In Java remove empty elements from a list of Strings

Question

In Java  I have an ArrayList of Strings like     Hi   How are you    I want to remove the null and empty elements  how to change it so it is like this    Hi How are you

User · Answer

lukastymo s answer seems the best one  But it may be worth mentioning this approach as well for it s extensibility  List lt String gt  list   new ArrayList lt String gt  Arrays asList  quot  quot    quot Hi quot   null   quot How quot    quot are quot      list   list stream               filter item - gt  item    null  amp  amp   item isEmpty                collect Collectors toList      System out println list    What I mean by that is you could then add additional filters  such as             filter item - gt   item startsWith  quot a quot         although of course that s not specifically relevant to the question

User · Answer

If you were asking how to remove the empty strings  you can do it like this  where l is an ArrayList lt String gt   - this removes all null references and strings of length 0   Iterator lt String gt  i   l iterator    while  i hasNext          String s   i next        if  s    null    s isEmpty                  i remove             Don t confuse an ArrayList with arrays  an ArrayList is a dynamic data-structure that resizes according to it s contents  If you use the code above  you don t have to do anything to get the result as you ve described it -if your ArrayList was      Hi      How   are   you    after removing as above  it s going to be exactly what you need -   Hi   How   are   you     However  if you must have a  sanitized  copy of the original list  while leaving the original as it is  and by  store it back  you meant  make a copy   then krmby s code in the other answer will serve you just fine

User · Answer

List lt String gt  list   new ArrayList lt String gt  Arrays asList      Hi   null   How     System out println list   list removeAll Arrays asList     null    System out println list     Output      Hi  null  How   Hi  How

User · Answer

There are a few approaches that you could use    Iterate over the list  calling Iterator remove   for the list elements you want to remove   This is the simplest  Repeatedly call List remove Object    This is simple too  but performs worst of all     because you repeatedly scan the entire list    However  this might be an option for a mutable list whose iterator didn t support remove     for some reason   Create a new list  iterate over the old list  adding elements that you want to retain to a new list    If you can t return the new list  as 3  above and then clear the old list and use addAll to add the elements of the new list back to it    Which of these is fastest depends on the class of the original list  its size  and the number of elements that need to be removed   Here are some of the factors    For an ArrayList  each individual remove operation is O N   where N is the list size   It is expensive to remove multiple elements from a large ArrayList using the Iterator remove   method  or the ArrayList remove element  method      By contrast  the Iterator remove method for a LinkedList is O 1   For an ArrayList  creating and copying a list is O N  and  relatively cheap  especially if you can ensure that the destination list s capacity is large enough  but not too large    By contrast  creating and copying to a LinkedList is also O N   but considerably more expensive    All of this adds up to a fairly complicated decision tree   If the lists are small  say 10 or less elements  you can probably get away with any of the approaches above   If the lists could be large  you need to weigh up all of the issues in the list of the expected list size and expected number of removals    Otherwise you might end up with quadratic performance

User · Answer

Going to drop this lil nugget in here   Stream of      Hi   null   How    are    you      filter t - gt   Strings isNullOrEmpty t      collect ImmutableList toImmutableList       I wish with all of my heart that Java had a filterNot

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private List cleanInputs String   inputArray            List lt String gt  result   new ArrayList lt String gt  inputArray length           for  String input   inputArray                if  input    null                    String str   input trim                    if   str isEmpty                          result add str                                                     return result

User · Answer

Another way to do this now that we have Java 8 lambda expressions   arrayList removeIf item - gt  item    null       equals item

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List lt String gt  list   new ArrayList lt String gt  Arrays asList  quot  quot    quot Hi quot    quot  quot    quot How quot                Stream lt String gt  stream   list  stream               Predicate lt String gt  empty   empt- gt  empt equals  quot  quot                 Predicate lt String gt  emptyRev   empty negate               list   stream filter emptyRev  collect Collectors toList      OR list    list  stream   filter empty- gt   empty equals  quot  quot     collect Collectors toList

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If you get UnsupportedOperationException from using one of ther answer above and your List is created from Arrays asList    it is because you can t edit such List   To fix  wrap the Arrays asList   inside new LinkedList lt String gt          List lt String gt  list   new LinkedList lt String gt  Arrays asList split      Source is from this answer

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If you are using Java 8 then try this using lambda expression and org apache commons lang StringUtils  that will also clear null and blank values from array input  public static String   cleanArray String   array        return Arrays stream array  filter x - gt   StringUtils isBlank x   toArray String    new       ref - https   stackoverflow com a 41935895 9696526

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Regarding the comment of Andrew Mairose - Although a fine solution  I would just like to add that this solution will not work on fixed size lists   You could attempt doing like so   Arrays asList new String    a    b   null   c                 removeIf item - gt  item    null       equals item      But you ll encounter an UnsupportedOperationException at java util AbstractList remove since asList returns a non-resizable List     A different solution might be this   List lt String gt  collect       Stream of new String    a    b    c   null                filter item - gt  item    null  amp  amp      equals item            collect Collectors toList       Which will produce a nice list of strings  -

User · Answer

This code compiles and runs smoothly  It uses no iterator so more readable  list is your collection  result is filtered form  no null no empty       public static void listRemove         List lt String gt  list   Arrays asList      Hi        How    are    you        List lt String gt  result   new ArrayList lt String gt          for  String str   list            if  str    null  amp  amp   str isEmpty                  result add str                        System out println result

User · Answer

Its a very late answer  but you can also use the Collections singleton   List lt String gt  list   new ArrayList lt String gt  Arrays asList      Hi   null   How     list removeAll Collections singleton null    list removeAll Collections singleton

[java] In Java, remove empty elements from a list of Strings

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