class PersonComparator implements Comparator lt Person gt        public int compare Person p1  Person p2                   Assume no nulls  and simple ordinal comparisons             First by campus - stop if this gives a result          int campusResult   p1 getCampus   compareTo p2 getCampus             if  campusResult    0                        return campusResult                        Next by faculty         int facultyResult   p1 getFaculty   compareTo p2 getFaculty             if  facultyResult    0                        return facultyResult                        Finally by building         return p1 getBuilding   compareTo p2 getBuilding               Basically you re saying   If I can tell which one comes first just by looking at the campus  before they come from different campuses  and the campus is the most important field  then I ll just return that result  Otherwise  I ll continue on to compare faculties  Again  stop if that s enough to tell them apart  Otherwise   if the campus and faculty are the same for both people  just use the result of comparing them by building

User · Answer

You have to write your own compareTo   method that has the Java code needed to perform the comparison   If we wanted for example to compare two public fields  campus  then faculty  we might do something like   int compareTo GraduationCeremony gc        int c   this campus compareTo gc campus        if  c    0                   sort by campus if we can         return c            else                 campus equal  so sort by faculty         return this faculty compareTo gc faculty             This is simplified but hopefully gives you an idea  Consult the Comparable and Comparator docs for more info

User · Answer

You just need to have your class inherit from Comparable   then implement the compareTo method the way you like

User · Answer

Your Comparator would look like this   public class GraduationCeremonyComparator implements Comparator lt GraduationCeremony gt        public int compare GraduationCeremony o1  GraduationCeremony o2            int value1   o1 campus compareTo o2 campus           if  value1    0                int value2   o1 faculty compareTo o2 faculty               if  value2    0                    return o1 building compareTo o2 building                 else                   return value2                                  return value1            Basically it continues comparing each successive attribute of your class whenever the compared attributes so far are equal     0

User · Answer

Hope this Helps   import java util ArrayList  import java util Collections  import java util Comparator  import java util Iterator   class Person implements Comparable     String firstName  lastName     public Person String f  String l        this firstName   f      this lastName   l         public String getFirstName         return firstName         public String getLastName         return lastName         public String toString         return    firstname     firstName     lastname     lastName               public int compareTo Object obj        Person emp    Person  obj      int deptComp   firstName compareTo emp getFirstName          return   deptComp    0    lastName compareTo emp getLastName      deptComp          public boolean equals Object obj        if    obj instanceof Person           return false            Person emp    Person  obj      return firstName equals emp getFirstName     amp  amp  lastName equals emp getLastName            class PersonComparator implements Comparator lt Person gt      public int compare Person emp1  Person emp2        int nameComp   emp1 getLastName   compareTo emp2 getLastName         return   nameComp    0    emp1 getFirstName   compareTo emp2 getFirstName      nameComp          public class Main     public static void main String args          ArrayList lt Person gt  names   new ArrayList lt Person gt         names add new Person  E    T         names add new Person  A    G         names add new Person  B    H         names add new Person  C    J          Iterator iter1   names iterator        while  iter1 hasNext            System out println iter1 next               Collections sort names  new PersonComparator         Iterator iter2   names iterator        while  iter2 hasNext            System out println iter2 next

[java] Sort a List of objects by multiple fields

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