How to maintain a Unique List in Java

Question

How to create a list of unique distinct objects  no duplicates  in Java    Right now I am using HashMap lt String  Integer gt  to do this as the key is overwritten and hence at the end we can get HashMap getKeySet   which would be unique  But I am sure there should be a better way to do this as the value part is wasted here

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HashSet lt String gt   or  any Set implementation may does the job for you  Set don t allow duplicates   Here is javadoc for HashSet

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I want to clarify some things here for the original poster which others have alluded to but haven t really explicitly stated  When you say that you want a Unique List  that is the very definition of an Ordered Set  Some other key differences between the Set Interface and the List interface are that List allows you to specify the insert index  So  the question is do you really need the List Interface  i e  for compatibility with a 3rd party library  etc    or can you redesign your software to use the Set interface  You also have to consider what you are doing with the interface  Is it important to find elements by their index  How many elements do you expect in your set  If you are going to have many elements  is ordering important    If you really need a List which just has a unique constraint  there is the Apache Common Utils class org apache commons collections list SetUniqueList which will provide you with the List interface and the unique constraint  Mind you  this breaks the List interface though  You will  however  get better performance from this if you need to seek into the list by index  If you can deal with the Set interface  and you have a smaller data set  then LinkedHashSet might be a good way to go  It just depends on the design and intent of your software   Again  there are certain advantages and disadvantages to each collection  Some fast inserts but slow reads  some have fast reads but slow inserts  etc  It makes sense to spend a fair amount of time with the collections documentation to fully learn about the finer details of each class and interface

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Use new HashSet lt String gt  An example   import java util HashSet  import java util Set   public class MainClass     public static void main String args          String   name1      Amy    Jose    Jeremy    Alice    Patrick          String   name2      Alan    Amy    Jeremy    Helen    Alexi          String   name3      Adel    Aaron    Amy    James    Alice          Set lt String gt  letter   new HashSet lt String gt          for  int i   0  i  lt  name1 length  i          letter add name1 i         for  int j   0  j  lt  name2 length  j          letter add name2 j         for  int k   0  k  lt  name3 length  k          letter add name3 k         System out println letter size       letters must be sent to      letter

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You can use a Set implementation   Some info from the JAVADoc      A collection that contains no duplicate elements  More formally  sets contain no pair of elements e1 and e2 such that e1 equals e2   and at most one null element  As implied by its name  this interface models the mathematical set abstraction       Note  Great care must be exercised if mutable objects are used as set elements  The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set  A special case of this prohibition is that it is not permissible for a set to contain itself as an element     These are the implementations    HashSet  This class offers constant time performance for the basic operations  add  remove  contains and size   assuming the hash function disperses the elements properly among the buckets  Iterating over this set requires time proportional to the sum of the HashSet instance s size  the number of elements  plus the  capacity  of the backing HashMap instance  the number of buckets   Thus  it s very important not to set the initial capacity too high  or the load factor too low  if iteration performance is important   When iterating a HashSet the order of the yielded elements is undefined  LinkedHashSet  Hash table and linked list implementation of the Set interface  with predictable iteration order  This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries  This linked list defines the iteration ordering  which is the order in which elements were inserted into the set  insertion-order   Note that insertion order is not affected if an element is re-inserted into the set   An element e is reinserted into a set s if s add e  is invoked when s contains e  would return true immediately prior to the invocation    So  the output of the code above      Set lt Integer gt  linkedHashSet   new LinkedHashSet lt  gt      linkedHashSet add 3    linkedHashSet add 1    linkedHashSet add 2     for  int i   linkedHashSet         System out println i           will necessarily be  3 1 2  TreeSet  This implementation provides guaranteed log n  time cost for the basic operations  add  remove and contains   By default he elements returned on iteration are sorted by their  natural ordering   so the code above      Set lt Integer gt  treeSet   new TreeSet lt  gt      treeSet add 3    treeSet add 1    treeSet add 2     for  int i   treeSet         System out println i           will output this   1 2 3    You can also pass a Comparator instance to a TreeSet constructor  making it sort the elements in a different order    Note that the ordering maintained by a set  whether or not an explicit comparator is provided  must be consistent with equals if it is to correctly implement the Set interface   See Comparable or Comparator for a precise definition of consistent with equals   This is so because the Set interface is defined in terms of the equals operation  but a TreeSet instance performs all element comparisons using its compareTo  or compare  method  so two elements that are deemed equal by this method are  from the standpoint of the set  equal  The behavior of a set is well-defined even if its ordering is inconsistent with equals  it just fails to obey the general contract of the Set interface

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You may want to use one of the implementing class of java util Set lt E gt  Interface e g  java util HashSet lt String gt   collection class      A collection that contains no duplicate elements  More formally  sets contain no pair of elements e1 and e2 such that e1 equals e2   and at most one null element  As implied by its name  this interface models the mathematical set abstraction

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I do not know how efficient this is  However worked for me in a simple context   List lt int gt  uniqueNumbers   new ArrayList lt  gt         public void AddNumberToList int num                if  uniqueNumbers  contains num                 uniqueNumbers  add num

User · Answer

You could just use a HashSet lt String gt  to maintain a collection of unique objects  If the Integer values in your map are important  then you can instead use the containsKey method of maps to test whether your key is already in the map

[java] How to maintain a Unique List in Java?

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