Is there a no-duplicate List implementation out there

Question

I know about SortedSet  but in my case I need something that implements List  and not Set  So is there an implementation out there  in the API or elsewhere   It shouldn t be hard to implement myself  but I figured why not ask people here first

User · Answer

Off the top of my head  lists allow duplicates  You could quickly implement a UniqueArrayList and override all the add   insert functions to check for contains   before you call the inherited methods  For personal use  you could only implement the add method you use  and override the others to throw an exception in case future programmers try to use the list in a different manner

User · Answer

So here s what I did eventually  I hope this helps someone else   class NoDuplicatesList lt E gt  extends LinkedList lt E gt         Override     public boolean add E e            if  this contains e                 return false                    else               return super add e                         Override     public boolean addAll Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll copy               Override     public boolean addAll int index  Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll index  copy               Override     public void add int index  E element            if  this contains element                 return                    else               super add index  element

User · Answer

Off the top of my head  lists allow duplicates  You could quickly implement a UniqueArrayList and override all the add   insert functions to check for contains   before you call the inherited methods  For personal use  you could only implement the add method you use  and override the others to throw an exception in case future programmers try to use the list in a different manner

User · Answer

Why not encapsulate a set with a list  sort like   new ArrayList  new LinkedHashSet       This leaves the other implementation for someone who is a real master of Collections  -

User · Answer

You should seriously consider dhiller s answer    Instead of worrying about adding your objects to a duplicate-less List  add them to a Set  any implementation   which will by nature filter out the duplicates  When you need to call the method that requires a List  wrap it in a new ArrayList set   or a new LinkedList set   whatever     I think that the solution you posted with the NoDuplicatesList has some issues  mostly with the contains   method  plus your class does not handle checking for duplicates in the Collection passed to your addAll   method

User · Answer

You should seriously consider dhiller s answer    Instead of worrying about adding your objects to a duplicate-less List  add them to a Set  any implementation   which will by nature filter out the duplicates  When you need to call the method that requires a List  wrap it in a new ArrayList set   or a new LinkedList set   whatever     I think that the solution you posted with the NoDuplicatesList has some issues  mostly with the contains   method  plus your class does not handle checking for duplicates in the Collection passed to your addAll   method

User · Answer

You should seriously consider dhiller s answer    Instead of worrying about adding your objects to a duplicate-less List  add them to a Set  any implementation   which will by nature filter out the duplicates  When you need to call the method that requires a List  wrap it in a new ArrayList set   or a new LinkedList set   whatever     I think that the solution you posted with the NoDuplicatesList has some issues  mostly with the contains   method  plus your class does not handle checking for duplicates in the Collection passed to your addAll   method

User · Answer

Why not encapsulate a set with a list  sort like   new ArrayList  new LinkedHashSet       This leaves the other implementation for someone who is a real master of Collections  -

User · Answer

Off the top of my head  lists allow duplicates  You could quickly implement a UniqueArrayList and override all the add   insert functions to check for contains   before you call the inherited methods  For personal use  you could only implement the add method you use  and override the others to throw an exception in case future programmers try to use the list in a different manner

User · Answer

There s no Java collection in the standard library to do this  LinkedHashSet lt E gt  preserves ordering similarly to a List  though  so if you wrap your set in a List when you want to use it as a List you ll get the semantics you want   Alternatively  the Commons Collections  or commons-collections4  for the generic version  has a List which does what you want already  SetUniqueList   SetUniqueList lt E gt

User · Answer

There s no Java collection in the standard library to do this  LinkedHashSet lt E gt  preserves ordering similarly to a List  though  so if you wrap your set in a List when you want to use it as a List you ll get the semantics you want   Alternatively  the Commons Collections  or commons-collections4  for the generic version  has a List which does what you want already  SetUniqueList   SetUniqueList lt E gt

User · Answer

You should seriously consider dhiller s answer    Instead of worrying about adding your objects to a duplicate-less List  add them to a Set  any implementation   which will by nature filter out the duplicates  When you need to call the method that requires a List  wrap it in a new ArrayList set   or a new LinkedList set   whatever     I think that the solution you posted with the NoDuplicatesList has some issues  mostly with the contains   method  plus your class does not handle checking for duplicates in the Collection passed to your addAll   method

User · Answer

Here is what I did and it works   Assuming I have an ArrayList to work with the first thing I did was created a new LinkedHashMap   LinkedHashSet lt E gt  hashSet   new LinkedHashSet lt E gt      Then I attempt to add my new element to the LinkedHashSet   The add method does not alter the LinkedHasSet and returns false if the new element is a duplicate   So this becomes a condition I can test before adding to the ArrayList    if  hashSet add E   arrayList add E     This is a simple and elegant way to prevent duplicates from being added to an array list   If you want you can encapsulate it in and override of the add method in a class that extends the ArrayList   Just remember to deal with addAll by looping through the elements and calling the add method

User · Answer

I needed something like that  so I went to the commons collections and used the SetUniqueList  but when I ran some performance test  I found that it seems not optimized comparing to the case if I want to use a Set and obtain an Array using the Set toArray   method    The SetUniqueTest took 20 1 time to fill and then traverse 100 000 Strings comparing to the other implementation  which is a big deal difference   So  if you worry about the performance  I recommend you to use the Set and Get an  Array instead of using the SetUniqueList  unless you really need the logic of the SetUniqueList  then you ll need to check other solutions     Testing code main method   public static void main String   args      SetUniqueList pq   SetUniqueList decorate new ArrayList     Set s   new TreeSet     long t1   0L  long t2   0L  String t    t1   System nanoTime    for  int i   0  i  lt  200000  i          pq add  a    Math random       while   pq isEmpty          t    String  pq remove 0     t1   System nanoTime   - t1   t2   System nanoTime    for  int i   0  i  lt  200000  i          s add  a    Math random        s clear    String   d    String    s toArray new String 0    s clear    for  int i   0  i  lt  d length  i          t   d i      t2   System nanoTime   - t2   System out println  double t1 1000 1000 1000     seconds System out println  double t2 1000 1000 1000     seconds System out println   double  t1    t2            comparing results      Regards  Mohammed Sleem

User · Answer

The documentation for collection interfaces says      Set     a collection that cannot contain duplicate elements    List     an ordered collection  sometimes called a sequence   Lists can contain duplicate elements    So if you don t want duplicates  you probably shouldn t use a list

User · Answer

in add method  why not using HashSet add   to check duplicates instead of HashSet consist    HashSet add   will return true if no duplicate and false otherwise

User · Answer

So here s what I did eventually  I hope this helps someone else   class NoDuplicatesList lt E gt  extends LinkedList lt E gt         Override     public boolean add E e            if  this contains e                 return false                    else               return super add e                         Override     public boolean addAll Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll copy               Override     public boolean addAll int index  Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll index  copy               Override     public void add int index  E element            if  this contains element                 return                    else               super add index  element

User · Answer

There s no Java collection in the standard library to do this  LinkedHashSet lt E gt  preserves ordering similarly to a List  though  so if you wrap your set in a List when you want to use it as a List you ll get the semantics you want   Alternatively  the Commons Collections  or commons-collections4  for the generic version  has a List which does what you want already  SetUniqueList   SetUniqueList lt E gt

User · Answer

Off the top of my head  lists allow duplicates  You could quickly implement a UniqueArrayList and override all the add   insert functions to check for contains   before you call the inherited methods  For personal use  you could only implement the add method you use  and override the others to throw an exception in case future programmers try to use the list in a different manner

User · Answer

Why not encapsulate a set with a list  sort like   new ArrayList  new LinkedHashSet       This leaves the other implementation for someone who is a real master of Collections  -

User · Answer

Why not encapsulate a set with a list  sort like   new ArrayList  new LinkedHashSet       This leaves the other implementation for someone who is a real master of Collections  -

User · Answer

in add method  why not using HashSet add   to check duplicates instead of HashSet consist    HashSet add   will return true if no duplicate and false otherwise

User · Answer

NOTE  it does not take subList implementation into account   import java util ArrayList  import java util Collection  import java util HashSet  import java util Set   public class UniqueList lt T gt  extends ArrayList lt T gt         private static final long serialVersionUID   1L           Unique elements SET        private final Set lt T gt  set new HashSet             Used by addAll methods        private Collection lt T gt  addUnique Collection lt   extends T gt  col            Collection lt T gt  unique new ArrayList            for T e  col               if  set add e   unique add e                     return unique              Override     public boolean add T e            return set add e    super add e    false              Override     public boolean addAll Collection lt   extends T gt  col            return super addAll addUnique col                Override     public void add int index  T e            if  set add e   super add index  e               Override     public boolean addAll int index  Collection lt   extends T gt  col            return super addAll index  addUnique col

User · Answer

I just made my own UniqueList in my own little library like this   package com bprog collections   my own little set of useful utilities and classes  import java util HashSet  import java util ArrayList  import java util List           author Jonathan    public class UniqueList    private HashSet masterSet   new HashSet    private ArrayList growableUniques  private Object   returnable   public UniqueList         growableUniques   new ArrayList       public UniqueList int size        growableUniques   new ArrayList size      public void add Object thing        if   masterSet contains thing             masterSet add thing           growableUniques add thing                   Casts to an ArrayList of unique values     return      public List getList        return growableUniques     public Object get int index        return growableUniques get index      public Object   toObjectArray         int size   growableUniques size        returnable   new Object size       for  int i   0  i  lt  size  i              returnable i    growableUniques get i             return returnable            I have a TestCollections class that looks like this   package com bprog collections  import com bprog out Out           author Jonathan    public class TestCollections       public static void main String   args           UniqueList ul   new UniqueList            ul add  Test            ul add  Test            ul add  Not a copy            ul add  Test               should only contain two things         Object   content   ul toObjectArray            Out pl  Array Content  content             Works fine  All it does is it adds to a set if it does not have it already and there s an Arraylist that is returnable  as well as an object array

User · Answer

So here s what I did eventually  I hope this helps someone else   class NoDuplicatesList lt E gt  extends LinkedList lt E gt         Override     public boolean add E e            if  this contains e                 return false                    else               return super add e                         Override     public boolean addAll Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll copy               Override     public boolean addAll int index  Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll index  copy               Override     public void add int index  E element            if  this contains element                 return                    else               super add index  element

User · Answer

There s no Java collection in the standard library to do this  LinkedHashSet lt E gt  preserves ordering similarly to a List  though  so if you wrap your set in a List when you want to use it as a List you ll get the semantics you want   Alternatively  the Commons Collections  or commons-collections4  for the generic version  has a List which does what you want already  SetUniqueList   SetUniqueList lt E gt

User · Answer

The documentation for collection interfaces says      Set     a collection that cannot contain duplicate elements    List     an ordered collection  sometimes called a sequence   Lists can contain duplicate elements    So if you don t want duplicates  you probably shouldn t use a list

User · Answer

I needed something like that  so I went to the commons collections and used the SetUniqueList  but when I ran some performance test  I found that it seems not optimized comparing to the case if I want to use a Set and obtain an Array using the Set toArray   method    The SetUniqueTest took 20 1 time to fill and then traverse 100 000 Strings comparing to the other implementation  which is a big deal difference   So  if you worry about the performance  I recommend you to use the Set and Get an  Array instead of using the SetUniqueList  unless you really need the logic of the SetUniqueList  then you ll need to check other solutions     Testing code main method   public static void main String   args      SetUniqueList pq   SetUniqueList decorate new ArrayList     Set s   new TreeSet     long t1   0L  long t2   0L  String t    t1   System nanoTime    for  int i   0  i  lt  200000  i          pq add  a    Math random       while   pq isEmpty          t    String  pq remove 0     t1   System nanoTime   - t1   t2   System nanoTime    for  int i   0  i  lt  200000  i          s add  a    Math random        s clear    String   d    String    s toArray new String 0    s clear    for  int i   0  i  lt  d length  i          t   d i      t2   System nanoTime   - t2   System out println  double t1 1000 1000 1000     seconds System out println  double t2 1000 1000 1000     seconds System out println   double  t1    t2            comparing results      Regards  Mohammed Sleem

User · Answer

I just made my own UniqueList in my own little library like this   package com bprog collections   my own little set of useful utilities and classes  import java util HashSet  import java util ArrayList  import java util List           author Jonathan    public class UniqueList    private HashSet masterSet   new HashSet    private ArrayList growableUniques  private Object   returnable   public UniqueList         growableUniques   new ArrayList       public UniqueList int size        growableUniques   new ArrayList size      public void add Object thing        if   masterSet contains thing             masterSet add thing           growableUniques add thing                   Casts to an ArrayList of unique values     return      public List getList        return growableUniques     public Object get int index        return growableUniques get index      public Object   toObjectArray         int size   growableUniques size        returnable   new Object size       for  int i   0  i  lt  size  i              returnable i    growableUniques get i             return returnable            I have a TestCollections class that looks like this   package com bprog collections  import com bprog out Out           author Jonathan    public class TestCollections       public static void main String   args           UniqueList ul   new UniqueList            ul add  Test            ul add  Test            ul add  Not a copy            ul add  Test               should only contain two things         Object   content   ul toObjectArray            Out pl  Array Content  content             Works fine  All it does is it adds to a set if it does not have it already and there s an Arraylist that is returnable  as well as an object array

User · Answer

So here s what I did eventually  I hope this helps someone else   class NoDuplicatesList lt E gt  extends LinkedList lt E gt         Override     public boolean add E e            if  this contains e                 return false                    else               return super add e                         Override     public boolean addAll Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll copy               Override     public boolean addAll int index  Collection lt   extends E gt  collection            Collection lt E gt  copy   new LinkedList lt E gt  collection           copy removeAll this           return super addAll index  copy               Override     public void add int index  E element            if  this contains element                 return                    else               super add index  element

User · Answer

The documentation for collection interfaces says      Set     a collection that cannot contain duplicate elements    List     an ordered collection  sometimes called a sequence   Lists can contain duplicate elements    So if you don t want duplicates  you probably shouldn t use a list

User · Answer

The documentation for collection interfaces says      Set     a collection that cannot contain duplicate elements    List     an ordered collection  sometimes called a sequence   Lists can contain duplicate elements    So if you don t want duplicates  you probably shouldn t use a list

User · Answer

Here is what I did and it works   Assuming I have an ArrayList to work with the first thing I did was created a new LinkedHashMap   LinkedHashSet lt E gt  hashSet   new LinkedHashSet lt E gt      Then I attempt to add my new element to the LinkedHashSet   The add method does not alter the LinkedHasSet and returns false if the new element is a duplicate   So this becomes a condition I can test before adding to the ArrayList    if  hashSet add E   arrayList add E     This is a simple and elegant way to prevent duplicates from being added to an array list   If you want you can encapsulate it in and override of the add method in a class that extends the ArrayList   Just remember to deal with addAll by looping through the elements and calling the add method

User · Answer

NOTE  it does not take subList implementation into account   import java util ArrayList  import java util Collection  import java util HashSet  import java util Set   public class UniqueList lt T gt  extends ArrayList lt T gt         private static final long serialVersionUID   1L           Unique elements SET        private final Set lt T gt  set new HashSet             Used by addAll methods        private Collection lt T gt  addUnique Collection lt   extends T gt  col            Collection lt T gt  unique new ArrayList            for T e  col               if  set add e   unique add e                     return unique              Override     public boolean add T e            return set add e    super add e    false              Override     public boolean addAll Collection lt   extends T gt  col            return super addAll addUnique col                Override     public void add int index  T e            if  set add e   super add index  e               Override     public boolean addAll int index  Collection lt   extends T gt  col            return super addAll index  addUnique col

[java] Is there a no-duplicate List implementation out there?

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