Finding all objects that have a given property inside a collection

Question

I have some complicated object  such as a Cat  which has many properties  such as age  favorite cat food  and so forth     A bunch of Cats are stored in a Java Collection  and I need to find all the Cats that are aged 3  or those whose favorite cat food is Whiskas   Surely  I can write a custom method that finds those Cats with a specific property  but this gets cumbersome with many properties  is there some generic way of doing this

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Just FYI there are 3 other answers given to this question that use Guava  but none answer the question  The asker has said he wishes to find all Cats with a matching property  e g  age of 3  Iterables find will only match one  if any exist  You would need to use Iterables filter to achieve this if using Guava  for example   Iterable lt Cat gt  matches   Iterables filter cats  new Predicate lt Cat gt           Override     public boolean apply Cat input            return input getAge      3

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you can search item from a list as follows function  good luck   int  searchList List lt T gt  list T item    int idx   -1  idx   Collections binarySearch list item          new Comparator lt T gt                  public int compare Titm1                      Titm2                        key1                 if  itm1 key1 compareTo itm2 key1     0                        return itm1 key2 compareTo itm2 key2                                         key2                 return itm1 key2                          compareTo itm2 key2                              return idx

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I suggest using Jxpath  it allows you to do queries on object graphs as if it where xpath like  JXPathContext newContext cats        getValue       drinks  milk

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Here s an idea for a searcher class you could parameterize with the specific values you want to look for   You could go further and store the names of the properties as well  probably in a Map with the desired values   In this case you would use reflection on the Cat class to call the appropriate methods given the property names   public class CatSearcher      private Integer ageToFind   null    private String  foodToFind   null     public CatSearcher  Integer age  String food         this ageToFind   age      this foodToFind   food         private boolean isMatch Cat cat        if   this ageToFind    null  amp  amp   cat getAge   equals this ageToFind             return false            if   this foodToFind    null  amp  amp   cat getFood   equals this foodToFind           return false            return true         public Collection lt Cat gt  search  Collection lt Cat gt  listToSearch            details left to the imagination  but basically iterate over        the input list  call isMatch on each element  and if true        add it to a local collection which is returned at the end

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Try the commons collections API   List lt Cat gt  bigList            master list  Collection lt Cat gt  smallList   CollectionUtils select bigList  new Predicate         public boolean evaluate Object o            Cat c    Cat o          return c getFavoriteFood   equals  Wiskas                 amp  amp  c getWhateverElse   equals Something               Of course you don t have to use an anonymous class every time  you could create implementations of the Predicate interface for commonly used searchs

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With Java 8 lambda expression you can do something like  cats stream        filter  c - gt  c getAge      3  amp  amp  c getFavoriteFood      WHISKAS        collect Collectors toList       Conceptually the same as the Guava Predicate approach  but it looks much cleaner with lambda   Probably not a valid answer for OP but worth to note for people with similar need

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You can use lambdaj  Things like these are trivial  the syntax is really smooth   Person me   new Person  Mario    Fusco   35   Person luca   new Person  Luca    Marrocco   29   Person biagio   new Person  Biagio    Beatrice   39   Person celestino   new Person  Celestino    Bellone   29   List lt Person gt  meAndMyFriends   asList me  luca  biagio  celestino   List lt Person gt  oldFriends   filter having on Person class  getAge    greaterThan 30    meAndMyFriends     and you can do much more complicated things  It uses hamcrest for the Matchers  Some will argue that this is not Java-style  but it s fun how this guy twisted Java to make a bit of functional programming  Have a look at the source code also  it s quite sci-fi

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You could write a method that takes an instance of an interface which defines a check Cat  method  where that method can be implemented with whatever property-checking you want   Better yet  make it generic   public interface Checker lt T gt        public boolean check T obj      public class CatChecker implements Checker lt Cat gt        public boolean check Cat cat            return  cat age    3      or whatever  implement your comparison here             put this in some class public static  lt T gt  Collection lt T gt  findAll Collection lt T gt  coll  Checker lt T gt  chk        LinkedList lt T gt  l   new LinkedList lt T gt         for  T obj   coll             if  chk check obj                l add obj             return l      Of course  like other people are saying  this is what relational databases were made for

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Solutions based on predicates  filters and custom iterators comparators are nice  but they don t provide analogue with database index  For instance  I would like to search collection of cats different ways  by sex and age and by age  so it looks like two indexes  1   sex  age  2   age    Values  accessed by that indexes could be hashed  to implement fast lookup  without iterating the whole collection  Is anywhere there such solution

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I have been using Google Collections  now called Guava  for this kind of problem  There is a class called Iterables that can take an interface called Predicate as a parameter of a method that is really helpful   Cat theOne   Iterables find cats  new Predicate lt Cat gt          public boolean apply Cat arg    return arg age      3          Check it here

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Sounds a lot like something you would use LINQ for in  NET  While there s no  real  LINQ implementation for java yet  you might want to have a look at Quaere which could do what you describe easily enough

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Again with the commons collections API  You get the  checker  type code when you Implement the Predicate Separately -  public class CatPredicate implements Predicate        private int age         public CatPredicate int age            super            this age   age               Override     public boolean evaluate Object o            Cat c  Cat o          return c getAge    this age             which gets used as  -  CollectionUtils filter catCollectionToFilter  new CatPredicate 3

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You could try some of the generic code in the Apache Commons project  The Collections subproject provides code for finding objects that match a particular Predicate  as well as a large number of predicates  equals  null  instanceof  etc   The BeanUtils subproject allows you to make predicates that test properties of beans   Use the CollectionUtils class to search within a collection  There are a few methods for this  but check out the select   method  in particular  Use the following classes to construct predicates  or write your own  Predicate  PredicateUtils  BeanPredicate   This is all sometimes a bit cumbersome  but at least it s generic   -

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Use Google Guava   final int lastSeq   myCollections size    Clazz target   Iterables find myCollections  new Predicate lt Clazz gt           Override     public boolean apply  Nullable Clazz input          return input getSeq      lastSeq              I think to use this method

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You can use something like JoSQL  and write  SQL  against your collections  http   josql sourceforge net    Which sounds like what you want  with the added benefit of being able to do more complicated queries

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A very common problem and I have used google collections and here is my code  public class FindByIdPredicate implements Predicate lt IDObject gt     private Long entityId   public FindByIdPredicate final Long entityId        this entityId   entityId       Override public boolean apply final IDObject input        return input getId   equals this entityId                 Pass in the Collection     param Collection     return IdObject if present or null     public IDObject getEntity final Collection lt   extends IDObject gt  collection         for  IDObject idObject   collection            if  this apply idObject                 return idObject                      return null              param Set     return IdObject if present or null      SuppressWarnings  unchecked   public  lt T gt  T getEntity final Set lt   extends IDObject gt  set         for  IDObject idObject   set            if  this apply idObject                 return  T  idObject                      return null          Hope this helps

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JFilter http   code google com p jfilter  suites your requirement   JFilter is a simple and high performance open source library to query collection of Java beans   Key features   Support of collection  java util Collection  java util Map and Array  properties  Support of collection inside collection of any depth  Support of inner queries  Support of parameterized queries  Can filter 1 million records in few 100 ms  Filter   query  is given in simple json format  it is like Mangodb queries  Following are some examples      id     le   10    where object id property is less than equals to 10      id      in    0    100      where object id property is 0 or 100     lineItems    lineAmount   1     where lineItems collection property of parameterized type  has lineAmount equals to 1       and     id    0      billingAddress    city   DEL       where id property is 0 and billingAddress city property is DEL     lineItems    taxes     key    code   GST     value     gt    1 01       where lineItems collection property of parameterized type  which has taxes    map type property of parameteriszed type  has code equals to GST value greater than 1 01      or     code   10     skus      and     price     in    20    40        code   RedApple         Select all products where product code is 10 or sku price in 20 and 40 and sku code is  RedApple

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Using Commons Collections   EqualPredicate nameEqlPredicate   new EqualPredicate 3   BeanPredicate beanPredicate   new BeanPredicate  age   nameEqlPredicate   return CollectionUtils filter cats  beanPredicate

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Guava has a very powerful searching capabilities when it comes to such type of problems  For example  if your area searching an object based on one of it properties you may consider    Iterables tryFind listOfCats  new Predicate lt Cat gt          Override     boolean apply  Nullable Cat input            return  tom  equalsIgnoreCase input name              or new Cat  Tom       in case it s possible that the Tom cat is not in the listOfCats  it will be returned  thus allowing you to avoid NPE

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You could store those objects on a database  If you don t want the overhead of a full blown database server you can use an embedded one like HSQLDB  Then you can use Hibernate or BeanKeeper  simpler to use  or other ORM to map objects to tables  You keep using the OO model and get the advanced storage and querying benefits from a database

[java] Finding all objects that have a given property inside a collection

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