Get unique values from arraylist in java

Question

I have an ArrayList with a number of records and one column contains gas names as CO2 CH4 SO2 etc Now i want to retrieve different gas names unique  only without repetation from the ArrayList  How can it be done

User · Answer

You should use a Set  A Set is a Collection that contains no duplicates   If you have a List that contains duplicates  you can get the unique entries like this   List lt String gt  gasList      create list with duplicates    Set lt String gt  uniqueGas   new HashSet lt String gt  gasList   System out println  Unique gas count      uniqueGas size       NOTE  This HashSet constructor identifies duplicates by invoking the elements  equals   methods

User · Answer

When I was doing the same query  I had hard time adjusting the solutions to my case  though all the previous answers have good insights    Here is a solution when one has to acquire a list of unique objects  NOT strings   Let s say  one has a list of Record object  Record class has only properties of type String  NO property of type int   Here implementing hashCode   becomes difficult as hashCode   needs to return an int   The following is a sample Record Class   public class Record       String employeeName      String employeeGroup       Record String name  String group             employeeName  name          employeeGroup   group                public String getEmployeeName            return employeeName            public String getEmployeeGroup            return employeeGroup            Override     public boolean equals Object o            if o instanceof Record               if    Record  o  employeeGroup equals employeeGroup   amp  amp                      Record  o  employeeName equals employeeName                    return true                                    return false              Override     public int hashCode       this should return a unique code         int hash   3    this could be anything  but I would chose a prime e g  5  7  11             again  the multiplier could be anything like 59 79 89  any prime         hash   89   hash   Objects hashCode this employeeGroup            return hash          As suggested earlier by others  the class needs to override both the equals   and the hashCode   method to be able to use HashSet   Now  let s say  the list of Records is allRecord List lt Record gt  allRecord     Set lt Record gt  distinctRecords   new HashSet lt  gt      for Record rc  allRecord       distinctRecords add rc       This will only add the distinct Records to the Hashset  distinctRecords   Hope this helps

User · Answer

Here s straightforward way without resorting to custom comparators or stuff like that   Set lt String gt  gasNames   new HashSet lt String gt     List lt YourRecord gt  records         for YourRecord record   records      gasNames add record getGasName           now gasNames is a set of unique gas names  which you could operate on  List lt String gt  sortedGasses   new ArrayList lt String gt  gasNames   Collections sort sortedGasses     Note  Using TreeSet instead of HashSet would give directly sorted arraylist and above Collections sort could be skipped  but TreeSet is otherwise less efficent  so it s often better  and rarely worse  to use HashSet even when sorting is needed

User · Answer

You can use Java 8 Stream API  Method distinct is an intermediate operation that filters the stream and allows only distinct values  by default using the Object  equals method  to pass to the next operation  I wrote an example below for your case     Create the list with duplicates  List lt String gt  listAll   Arrays asList  quot CO2 quot    quot CH4 quot    quot SO2 quot    quot CO2 quot    quot CH4 quot    quot SO2 quot    quot CO2 quot    quot CH4 quot    quot SO2 quot        Create a list with the distinct elements using stream  List lt String gt  listDistinct   listAll stream   distinct   collect Collectors toList         Display them to terminal using stream  collect with a build in Collector  String collectAll   listAll stream   collect Collectors joining  quot    quot     System out println collectAll       gt  CO2  CH4  SO2  CO2  CH4 etc   String collectDistinct   listDistinct stream   collect Collectors joining  quot    quot     System out println collectDistinct       gt  CO2  CH4  SO2

User · Answer

public static List getUniqueValues List input          return new ArrayList lt  gt  new LinkedHashSet lt  gt  incoming           dont forget to implement your equals method first

User · Answer

If you have an array of a some kind of object  bean  you can do this   List lt aBean gt  gasList   createDuplicateGasBeans    Set lt aBean gt  uniqueGas   new HashSet lt aBean gt  gasList     like said Mathias Schwarz above  but you have to provide your aBean with the methods hashCode   and equals Object obj  that can be done easily in Eclipse by dedicated menu  Generate hashCode   and equals     while in the bean Class   Set will evaluate the overridden methods to discriminate equals objects

User · Answer

you can use this for making a list Unique  ArrayList lt String gt  listWithDuplicateValues   new ArrayList lt  gt     list add  first    list add  first    list add  second     ArrayList uniqueList    ArrayList  listWithDuplicateValues stream   distinct   collect Collectors toList

User · Answer

ArrayList values          your values Set uniqueValues   new HashSet values     now unique

User · Answer

I hope I understand your question correctly  assuming that the values are of type String  the most efficient way is probably to convert to a HashSet and iterate over it   ArrayList lt String gt  values         Your values HashSet lt String gt  uniqueValues   new HashSet lt  gt  values   for  String value   uniqueValues             Do something

[java] Get unique values from arraylist in java

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