How to find an object in an ArrayList by property

Question

How can I find an object  Carnet  in a ArrayList lt Carnet gt  knowing its property codeIsin   List lt Carnet gt  listCarnet   carnetEJB findAll     public class Carnet        private String codeTitre      private String nomTitre      private String codeIsin          Setters and getters

User · Answer

For finding objects which are meaningfully equal, you need to override equals and hashcode methods for the class. You can find a good tutorial here.

http://www.thejavageek.com/2013/06/28/significance-of-equals-and-hashcode/

User · Answer

If you use Java 8 and if it is possible that your search returns null  you could try using the  Optional class    To find a carnet   private final Optional lt Carnet gt  findCarnet Collection lt Carnet gt  yourList  String codeIsin          This stream will simply return any carnet that matches the filter  It will be wrapped in a Optional object         If no carnets are matched  an  Optional empty  item will be returned     return yourList stream   filter c - gt  c getCodeIsin   equals codeIsin   findAny        Now a usage for it   public void yourMethod String codeIsin       List lt Carnet gt  listCarnet   carnetEJB findAll         Optional lt Carnet gt  carnetFound   findCarnet listCarnet  codeIsin        if carnetFound isPresent                You use this   get    method to actually get your carnet from the Optional object         doSomething carnetFound get               else          doSomethingElse

User · Answer

Here is a solution using Guava  private User findUserByName List lt User gt  userList  final String name        Optional lt User gt  userOptional               FluentIterable from userList  firstMatch new Predicate lt User gt                       Override                 public boolean apply  Nullable User input                        return input getName   equals name                                         return userOptional isPresent     userOptional get     null     return user if found otherwise return null if user name don t exist in user list

User · Answer

Here is another solution using Guava in Java 8 that returns the matched element if one exists in the list  If more than one elements are matched then the collector throws an IllegalArgumentException  A null is returned if there is no match   Carnet carnet   listCarnet stream                        filter c - gt  c getCodeIsin   equals wantedCodeIsin                        collect MoreCollectors toOptional                         orElse null

User · Answer

You can t without an iteration   Option 1  Carnet findCarnet String codeIsIn        for Carnet carnet   listCarnet            if carnet getCodeIsIn   equals codeIsIn                 return carnet                      return null      Option 2  Override the equals   method of Carnet   Option 3  Storing your List as a Map instead  using codeIsIn as the key   HashMap lt String  Carnet gt  carnets   new HashMap lt  gt        setting map Carnet carnet   carnets get codeIsIn

User · Answer

In Java8 you can use streams   public static Carnet findByCodeIsIn Collection lt Carnet gt  listCarnet  String codeIsIn        return listCarnet stream   filter carnet - gt  codeIsIn equals carnet getCodeIsin     findFirst   orElse null       Additionally  in case you have many different objects  not only Carnet  or you want to find it by different properties  not only by cideIsin   you could build an utility class  to ecapsulate this logic in it   public final class FindUtils       public static  lt T gt  T findByProperty Collection lt T gt  col  Predicate lt T gt  filter            return col stream   filter filter  findFirst   orElse null            public final class CarnetUtils       public static Carnet findByCodeTitre Collection lt Carnet gt  listCarnet  String codeTitre            return FindUtils findByProperty listCarnet  carnet - gt  codeTitre equals carnet getCodeTitre                 public static Carnet findByNomTitre Collection lt Carnet gt  listCarnet  String nomTitre            return FindUtils findByProperty listCarnet  carnet - gt  nomTitre equals carnet getNomTitre                 public static Carnet findByCodeIsIn Collection lt Carnet gt  listCarnet  String codeIsin            return FindUtils findByProperty listCarnet  carnet - gt  codeIsin equals carnet getCodeIsin

User · Answer

Following with Oleg answer  if you want to find ALL objects in a List filtered by a property  you could do something like    Search into a generic list ALL items with a generic property public final class SearchTools          public static  lt T gt  List lt T gt  findByProperty Collection lt T gt  col  Predicate lt T gt  filter               List lt T gt  filteredList    List lt T gt   col stream   filter filter  collect Collectors toList                return filteredList            Search in the list  quot listItems quot  ALL items of type  quot Item quot  with the specific property  quot iD item itemID quot  public static final class ItemTools            public static List lt Item gt  findByItemID Collection lt Item gt  listItems  String itemID                 return SearchTools findByProperty listItems  item - gt  itemID equals item getiD Item                           and similarly if you want to filter ALL items in a HashMap with a certain Property   Search into a MAP ALL items with a given property public final class SearchTools          public static  lt T gt  HashMap lt String T gt  filterByProperty HashMap lt String T gt  completeMap  Predicate lt   super Map Entry lt String T gt  gt  filter               HashMap lt String T gt  filteredList    HashMap lt String T gt   completeMap entrySet   stream                                    filter filter                                   collect Collectors toMap map - gt  map getKey    map - gt  map getValue                 return filteredList                 Search into the MAP ALL items with specific properties public static final class ItemTools            public static HashMap lt String Item gt  filterByParentID HashMap lt String Item gt  mapItems  String parentID                return SearchTools filterByProperty mapItems  mapItem - gt  parentID equals mapItem getValue   getiD Parent                         public static HashMap lt String Item gt  filterBySciName HashMap lt String Item gt  mapItems  String sciName                return SearchTools filterByProperty mapItems  mapItem - gt  sciName equals mapItem getValue   getSciName

[java] How to find an object in an ArrayList by property

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