how to sort an ArrayList in ascending order using Collections and Comparator

Question

How to sort an ArrayList in ascending order using Comparator  I know how to sort it in descending order using    Comparator mycomparator   Collections reverseOrder      then  Collections sort myarrayList mycomparator     just want to know how to sort it in ascending order using Collections and comparator  Thanks

User · Answer

Sort By Value    public Map sortByValue Map map  final boolean ascending                Map result   new LinkedHashMap                try                   List list   new LinkedList map entrySet                      Collections sort list  new Comparator                          Override                     public int compare Object object1  Object object2                            if  ascending                              return   Comparable    Map Entry   object1   getValue                                         compareTo   Map Entry   object2   getValue                             else                             return   Comparable    Map Entry   object2   getValue                                         compareTo   Map Entry   object1   getValue                                                                 for  Iterator it   list iterator    it hasNext                           Map Entry entry    Map Entry  it next                        result put entry getKey    entry getValue                                      catch  Exception e                    Log e  Error   e getMessage                                return result

User · Answer

Use the default version   Collections sort myarrayList     Of course this requires that your Elements implement Comparable  but the same holds true for the version you mentioned   BTW  you should use generics in your code  that way you get compile-time errors if your class doesn t implement Comparable  And compile-time errors are much better than the runtime errors you ll get otherwise   List lt MyClass gt  list   new ArrayList lt MyClass gt        now fill up the list     compile error here unless MyClass implements Comparable Collections sort list

User · Answer

This might work   Comparator mycomparator        Collections reverseOrder Collections reverseOrder

User · Answer

Just throwing this out there   Can t you just do   Collections sort myarrayList     It s been awhile though

User · Answer

Two ways to get this done   Collections sort myArray    given elements inside myArray implements Comparable  Second  Collections sort myArray  new MyArrayElementComparator       where MyArrayElementComparator is Comparator for elements inside myArray

User · Answer

Here a complete example    Suppose we have a Person class like    public class Person       protected String fname      protected String lname       public Person                     public Person String fname  String lname                this fname   fname          this lname   lname             public boolean equals Object objet                if objet instanceof Person                        Person p    Person  objet              return  p getFname   equals this fname    amp  amp  p getLname   equals this lname                      else return super equals objet               Override     public String toString                 return  Person fname       getFname      lname       getLname                       Getters and Setters         Now we create a comparator     import java util Comparator   public class ComparePerson implements Comparator lt Person gt         Override     public int compare Person p1  Person p2                if p1 getFname   equalsIgnoreCase p2 getFname                           return p1 getLname   compareTo p2 getLname                       return p1 getFname   compareTo p2 getFname               Finally suppose we have a group of persons     import java util ArrayList  import java util Collections  import java util Comparator  import java util List   public class Group       protected List lt Person gt  listPersons       public Group                 this listPersons   new ArrayList lt Person gt                public Group List lt Person gt  listPersons                this listPersons   listPersons             public void order boolean asc                Comparator lt Person gt  comp   asc   new ComparePerson     Collections reverseOrder new ComparePerson             Collections sort this listPersons  comp              public void display                 for Person p   this listPersons                        System out println p                            Getters and Setters         Now we try this    import java util ArrayList  import java util List   public class App       public static void main String   args                Group g   new Group            List listPersons   new ArrayList lt Person gt             g setListPersons listPersons            Person p           p   new Person  A    B            listPersons add p            p   new Person  C    D            listPersons add p                you can add Person as many as you want              g display             g order true           g display             g order false           g display

[java] how to sort an ArrayList in ascending order using Collections and Comparator

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