Java List contains Object with field value equal to x

Question

I want to check whether a List contains an object that has a field with a certain value  Now  I could use a loop to go through and check  but I was curious if there was anything more code efficient    Something like   if list contains new Object   setName  John            Do some stuff     I know the above code doesn t do anything  it s just to demonstrate roughly what I am trying to achieve   Also  just to clarify  the reason I don t want to use a simple loop is because this code will currently go inside a loop that is inside a loop which is inside a loop  For readability I don t want to keep adding loops to these loops  So I wondered if there were any simple ish  alternatives

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Streams  If you are using Java 8  perhaps you could try something like this   public boolean containsName final List lt MyObject gt  list  final String name       return list stream   filter o - gt  o getName   equals name   findFirst   isPresent        Or alternatively  you could try something like this   public boolean containsName final List lt MyObject gt  list  final String name       return list stream   map MyObject  getName  filter name  equals  findFirst   isPresent        This method will return true if the List lt MyObject gt  contains a MyObject with the name name  If you want to perform an operation on each of the MyObjects that getName   equals name   then you could try something like this   public void perform final List lt MyObject gt  list  final String name       list stream   filter o - gt  o getName   equals name   forEach              o - gt                                                   Where o represents a MyObject instance   Alternatively  as the comments suggest  Thanks MK10   you could use the Stream anyMatch method   public boolean containsName final List lt MyObject gt  list  final String name       return list stream   anyMatch o - gt  o getName   equals name

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Binary Search  You can use Collections binarySearch to search an element in your list  assuming the list is sorted    Collections binarySearch list  new YourObject  a1    b                    c    new Comparator lt YourObject gt                    Override             public int compare YourObject o1  YourObject o2                    return o1 getName   compareTo o2 getName                                 which will return a negative number if the object is not present in the collection or else it will return the index of the object  With this you can search for objects with different searching strategies

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Despite JAVA 8 SDK there is a lot of collection tools libraries can help you to work with  for instance   http   commons apache org proper commons-collections   Predicate condition   new Predicate        boolean evaluate Object obj            return   Sample obj  myField equals  myVal            List result   CollectionUtils select  list  condition

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Eclipse Collections  If you re using Eclipse Collections  you can use the anySatisfy   method  Either adapt your List in a ListAdapter or change your List into a ListIterable if possible   ListIterable lt MyObject gt  list         boolean result       list anySatisfy myObject - gt  myObject getName   equals  John       If you ll do operations like this frequently  it s better to extract a method which answers whether the type has the attribute   public class MyObject       private final String name       public MyObject String name                this name   name             public boolean named String name                return Objects equals this name  name             You can use the alternate form anySatisfyWith   together with a method reference   boolean result   list anySatisfyWith MyObject  named   John      If you cannot change your List into a ListIterable  here s how you d use ListAdapter   boolean result        ListAdapter adapt list  anySatisfyWith MyObject  named   John      Note  I am a committer for Eclipse ollections

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Here is a solution using Guava  private boolean checkUserListContainName List lt User gt  userList  final String targetName        return FluentIterable from userList  anyMatch new Predicate lt User gt               Override         public boolean apply  Nullable User input                return input getName   equals targetName

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If you need to perform this List contains Object with field value equal to x  repeatedly  a simple and efficient workaround would be   List lt field obj type gt  fieldOfInterestValues   new ArrayList lt field obj type gt   for Object obj   List        fieldOfInterestValues add obj getFieldOfInterest         Then the List contains Object with field value equal to x  would be have the same result as fieldOfInterestValues contains x

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Map  You could create a Hashmap lt String  Object gt  using one of the values as a key  and then seeing if yourHashMap keySet   contains yourValue  returns true

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Predicate  If you dont use Java 8  or library which gives you more functionality for dealing with collections  you could implement something which can be more reusable than your solution   interface Predicate lt T gt           boolean contains T item              static class CollectionUtil           public static  lt T gt  T find final Collection lt T gt  collection final  Predicate lt T gt  predicate               for  T item   collection                   if  predicate contains item                        return item                                              return null                   and many more methods to deal with collection             i m using something like that  i have predicate interface  and i m passing it implementation to my util class   What is advantage of doing this in my way  you have one method which deals with searching in any type collection  and you dont have to create separate methods if you want to search by different field  alll what you need to do is provide different predicate which can be destroyed as soon as it no longer usefull   if you want to use it  all what you need to do is call method and define tyour predicate  CollectionUtil find list  new Predicate lt MyObject gt       public boolean contains T item           return  John  equals item getName

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Collection contains   is implemented by calling equals   on each object until one returns true   So one way to implement this is to override equals   but of course  you can only have one equals   Frameworks like Guava therefore use predicates for this  With Iterables find list  predicate   you can search for arbitrary fields by putting the test into the predicate   Other languages built on top of the VM have this built in  In Groovy  for example  you simply write   def result   list find  it name     John      Java 8 made all our lives easier  too   List lt Foo gt  result   list stream        filter it - gt   John  equals it getName         collect Collectors toList       If you care about things like this  I suggest the book  Beyond Java   It contains many examples for the numerous shortcomings of Java and how other languages do better

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You have two choices   1   The first choice  which is preferable  is to override the  equals    method in your Object class   Let s say  for example  you have this Object class   public class MyObject       private String name      private String location        getters and setters     Now let s say you only care about the MyObject s name  that it should be unique so if two  MyObject s have the same name they should be considered equal   In that case  you would want to override the  equals    method  and also the  hashcode    method  so that it compares the names to determine equality   Once you ve done this  you can check to see if a Collection contains a MyObject with the name  foo  by like so   MyObject object   new MyObject    object setName  foo    collection contains object     However  this might not be an option for you if    You are using both the name and location to check for equality  but you only want to check if a Collection has any  MyObject s with a certain location   In this case  you ve already overridden  equals      MyObject  is part of an API that you don t have liberty to change    If either of these are the case  you ll want option 2   2   Write your own utility method   public static boolean containsLocation Collection lt MyObject gt  c  String location        for MyObject o   c            if o    null  amp  amp  o getLocation equals location                 return true                      return false      Alternatively  you could extend ArrayList  or some other collection  and then add your own method to it   public boolean containsLocation String location        for MyObject o   this            if o    null  amp  amp  o getLocation equals location                     return true                                  return false          Unfortunately there s not a better way around it

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This is how to do it using Java 8     boolean isJohnAlive   list stream   anyMatch o - gt  o getName   equals  John

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contains method uses equals internally  So you need to override the equals method for your class as per your need   Btw this does not look syntatically correct    new Object   setName  John

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Google Guava  If you re using Guava  you can take a functional approach and do the following  FluentIterable from list  find new Predicate lt MyObject gt         public boolean apply MyObject input          return  John  equals input getName             Any      which looks a little verbose  However the predicate is an object and you can provide different variants for different searches  Note how the library itself separates the iteration of the collection and the function you wish to apply  You don t have to override equals   for a particular behaviour   As noted below  the java util Stream framework built into Java 8 and later provides something similar

[java] Java List.contains(Object with field value equal to x)

Streams

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