Check if one list contains element from the other

Question

I have two lists with different objects in them   List lt Object1 gt  list1  List lt Object2 gt  list2    I want to check if element from list1 exists in list2  based on specific attribute  Object1 and Object2 have  among others    one mutual attribute  with type Long   named attributeSame    right now  I do it like this   boolean found   false  for Object1 object1   list1      for Object2 object2  list2          if object1 getAttributeSame      object2 getAttributeSame                found   true               also do something                    if  found             do something           found   false      But I think there is a better and faster way to do this    Can someone propose it   Thanks

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There is one method of Collection named retainAll but having some side effects for you reference     Retains only the elements in this list that are contained in the   specified collection  optional operation   In other words  removes   from this list all of its elements that are not contained in the   specified collection       true if this list changed as a result of the call   Its like      boolean b   list1 retainAll list2

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org springframework util CollectionUtils  boolean containsAny java util Collection lt   gt  source  java util Collection lt   gt  candidates   Return true if any element in  candidates  is contained in  source   otherwise returns false

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According to the JavaDoc for the  contains Object obj       Returns true if this list contains the specified element  More   formally  returns true if and only if this list contains at least one   element e such that  o  null   e  null   o equals e      So if you override your  equals   method for your given object  you should be able to do  if list1 contains object2       If the elements will be unique  ie  have different attributes  you could override the  equals   and  hashcode   and store everything in  HashSets  This will allow you to check if one contains another element in constant time

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faster way will require additional space     For example    put all items in one list into a HashSet   you have to implement the hash function by yourself to use object getAttributeSame     Go through the other list and check if any item is in the HashSet    In this way each object is visited at most once  and HashSet is fast enough to check or insert any object in O 1

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Loius answer is correct  I just want to add an example   listOne add  A    listOne add  B    listOne add  C     listTwo add  D    listTwo add  E    listTwo add  F           boolean noElementsInCommon   Collections disjoint listOne  listTwo      true

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With java 8  we can do like below to check if one list contains any element of other list  boolean var   lis1 stream   filter element - gt  list2 contains element   findFirst   isPresent

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If you just need to test basic equality  this can be done with the basic JDK without modifying the input lists in the one line   Collections disjoint list1  list2     If you need to test a specific property  that s harder   I would recommend  by default   list1 stream       map Object1  getProperty      anyMatch       list2 stream           map Object2  getProperty          collect toSet             contains       which collects the distinct values in list2 and tests each value in list1 for presence

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Can you define the type of data you hold   is it big data   is it sorted   I think that you need to consider different efficiency approaches depending on the data   For example  if your data is big and unsorted you could try and iterate the two lists together by index and store each list attribute in another list helper   then you could cross check by the current attributes in the helper lists   good luck  edited   and I wouldn t recommend overloading equals  its dangerous and probably against your object oop meaning

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You can use Apache Commons CollectionUtils   if CollectionUtils containsAny list1 list2              do whatever you want   else           do other thing        This assumes that you have properly overloaded the equals functionality for your custom objects

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To shorten Narendra s logic  you can use this   boolean var   lis1 stream   anyMatch element - gt  list2 contains element

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to make it faster  you can add a break  that way the loop will stop if found is set to true   boolean found   false  for Object1 object1   list1      for Object2 object2  list2          if object1 getAttributeSame      object2 getAttributeSame                found   true               also do something              break                     if  found             do something           found   false      If you would have maps in stead of lists with as keys the attributeSame  you could check faster for a value in one map if there is a corresponding value in the second map or not

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If you want to check if an element exists in a list  use the contains method  if  list1 contains Object o          do this

[java] Check if one list contains element from the other

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