Sorting an ArrayList of objects using a custom sorting order

Question

I am looking to implement a sort feature for my address book application   I want to sort an ArrayList lt Contact gt  contactArray  Contact is a class which contains four fields  name  home number  mobile number and address  I want to sort on name   How can I write a custom sort function to do this

User · Answer

By using lambdaj you can sort a collection of your contacts (for example by their name) as it follows

sort(contacts, on(Contact.class).getName());

or by their address:

sort(contacts, on(Contacts.class).getAddress());

and so on. More in general, it offers a DSL to access and manipulate your collections in many ways, like filtering or grouping your contacts based on some conditions, aggregate some of their property values, etc.

User · Answer

The Collections sort is a good sort implementation  If you don t have The comparable implemented for Contact  you will need to pass in a Comparator implementation  Of note      The sorting algorithm is a modified mergesort  in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist   This algorithm offers guaranteed n log n  performance  The specified list must be modifiable  but need not be resizable  This implementation dumps the specified list into an array  sorts the array  and iterates over the list resetting each element from the corresponding position in the array  This avoids the n2 log n  performance that would result from attempting to sort a linked list in place     The merge sort is probably better than most search algorithm you can do

User · Answer

You need make your Contact classes implement Comparable  and then implement the compareTo Contact  method   That way  the Collections sort will be able to sort them for you   Per the page I linked to  compareTo  returns a negative integer  zero  or a positive integer as this object is less than  equal to  or greater than the specified object    For example  if you wanted to sort by name  A to Z   your class would look like this   public class Contact implements Comparable lt Contact gt         private String name          all the other attributes and methods      public compareTo Contact other            return this name compareTo other name

User · Answer

You shoud use the Arrays sort function  The containing classes should implement Comparable

User · Answer

I did it by the following way  number and name are two arraylist  I have to sort name  If any change happen to name arralist order then the number arraylist also change its order   public void sortval             String tempname    tempnum               if  name size   gt 1     check if the number of orders is larger than 1                               for  int x 0  x lt name size    x       bubble sort outer loop                                       for  int i 0  i  lt  name size  -x-1  i                              if  name get i  compareTo name get i 1    gt  0                                                         tempname   name get i                                tempnum number get i                                name set i name get i 1                                name set i 1  tempname                                number set i number get i 1                                 number set i 1  tempnum

User · Answer

Here s a tutorial about ordering objects   The Java Tutorials - Collections - Object Ordering  Although I will give some examples  I would recommend to read it anyway   There are various way to sort an ArrayList  If you want to define a natural  default  ordering  then you need to let Contact implement Comparable  Assuming that you want to sort by default on name  then do  nullchecks omitted for simplicity   public class Contact implements Comparable lt Contact gt         private String name      private String phone      private Address address        Override     public int compareTo Contact other            return name compareTo other name                 Add generate getters setters and other boilerplate     so that you can just do List lt Contact gt  contacts   new ArrayList lt Contact gt        Fill it   Collections sort contacts     If you want to define an external controllable ordering  which overrides the natural ordering   then you need to create a Comparator  List lt Contact gt  contacts   new ArrayList lt Contact gt        Fill it      Now sort by address instead of name  default   Collections sort contacts  new Comparator lt Contact gt          public int compare Contact one  Contact other            return one getAddress   compareTo other getAddress                  You can even define the Comparators in the Contact itself so that you can reuse them instead of recreating them everytime  public class Contact        private String name      private String phone      private Address address                   public static Comparator lt Contact gt  COMPARE BY PHONE   new Comparator lt Contact gt              public int compare Contact one  Contact other                return one phone compareTo other phone                         public static Comparator lt Contact gt  COMPARE BY ADDRESS   new Comparator lt Contact gt              public int compare Contact one  Contact other                return one address compareTo other address                        which can be used as follows  List lt Contact gt  contacts   new ArrayList lt Contact gt        Fill it      Sort by address  Collections sort contacts  Contact COMPARE BY ADDRESS       Sort later by phone  Collections sort contacts  Contact COMPARE BY PHONE     And to cream the top off  you could consider to use a generic javabean comparator  public class BeanComparator implements Comparator lt Object gt         private String getter       public BeanComparator String field            this getter    quot get quot    field substring 0  1  toUpperCase     field substring 1              public int compare Object o1  Object o2            try               if  o1    null  amp  amp  o2    null                    o1   o1 getClass   getMethod getter  new Class 0   invoke o1  new Object 0                    o2   o2 getClass   getMethod getter  new Class 0   invoke o2  new Object 0                            catch  Exception e                   If this exception occurs  then it is usually a fault of the developer              throw new RuntimeException  quot Cannot compare  quot    o1    quot  with  quot    o2    quot  on  quot    getter  e                      return  o1    null    -1     o2    null    1     Comparable lt Object gt   o1  compareTo o2              which you can use as follows     Sort on  quot phone quot  field of the Contact bean  Collections sort contacts  new BeanComparator  quot phone quot       as you see in the code  possibly null fields are already covered to avoid NPE s during sort

User · Answer

use this method   private ArrayList lt myClass gt  sortList ArrayList lt myClass gt  list        if  list    null  amp  amp  list size    gt  1            Collections sort list  new Comparator lt myClass gt                  public int compare myClass o1  myClass o2                    if  o1 getsortnumber      o2 getsortnumber    return 0                  return o1 getsortnumber    lt  o2 getsortnumber     1   -1                                      return list         and use  mySortedlist   sortList myList   No need to implement comparator in your class  If you want inverse order swap 1 and -1

User · Answer

This page tells you all you need to know about sorting collections  such as ArrayList   Basically you need to   make your Contact class implement the Comparable interface by   creating a method public int compareTo Contact anotherContact  within it   Once you do this  you can just call Collections sort myContactList      where myContactList is ArrayList lt Contact gt   or any other collection of Contact      There s another way as well  involving creating a Comparator class  and you can read about that from the linked page as well   Example   public class Contact implements Comparable lt Contact gt                     return -1 for less than  0 for equals  and 1 for more than     public compareTo Contact anotherContact            int result   0          result   getName   compareTo anotherContact getName             if  result    0                        return result                    result   getNunmber   compareTo anotherContact getNumber             if  result    0                        return result

User · Answer

In addition to what was already posted you should know that since Java 8 we can shorten our code and write it like   Collection sort yourList  Comparator comparing YourClass  getFieldToSortOn      or since List now have sort method   yourList sort Comparator comparing YourClass  getFieldToSortOn      Explanation   Since Java 8  functional interfaces  interfaces with only one abstract method - they can have more default or static methods  can be easily implemented using    lambdas arguments - gt  body  or method references source  method    Since Comparator lt T gt  has only one abstract method int compare T o1  T o2  it is functional interface   So instead of  example from  BalusC answer   Collections sort contacts  new Comparator lt Contact gt          public int compare Contact one  Contact other            return one getAddress   compareTo other getAddress                  we can reduce this code to   Collections sort contacts   Contact one  Contact other  - gt         return one getAddress   compareTo other getAddress           We can simplify this  or any  lambda by skipping    argument types  Java will infer them based on method signature   or  return         So instead of     Contact one  Contact other  - gt         return one getAddress   compareTo other getAddress        we can write   one  other  - gt  one getAddress   compareTo other getAddress      Also now Comparator has static methods like comparing FunctionToComparableValue  or comparing FunctionToValue  ValueComparator  which we could use to easily create Comparators which should compare some specific values from objects    In other words we can rewrite above code as  Collections sort contacts  Comparator comparing Contact  getAddress       assuming that Address implements Comparable  provides default order

User · Answer

Ok  I know this was answered a long time ago    but  here s some new info   Say the Contact class in question already has a defined natural ordering via implementing Comparable  but you want to override that ordering  say by name   Here s the modern way to do it   List lt Contact gt  contacts         contacts sort Comparator comparing Contact  getName  reversed   thenComparing Comparator naturalOrder       This way it will sort by name first  in reverse order   and then for name collisions it will fall back to the  natural  ordering implemented by the Contact class itself

User · Answer

BalusC and bguiz have already given very complete answers on how to use Java s built-in Comparators   I just want to add that google-collections has an Ordering class which is more  powerful  than the standard Comparators  It might be worth checking out  You can do cool things such as compounding Orderings  reversing them  ordering depending on a function s result for your objects     Here is a blog post that mentions some of its benefits

[java] Sorting an ArrayList of objects using a custom sorting order

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